The g Factor   General Intelligence and its Implications   Christopher BRAND
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III NATURE IN NURTURE - the developmental origins of g differences


  • Positions and phobias

  • Eugenics, Sir Cyril Burt. and Leon Kamin

  • Obstacles and objections to psychogenetic research

  • Seven major research programmes, eight new findings

  • Heritability estimates - why all the fuss?


    How do individual differences in intelligence come about? - For example, why is there (on average) a difference of 12 IQ points between ordinary children who grow up in the same biological family? Perhaps researchers will never find out. Perhaps it is a 'meaningless question'. And perhaps that is a very good thing.

    This chapter concerns the legendary 'nature-nurture controversy' about general intelligence. This long-running debate - or, sometimes, non-debate - has been beset by:

    1. (a) anxieties about the social and political implications that might seem to follow if g differences were thought largely genetic;
    2. (b) accusations of fraud against the leading mid-twentieth-century exponent of a largely 'hereditarian' account of the g factor;
    3. (c) protests that psychogenetic research is anyhow pointless. For these reasons geneticists have fought shy of studying g ; and most psychologists have preferred their homely offices and high-IQ subjects to the real-life study of twins and adoptees.

    Anxieties about possible state interference in family life are considered in the context of interventions that are made already, of future possibilities of genetic engineering, and of the improving aspirations of social environmentalists. The question of fraud by researchers is serious - not least when it results in criminal conviction and imprisonment; but science has its own answers to fraud so long as enquiry is allowed to flourish. Methodological problems about nature-nurture questions are probably the most grievous for science, so they are examined. The chief concern is with the popular suggestion that nature and nurture are so complex in their 'ongoing interaction' as necessarily to defy the scientist.

    The past decade has brought new researches from the USA, France and Finland. These allow estimates of the importance of the parentally supplied environment, of the 'micro-environments' that children choose for themselves, of genetic influences (some of them heritable), and of such 'interaction effects' as can be specified by would-be devotees. Ideological resistance to psychogenetic research has always been a matter for shame and is now pointless. Today it is increasingly clear that the truth about g 's degree of heritability must be sought, not shunned. Modern research into other human psychological differences suggests that no alterations at all of heredity or environment would have much effect on average trait levels in the population: g itself might be similar. Learning the truth is now easier than obscurantist endeavour - not least because discouraging irresponsible parenthood is anyhow a natural objective for theorists of both hereditarian and environmentalist persuasions.

    There have always been psychologists who disagreed with Spearman and his intellectual descendants in the London School. Many of today's psychologists view with alarm the possibilities that intelligence is either substantially unitary or closely associated with speed of intake of elementary information. For many psychologists, it has been a cardinal principle to dispute (if they could not ignore) both the 'positive manifold' of mental test correlations that licenses talk of g (see Chapter 1) and the association of general intelligence (g ) with 'inspection time' (IT) (see Chapter 2). Such leading American educational psychologists as Howard Gardner and Robert Sternberg continue to cavil at the London School claim for the centrality and underlying simplicity of g .

    However, though he is the best-known student of Louis Thurstone, the original disunitarian, John Carroll (1993) has concluded that g differences create correlations between hundreds of mental tests in hundreds of studies (see Chapter 1); and the British experimental psychologist, Patrick Rabbitt, has acknowledged a strong link between g and Inspection Time (Nettelbeck & Rabbitt, 1992) (see Chapter 2). Despite objections and fixed ideas, research on test inter-correlations and basic processes has at least continued; and today there is probably increasing agreement on the importance of g differences across lower ability levels and on the 'differentiation' of the more varied ability and personality profiles of higher-g people (e.g. Brand et al., 1994). It was only on another type of question, about the developmental origins of g differences, that twentieth-century psychologists would quite largely abjure their usual principle that 'further research is necessary' - at least so long as the taxpayer foots the bill.

    This question, about g 's heritability, concerns the respective involvement of three broad influences: genetic (G) differences, environmental (E) differences, and more or less complex 'interactions' and 'covariation' of g and E variables (especially 'G x E ' and 'G,ECOV') are the main ways in which people's g differences might be produced. These three broad types of influence bear comparison with those that have appealed respectively to people of differing philosophical persuasions.

    1. (i) Rationalists (who find key truths 'self-evident' to human reason) and hereditarians put more stress on 'hard-wired' human faculties and individual differences and on the fundamental abilities (or inabilities) that result: they incline to accept genetic provision of abilities.

    2. (ii) Empiricists stress the role of the environment and of our conditioning (or other similarly passive experience): they presume experience is the prime contibutor to our kowledge and to our natures.

    3. (iii) Idealists prefer to invoke more active, on-going constructions that usually involve language and society and often a semblance of a person (of unspecified genetic ancestry, but still available to do all the necessary perceiving and interacting).

    Given the relatively distinct nature of the three options, an important stratagem for any one party to the argument is to dismiss the other two options as old-fashioned or simple-minded: if any two of the theoretical sources of human variation cannot deliver, then the remaining option has a field day. Importantly, preventing new research can be a useful tactic in this process.

    Sensitivity about nature / nurture enquiries is very proper. Before the Nazi period, there had been optimism among progressive people that genetically based features of humanity could be steadily improved - even if not transformed overnight, as the discredited doctrines of Jean Lamarck (1744-1829) had once promised. Britain's leading Liberal, Asquith (later Prime Minister), spoke for many when he asked "What is the use of an Empire if it does not breed and maintain in the truest sense of the word an Imperial race?" (Webb, 1901).(1) Eugenics (selective procreation)(2) offered one answer to the growing urban squalor of turn-of-the-century Europe. (Prohibition of alcohol seemed the obvious alternative for impoving the human condition without vastly increasing the burden of debt on future generations; but the USA would try even this drastic measure without success.) Thus Ronald Fisher (1890-1962), the British statistician and pioneer of the science of population genetics declared (1913/14): "Darwinism is not content to reveal the possible, perhaps the necessary destiny of our race; in this case the method is as clear as the detail; the best are to become better by survival." To Fisher, as to Galton, genetic factors contributed at once to the individualism, the hierarchy and the successful functioning of human society; and they pointed the way to improvement, if desired. In the 1930's, Aldous Huxley, who had memorably satirized statist utopianism in his Brave New World (Huxley, 1932), embraced eugenics as the long-term answer to rising unemployment;(3) and the Marxist geneticist, J.B.S.Haldane (1938), was another of many to see eugenic policies as useful in counteracting what, in the aftermath of the attrition of talent in World War I, was feared to be a declining level of national intelligence. Such ideas were to have wide popularity and lasting impact: compulsory sterilization of supposedly mentally defective girls was still being practised in Virginia as late as 1972. After 1945, however, among the intelligentsia, to deem a characteristic 'genetic' seemed relatively pessimistic: for by then the drawbacks of eugenics were becoming clear, as follows.

    1. (i) Because of such complexities as recessive genes, which could transmit characteristics invisibly across generations, enouraging responsible parenthood might take a long time to have any effect on national levels of psychopathology and mental retardation.

    2. (ii) It is not clear how to motivate people decisively towards eugenic parenting when welfare states shoulder much of the financial responsibility for medical and educational casualties. (Not even munificence is required to support welfare states: present-day voters provide the funds chiefly on the assumption that they and their children will be well looked after in their turn.)

    3. (iii) Today, 'reform eugenics' (e.g. Kevles, 1985; Paintin, 1995) involves only voluntaristic measures that involve no state inducements: its procedures are those of carrier detection, eugenic counselling, control of environmental mutagens, preimplantation diagnosis of inherited abnormality, legal (and, in Britain, state-funded) abortion, the provision of opportunities for women themselves (or, in Britain, their medical advisers) to select suitable sperm donors, and ova transplants. Even so, eugenics necessarily involves some people realizing that their own attributes and tendencies are effectively being deselected by others - whether the disfavoured conditions are thalassemia, cystic fibrosis, Duchenne muscular dystrophy, adenosine deaminese deficiency, Lesch-Nyhan syndrome, premature baldness or short-sightedness. At present the wishes of non-Western women for fewer daughters and more sons are held in check by the expense of artificial insemination - otherwise, paralleling debates about abortion, there would already be a second controversy about 'a woman's right to choose.'

    4. (iv) However tactfully, sensibly and voluntarily eugenic measures are introduced, there is always the possibility of the 'improvers' becoming impatient with human rights and instead pursuing compulsory eugenics - notably, compulsory sterilisation. India's campaign of mass vasectomy in the 1970's provided men with the inducement of free transistor radios but it was widely experienced as involving social pressures bordering on the compulsory. Indeed, beyond programmes of sterilisation, the exterminative programmes of state-enforced euthanasia and the Holocaust had soon turned out to be preferred in Nazi Germany by quite a few of the politicans and experts who had at first seemed to advocate only eugenics; and apparent eugenic concern for human improvement had given way to racial triumphalism and the arbitrary suppression of minorities.(4) Doctors may today practise euthanasia in Holland (when three doctors agree with persistent requests from a patient or, if the patient is unable to act, from relatives); but anxieties about 'undue pressure' being put on patients will always arise so long as it is relatives or the state that must foot the bill when a very sick patient prefers to postpone death with the help of doctors or surgeons.

    Today, when millions of people have been the victims of genocide in Cambodia, ex-Yugoslavia and Rwanda-Burundi, and when millions of aborted human foetuses are incinerated annually in the West, anxieties about any possible 'slippery slope' from voluntary eugenics to involuntary death camps have to be balanced by a proper concern for all-round reduction of present and anticipatable horrors. Scientific, social and political changes may have lately made eugenic options relatively more acceptable. Seven developments are especially relevant.

    1. (i) Human genetics is on the brink of offering gene replacement therapy just like any other medicine to help the unhealthy themselves and not just their descendants. Already the severity of the consequences of some adverse genetic mutations is moderated, in individuals, by the somatic introduction of unmutated genes (Postgate, 1995).

    2. (ii) It seems clear that some gene therapies will involve alterations to the sex cells, and thus to the 'germ line' - allowing changes to be passed on naturally to patients' offspring. Advisers to the European Commission have apparently suggested that germ-line therapy that made changes in people's DNA would be "foolish and irresponsible."(5) However, according to a report in Nature (Butler,1994) a UNESCO International Bioethics Committee working group says that "debates about germline gene therapy and 'directed evolution' amount to much ado about nothing. The only application envisaged, namely to spare descendants a serious disease, could ultimately be achieved more cheaply by sorting sperm or by selecting embryos, as is already done in several countries."

    3. (iii) As is indicated in the foregoing observation, eugenics is already practised clinically in modern medicine. (This is why the UNESCO Committee maintains that gene therapy will only be an extension of exisiting prophylactic practice.)

    4. (iv) There is now wide acceptance of nation-states spending around a quarter of their citizens' gross national products on their behalf, via taxation, with the intention of improving population levels of health, educational attainment and employability.

    5. (v) There is greater realization of the extent to which ordinary parents who have handicapped children limit distribution of attention and affection to such children quite unconsciously: even though parents may insist verbally that they treat all their children equally, careful observation shows that they make decisive choices about when to lavish more nursing care and when to restrict further investment of time and effort in an unpromising child (Mann, 1992).

    6. (vi) Any who have genuine anxieties about counterproductive and dangerously escalating state intervention and spending can today support libertarian campaigns to privatize welfare into state-aided private insurance schemes.(6) (Even beyond private insurance for health and medical costs, it could be arranged that criminals paid the full costs of their crimes from insurance - including the recompense of victims.(7))

    7. (vii) It is possible to articulate a coherent and workable code of eugenic practice in which respect for human autonomy and reasoning are unreservedly respected as the sources of all specifically human activity (Bayertz, 1995). Instead of relying on ancient rules or impossible calculations of future gains from eugenic interference, Bayertz''s 'GenEthics' provides a locus of value in human intelligence.

    Nevertheless, these six developments are modern novelties. Immediately after 1945, people were bound to ask what was the point of research which might sometimes result in our knowing that important human differences are partly genetic. Were hereditarianism, eugenics, elitism, racism, ignorance, paranoia and barbarism not fatally intertwined and incarnated in Hitler's Reich?(8) What was the point of trying to disentangle them? Who could wish to sort wheat from so much chaff?

    In this regard, the British psychologist, Cyril Burt (1883-1971) was a singularly re-assuring figure. After studying in Oxford with the Lancastrian instinct psychologist and forerunner of sociobiology, William McDougall(9), yet also liaising with the cognitively oriented Spearman, Burt first lived and worked for five years with delinquent boys in the University Settlement serving Liverpool docklands. Alert to issues of what could be proved by measurement, he particularly noticed that Binet's tests pointed away from authoritarian traditions of the past and towards the understanding of individuals. Burt's next appointment, in 1913, as the first-ever educational psychologist for London County Council, extended his contact with children and led to what was to be his most popular book, The Young Delinquent. Here Burt advocated social and environmental improvements, welfare spending and expanded employment of educational psychologists - especially since "the problem never lies in the 'problem child' alone: it lies always in the relations between that child and his environment" (Burt, 1940, p.243). Burt also urged the use of intelligence tests - while recognizing other ability factors (especially beyond MA 11) and being aware that g might possibly reflect only "the irrepressible disposition of the human mind to reify" or a too simple reduction of g "to single atomlike existents" (Burt, 1940, pp. 66, 237).(10) From the time of IQ tests' first official use in Britain (by Bradford's progressive (Labour-controlled) education authority, to award free grammar school places (Burt, 1924)), Burt's ideas enjoyed steadily increasing official acceptance. Notably, in 1938, British government committees accepted Burt's evidence that the increasing range of mental differences with age through childhood required separate teaching for children of different abilities, at least by age twelve (Evans & Waites, 1981, p.93).

    Representative of British rejection(11) of compulsory sterilization, ethnic segregation and euthanasia, Burt was no sympathiser with Nazi Germany. More than Galton and Spearman, Burt was 'politically correct' about racial differences in intelligence: in 1923 he told the British Association that any "innate group differences" were small in comparison to the variation that was found between individuals (Blinkhorn, 1994). For a man of his age, he played a full part in the 1939-1945 British war effort, and he was knighted for his advisory work in education and propaganda by the post-War Labour Government. As interested in parapsychology as in the dispiriting question of whether national IQ might be falling, Burt became best known for the educational revolution that he enabled.(12) Whereas primary school teachers understandably prefer polite middle class children who are interested in their lessons, Burt advocated that all bright children, regardless of their backgrounds and records in primary school, should have free access to a fully academic, grammar school education. This proposal had the merits of economizing in the distribution of resources, individualizing education according to a sensible principle, and seeking maximum 'value added' attainment for all children. Thus it was popular in its day and was implemented by most UK Local Education Authorities in Britain soon after the War.(13)

    Burt was a hereditarian about intelligence; but he believed that improving the match between educational goals and IQ levels would prove beneficial to the life-chances of all children. For an academic, his achievement was remarkable - even though, from the beginning, local educational authorities showed little interest in setting up the technical schools that Burt had wanted (alongside grammar schools) for children of greater practical that verbal abilities. (By contrast, West Germany followed Burt's recommendations - see Chapter IV.) More problematic, middle-class parents would become resentful when their own children not infrequently 'failed' the 11+ examination, with its new-fangled IQ component, while some local children passed despite a poor record of attendance, behaviour and achievement at primary school. Some L.E.A.'s (e.g. Hertfordshire, in 1954) reacted to mounting pressures from the expectant middle classes by abandoning the IQ part of the 11+ examination and relying on conventional examinations and teachers' assessments. However, this reduced the percentage of working class children going on to grammar school (Floud & Halsey, 1957). Now even harder to defend, selection itself was progressively abandoned: by 1980, the vast majority of state secondary schools in Britain were 'comprehensive'. Nevertheless, Burt's memorial is that increase through the 1950's of candidates from modest social backgrounds who finally met traditional university entrance requirements in competition with children who had enjoyed the privilege of private education; and Burt's achievement would have been still greater had his proposals been followed in full. Today's universities may take in many more undergraduates than was ever envisaged in Burt's day; but such education is no longer a guarantee of a tenured post in the Church or Civil Service - or anywhere else.(14)

    Burt's achievement was to realize the full potential of brighter children. Unlike later educational gurus, Burt's revolution did not simply lower the system's standards to disguise underachievement. Most notably, Burt's protégés included girls, whose unsuitability for academic learning was taken for granted until the arrival of IQ testing. While Galton had thought his Science Museum data showed women were "on all counts inferior to men" (Beloff, 1973), Burt & Moore (1912) had arrived at what would prove at once the most revolutionary and the best agreed conclusion of twentieth-century psychology: "with few exceptions, innate sex differences of mental constitution are astonishingly small - far smaller than common belief and common practice would lead us to expect."

    After 1950, in retirement from what had once been Spearman's Chair of Psychology at University College London, Burt was to see the most central of his considered views come under increasing challenge as the fashion of academic psychology favoured environmentalism and the multifactorialism of Thurstone and Guilford. Burt (1955) was especially unhappy with the idea that "the examination at 11 plus can best be run on the principle of the caucus-race in Wonderland, where everybody wins and each get some kind of prize." Remarkably, in 1955, and again, eleven years later, at age 83, Burt (1966) wrote papers claiming that, with the help of female assistants and colleagues, he had, since 1940, been able to amass psychometric data on some rare and theoretically crucial subjects. He had studied a growing number of pairs of identical twins who had been separated early in life: by 1966 he was able to report on 53 pairs of 'monozygotic apart twins' (MZa's). Apparently, Burt's MZa twins were correlated very strongly for IQ: their within-pair, 'intraclass' r (.771) for IQ was almost as high as were their r's for height and weight; and the twins differed little more from each other in g than do individual testees differ from themselves on a 20-minute test when re-tested over six months. With the help of adjustments "to reduce the disturbing effects of the environment to relatively slight proportions", the MZa's correlated almost as highly as the reliability of IQ testing allowed. Whatever educationists of the 1960's preferred to do, Burt was confirming the rationale of the educational revolution which he had urged: deep-seated intelligence differences could not be neglected by educators without a price being paid by children themselves .

    Shortly before Burt's death, the scholarly American behaviourist and former Communist, Leon Kamin , returned to the USA from the refuge he had found in Canada during the McCarthy years. Himself an international expert on the conditioning of the laboratory rat, he was intrigued by the persistence of Burt's influence in psychology - not least on his fellow learning theorist, Richard Herrnstein (see Chapter IV), who had defended London School views in The Atlantic Monthly. To find out where the truth lay, Kamin began to examine Burt's papers. Fortunately he was something of a number-lover. Like James Shields, the London researcher who had conducted the biggest study of MZa's and had himself discounted Burt's earlier work (Shields,1962), Kamin soon discovered many flaws - to be detailed in The Science and Politics of IQ. Anticipating Kamin's (1974) book, London School insiders admitted that Burt's 'classic' twin study would have to be discounted for scientific purposes (Jensen, 1974). A closer reading of his work than Burt had received during his lifetime(15) had revealed it was far from clear who his MZ twins were; or where and how or by whom they had been tested. Still worse, some of Burt's results were most unlikely - notably the constancy of his MZa correlation which stayed the same, at .771, over the years, despite Burt claiming to have tripled the number of pairs in his MZa sample between 1943 and 1966. Thus the best known British psychologist of his day was posthumously denounced for fraud by the British Psychological Society in 1979;(16) and the 'hereditarian'(17) cause about IQ and g that Burt had espoused suffered a corresponding setback. Subsequently, two books appeared (Joynson, 1989; Fletcher, 1991) urging that the correct verdict should have been 'not proven' or even 'innocent' (for reviews see Brand, 1990, and Aldhous, 1992). The basic problem was to explain how a gifted methodologist like Burt, if he had really intended outright deception, had not been able to make a better job of it. However, most psychologists remained impressed that Burt had been at least a brazen rogue even if he was not a devious fraud. In understandable hubris, his opponents would claim him to have been a reactionary, an elitist and a racist. Such labels provide no fit to Burt's involvements or pronouncements of a lifetime; but the penalty for bold claims from slipshod science is rightly a heavy one in a world where truth-claims are largely adjudicated by scientists. So, by 1974, the hereditarian cause about IQ differences had a new, high hurdle to clear.

    Although there were plenty of other studies of twins, and even of MZa twins, in the scientific record, only Burt's MZa's met the important methodological criterion of having been reared in uncorrelated homes. Burt's MZa's had apparently been raised in homes that differed (on average) as much as homes throughout the entire population differ (on average) in levels of affluence and social class (socio-economic status (SES)). Thus, for a short while, Burt's figures ruled out the most familiar explanation usually offered for MZa similiarities by social-environmentalists - that the twins have been brought up in similar home environments. By 1980, other types of study, too, needed re-doing. The crucial correlation of .25 between the IQ's of unrelated adopted children growing up together suggested at least some influence of family environment on intelligence; yet, like many twins reared apart, adoptees may have been somewhat 'selectively placed' (by adoption agencies) with parents who were thought similar to adoptees' biological parents.(18)

    However, by 1980, the interest in psychogenetic studies was limited - at least in Britain. Professional geneticists preferred to progress towards quite specific gene therapies for rare and manifestly undesirable conditions, and to avoid controversial topics; and Burt's psychological critics wanted to close the account with the chapter on Burt. Indeed, many were coming to deny the very possibility of doing any useful work on heritability. Their rallying proposition was essentially: 'Intelligence is the result of an interaction between nature and nurture so complex as to be (mercifully) impossible to untangle in any meaningful way - any more than eggs can be unscrambled'. Even today, according to Hirsch (1991) and Wahlsten (1990), development is considered so complicated and g x E interactions so abundant that no estimates can ever be made of the relative contribution of genetic and environmental factors. Similarly, Lewontin (1992) and Lerner (1992) call for recognition of what is apparently Karl Marx's idea that organism and environment are so completely "fused" that it would be meaningless to think of disentangling them. After the Burt débacle, uninhibited delight in complexity came to be offered as a substitute for finding the truth.

    In fact, behind this smokescreen of 'interactionism' there can usually be found one of four quite different broad claims. Properly considered, none of these claims, diminishes the interest and value of twin and adoption studies.

    1. Inseparability. It is often said that everyone has 'both genes and an environment'; that development must be a product of both - rather as the area of a field depends upon both its length and breadth; and that genes and environment are thus inseparable in their 'complex interaction'. However, this popular claim itself turns out, on consideration, to mean any of four quite different things - all of them trivial, routine or wrong.


      1. Human differences are typically affected by both genetic and environmental differences. This claim is plainly true, but it cannot be proved to be so except by population-genetic studies. Anyhow, both is too small a word to cover the range of possibilities: to plan further welfare endeavours, for example, it is plainly important to have an informed view as to whether genetic factors account for 75% or for 25% of existing observable ('phenotypic'(19)) variance in the general population.
      2. Development occurs, as envisaged by Piaget, in a series of ongoing interactions, i.e. interchanges with the environment. This claim concerns the intra-individual development of all children, not the inter-individual differences between children. Its correctness - or, more commonly, incorrectness (see Chapter II) - is quite independent of how phenotypic differences come about: all children might learn (or remain ignorant) from asking questions of their parents, yet they might all make such similar gains as to remain equal to each other in understanding. Psychogeneticists cannot be criticized for neglecting interaction unless the hypothesized interaction is advanced as relevant to explaining eventual phenotypic differences.

      3. Genetic and environmental factors sometimes have their effects multiplicatively (in calculable statistical interaction with each other). This would happen, for example, if a child's musical skills in adolescence had depended on the child's having both a few 'genes for music' (and for persisting with music practice) and on its having had parents who provided encouragement and paid for tuition. It is what psychogeneticists themselves mean by g x E ; and there is a clear criterion for its occurrence. If similar genes and certain similar environments are both necessary, in combination, for a particular phenotypic similarity to occur, the similarity of MZ twins reared together (MZt's) should significantly exceed what would be expected merely from summing the similarities of MZa's and of unrelated adoptees reared together. Again, in adoption studies, a g x E effect could be detected if adoptees achieve a certain phenotypic level (e.g. college entrance) only when particular features (e.g. having been to college) are present in both the biological and the adoptive parents. (It has sometimes been thought that environmental features are particularly important to higher-IQ children - i.e. that their educational outcomes especially involve a g x E multiplication effect. A multiplicative influence of a high IQ was thought by Burt to require a more demanding educational exposure for bright children: in particular, it explained the very high results in educational attainment and real-life achievements that accrue when an adequate educational input is supplied to such children (Burt, 1943). Today, such results are sometimes claimed in studies of gifted children - see Chapter IV.)

      4. G and E differences sometimes 'covary' during development. That is: the genetic and environmental levels that are operative may themselves be correlated - such covariation being symbolized as G, ECOV . Such G, ECOV creates more final phenotypic variance in the population than would have arisen if the relevant g and E influences had been independent of each other. Examples of G, ECOV would be: if children with 'genes for violining skill' tend also to be born into homes where the parents are musical and will particularly encourage the children to persist with violin practice; or if parents notice that their child seems to appreciate music, and then respond to this by supplying suitable instruction - though they would not have done so just for any child of theirs; or if the child, being musical, enjoys music, welcomes music lessons and seeks its own violin for a birthday present - thus changing its own environment (or microenvironment, or milieu), which now comes to include the child's own violin. It is harder to detect all possible forms of G, ECOV , though the more interesting, 'active' forms should yield relatively high r's for MZa twins (versus DZt's). (MZa's will tend to make their originally different environments similar in relevant ways, whereas the DZ 's, being on average only 50% genetically similar, will push their initially similar supplied environments apart in so far as active G, ECOV occurs). Like g x E interaction, and contrary to what some devotees of it may like to imply, G, ECOV cannot have any effect without there being substantial 'main' effects of both g and E . Moreover, active G, ECOV is not some novel type of effect with which to amaze hereditarians; rather, it is just one of the routes by which hereditarian theorists have always supposed genes to yield final phenotypic differences. Genes can (however indirectly) guide people to select and create particular micro-environments for themselves which, once created (whether in the form of 'noisy friends' or 'a collection of tapes about Zen Buddhism') will have their own causal effects back upon them. Such 'active G,E COV ' deserves its own title of 'transaction' in so far as people's own 'nature' comes to influence their 'nurture' - a process for which modern psychogenetic work offers hard evidence (see Scarr, 1992; Plomin et al.,1994; and see below).

      More generally, regarding the 'analogy' with areas of fields beloved of professing interactionists, it is in fact perfectly possible to distinguish between rectangles in terms of their linearity versus squatness; and to attribute size differences amongst any set of rectangles to their differences in (1) height, (2) width or (3) the multiplicative interaction of height and width with each other. All rectangles 'necessarily have heights and widths'; but they need not, of course, have the same heights and widths. Any number of rectangles can differ among themselves chiefly in height, breadth or in both aspects; likewise, phenotypic differences can arise because of G, because of E , or because of both types of differences in ('interactive') multiplication with each other.(20)

    2. Specificity. Plant and animal geneticists sometimes say that heritability estimates are 'essentially meaningless' because they can only be made for particular populations under particular conditions. Especially, a genetic effect will often work via an 'epigenetic route' that depends on environmental factors taking certain particular values. Each genotype may thus have its own 'reaction range' of environments: for some genotypes, how well they fare will depend markedly on the environment in which they are placed, whereas other genotypes will have a smaller range of reactions (Gottesman, 1963) (see Figure).

      Across a sufficiently wide range of genotypes and environments, it may happen that "the contribution of nature is a function of nurture and the contribution of nurture is a function of nature, the one varying in dependence on the other, so that a statement that might be true in one context of environment and upbringing might not necessarily be true in another" (Medawar, 1977). Thus, if all possible environments are considered, "there are always circumstances that might have altered a cell's fate" (Purves, 1995). The implication of such observations is meant to be that knowledge about the heritability of human IQ differences would be of only local and short-term significance and so not worth collecting - especially given the dangers of pessimistic misinterpretation; and that genetic effects can never be disentangled from the myriad epigenetic routes and factors on which they depend for their usual expression.

      Which plants thrive, having which genes, may well depend on what conditions of temperature are arranged in the experimental laboratory. Likewise, suppose a random 50% of a country's babies were given some crucial protein boost at birth: actual levels of later intelligence - which may have previously been determined largely by genetic factors - would now be found by researchers to have come under overwhelming environmental control. However, though this is a valuable theoretical point for plant geneticists, it neglects the fact that human beings do not in fact live in a laboratory where an experimenter can thus play around with the determining parameters of human experience - whether such parameters are chiefly genetic, chiefly environmental, or whether they are epigenetic constellations (which vary as little between members of a species as do genes themselves). State provision of free education for all might once have raised the heritability of educational achievements - since a major environmental inequality of the past, in terms of access to education, was being reduced. But human 'populations' are not like plants: they act and choose, maintain valued traditions, and, though there are occasional 'revolutions', there is still much continuity. People do not experience their conditions of existence at the whim of a laboratory geneticist; and most g and E differences that are relevant to development are unlikely to be capable, within usual ethical provisos, of being suddenly created or abolished.

      In any case, even among plants, dramatic shifts in the relevance of genotypes at different environmental extremes are strangely hard to find: would-be critics of heritability estimation (Lewontin, 1975; Byne, 1994) need to have recourse to data collected in 1940 from races (of the California Achillea plant) which apparently thrive differently at dramatically different altitudes (30m vs 3050m above sea level) (Bouchard, 1995/6). What can possibly be the implications of such a distant example for the study of individual differences between people across environments between which voluntary migration freely occurs? The fact that a heritability estimate might, evidently rarely, have such limitations seems a reason for more genetic studies of populations, not for fewer. Most likely, environment-specific heritabilities are a rarity and those that are 'discovered' are methodological artefacts. One frequently quoted example of reaction range effects in animal learning is Cooper & Zubeck's (1958) finding that 'dull' and 'bright' rat strains were equalized, so that heritability became zero, in an 'enriched' environment. Yet Bouchard (1995/6) observes that the authors of this study expressly explained that the apparent disappearance of their strain difference under enriched conditions could have occurred simply because "the ceiling of the test may have been too low to differentiate the animals, that is, the problems may not have been sufficiently difficult to tax the ability of the bright rats." Moreover, the usual idea of advocates of state welfare is precisely that current, local inequalities in intelligence and self-control have their origins in clear environmental differences and can therefore be corrected by environmental equalization. This important claim surely deserves repeated evaluation: the case for new provision of redistributive welfare expenditure is more easily made when a twin study finds clear environmental effects. An ideological environmentalist may fear that no such environmental influence will be found: only in this case is there an intelligible reason for not wanting research. It is certainly no argument against a trait's heritability to say that new environmental variations could conceivably be created. The same arguments apply about 'epigenetic' effects: staying within ethical limits, the human 'epigenetic landscape' in which cells develop is not readily alterable by experimental intervention; and, even if genetic effects operate via such a landscape in ways that cannot be decisively unravelled, both genetic and epigenetic influences can still be contrasted as a package with the relative influences of observable variations in external environments. If MZa twins differ, this must reflect conventional environmental differences (or very rare genetic mutations, or the very special environmental challenges that twins present to each other in the battle for maternal resources in the womb); and if MZa's are phenotypically similar, this must reflect their genetic similarity (and the environment's response to it) no matter how different epigenetic landscapes might have altered everything in the brave new world of an experimenter who wants to distract attention from the issue of heritability.

    3. Non-heritability of fitness characters. It is sometimes thought that the heritability of IQ cannot be high because intelligence is an important quality for the human race as a whole. If intelligence had been thus important in the course of evolution, downward variations from the population norm would have been disadvantageous to long-term fertility. Natural selection would have reduced heritable differences in the usual way - especially deselecting genes for low intelligence in those human societies based upon agriculture, manufacture and literacy. Thus genetic variance in the population would have become restricted; and remaining phenotypic variance would owe more to environmental variations. By contrast with what is thus expected for intelligence, strongly heritable differences between people are still found for the variable of height. This is presumably because the present range of height differences has had no specific relation to fitness and has thus not been subject of natural selection. (To judge by requirements advertised lonelyhearts columns, being tall is probably advantageous to the mating prospects of males but not generally to those of females.)

      However, this argument against a substantial heritability for IQ differences neglects three special considerations:

      1. the help offered to lower-IQ people by individual benefactors, churches, charities and governments;
      2. the economic demand (till very recently) for manual labour and basic soldiering and domestic skills; and
      3. the rewards available for criminality (even when the criminal is detected, if proceeds have been transferred to kin, and if there are few penalties for a criminal's biological relatives).

      So perhaps there need have been no general relation between IQ and fertility over at least the past century. Whatever may have happened across evolutionary history, intellectual limitations could easily have remained perfectly heritable across recent generations in the West; and, since 1970, at least in the USA, out-of-wedlock births have risen sharply among girls and women of below-average IQ (see Chapter IV).(21) In short, g has not lately been a fitness character: genes for the full range of IQ differences can easily have been passed on.

    4. Irrelevance. There is one last reason for professional geneticists' unwillingness to research IQ's heritability. Individual differences on g-loaded tests are are normally distributed, so IQ is likely to depend on quite a number of different causal variables, both genetic and environmental (and on a limited number of multiplicative interaction effects between the relevant variables).(22) It is sometimes said that perhaps a hundred independent causal factors may be involved in IQ; and no single gene is likely to be crucial. Thus, since no gene therapy can be just around the corner, what really is the point of potentially divisive work on heritability?

      There are however, are four replies to this excuse.

      1. (a) If gene-gene interaction (or epistasis, i.e. G x G) influences a trait, MZ twins will be markedly more similar than are DZ twins. If a trait were simply, 'additively' heritable, MZ 's would be twice as similar as DZ 's because MZ 's share twice as many gene variations as to DZ 's. (DZ twins share, on average, only 50% of the genetic variaions that can occur between human beings: they are no more genetically similar than are ordinary siblings. In addition, being of the same age, they will, over the years, have experienced a slightly more similar family environment than do ordinary siblings.) However, MZ twins have in common not only twice as many gene variations but all the possible multiplier effects that can occur between all these genes. (If some entire package of, say, six genes is needed to provide some particular push in the direction of higher intelligence, it is unlikely that two DZ 's will share the effect: most likely they will share three of these genetic variations, but there would only be a 1-in-36 chance that they would share all six.) The phenomenon of more-than-additive similarity for MZ 's does in fact occur for self-reported personality traits. It is now well known that MZ twins correlate at around .40 to .60 for personality traits (depending largely on reliability of measurement) while DZ 's correlate at little more than .20 (e.g. Brand, 1989; Baker & Daniels, 1990; Bouchard, 1992, 1994). Thus, at least for the 'Big 5' personality dimensions (see the bipolar ability contrasts of Chapter 1), MZ correlations are twice those found amongst DZ 's, even though the MZ correlations themselves (around .50) show that not more than a half of personality variation can be heritable.(23) By contrast, for general intelligence, DZ twins show substantial similarity of their own . This indicates that, for intelligence, at least, additive genetic effects, unsupplemented by genetic interactions, can account quite adequately for the observed similarities. (Only for intelligence differences found in late middle age is there evidence of epistasis playing a significant role - see below {III, C, vi} and Pedersen et al., 1992.)

      2. (b) If there is substantial involvement of g x E interaction in causation (see above, {Draft 3, Chp III, p.14}), MZt twins should be markedly more similar than MZa twins. In particular, this difference in similiarity should exceed that between DZt's and Dza's. However, there is no sign of such a phenomenon for intelligence.

      3. (c) Good scientific practice would surely test for the occurrence of epistasis and G x E interaction before discarding the possibility that major genes influence intelligence.

      4. (d) The Leipzig psychogeneticist, Volkmar Weiss, has argued (as had Goddard(24) - see Chapter I) that there may even be just one recessive gene having a rather large effect on IQ levels. Weiss (1992) suggests that some aspects of intelligence, instead of showing the familiar, unimodal, bell-shaped frequency curve, actually show, like eye-colour, a skewed distribution. In some large data sets, it appears that individual intellectual levels might depend importantly on whether gene combinations involve:
        1. the two recessive forms of the gene (and thus IQ's around an average of 130);
        2. the recessive-dominant combination (IQ's around 106); or
        3. the most commonly observed dominant-dominant combination (hypothesized to yield IQ's around 94). (Weiss had to work hard to make his views known. An English geneticist actually reported Weiss's hereditarian 'error' to the East German 'Stasi' - the secret police of the pre-1989 Communist régime - and thus cost Weiss his job as an educational researcher.)

    Unlike much of what passes for research today, psychogenetic methods are able to mount reasonably decisive tests of causal hypotheses. In particular, it is of the greatest importance to conduct more psychogenetic research before the supply of adopted-away children from mothers of relatively normal intelligence dries up altogether. - Studies of children separated by divorce in modern times will provide a substitute; but such separations, though far more numerous, are much less clear-cut and thus less able to yield definitive conclusions - even if members of such emotionally divided families were willing to be re-united and co-operate for research purposes. Unfortunately, the sorry tale of Western geneticists and psychologists doing so little to research Burt's eminently viable hypotheses is only just drawing to a close. Psychogenetic research is now achieving funding in the USA and Britain; but this is still for work that would lead lead on to genetic engineering for rare conditions rather than to mainstream genetic counselling. The setting up of the Centre for Social, Genetic and Developmental Psychiatry at the Maudsley Hospital in London is a notable milestone in Britain (Burne, 1994); but even this unit has yet to announce any plans to study general intelligence. In Germany, the only author of a textbook on psychogenetics does not even have a position in a university. Though Weiss's observations may reflect other interaction effects than those of genetic dominance and recession, his having to pass his days as a state genealogist in psychology's home city of Leipzig speaks volumes about the ideological cultivation of ignorance.

    Notwithstanding such prevarication, major researches into the heritability of g and other psychological dimensions have appeared - all outside Britain and beyond the reach of its institutionalized anti-hereditarianism. Since the setting-aside of Burt's flawed papers, programmes of psychogenetic work involving twins, adoptees or separated half-siblings have been reported from Colorado, Denmark, France, Minnesota, Sweden,

    Texas and Virginia. They have had eight notable results, as follows.

    1. From Paris, a team headed by former nuclear physicist, Maurice Schiff, used the centralized French educational system to mount a remarkable study of separated siblings (Schiff & Lewontin, 1986; Capron & Duyme, 1989). They found and tested adopted children whose biological mothers they could trace. They selected mothers who had themselves gone on to bear and, this time, raise other children - these being full- or half-siblings of the adoptees. (Most commonly, after a first, unwanted child had been adopted, the women went on to marry and to begin families of their own in the usual way - sometimes with the father of the child who had been adopted.) The researchers located thirty-two such separated pairs where the adopted and biological children had reached the beginning of adolescence in homes that were strikingly different in social class. The adoptive parents were of decidedly upper-middle-class status - on average enjoying income levels found in the top ten per cent of French households. By contrast, the biological mothers and their partners provided for their non-adopted children homes that were typically in the bottom half of the range of French income levels.(25) Despite the environmentalistic expectations of Schiff's research team in the early days of the research, the effect of the adoptees' 'social advantage' on IQ proved to be slight. By about age twelve, as few as eight IQ points (of fluid intelligence, gf) distinguished the privileged adoptees from their brothers and sisters.(26) Although the homes of the two groups of children were separated (on average) by 60% of the population range of class differences, the resulting g difference between the children spanned only 12% of the population range of IQ's. Such a slight environmental effect of what sociologists classically take as the crucial feature of a child's home environment was in line with the general (not 100%) hereditarianism of the London School. (It is not always appreciated that Burt, Jensen and Eysenck always reckoned that social class differences accounted, environmentally, for some 15% of population variance in measured IQ - not least because IQ assessment normally draws on a number of techniques, none of which is a pure measure of g . Schiff et al. were themselves unwilling to admit the compatibility of their findings with Jensen's estimates, but it is readily seen in their work (see Brand, 1987b).)

    2. (ii) In Minneapolis, during the 1980's, Tom Bouchard, David Lykken and co-workers brought together the most widely separated pairs of MZ twins to have been studied by psychologists.(27) Quite often one of the twins, or a friend, wrote in to Minneapolis after reading an article in a newspaper about the project or seeing an advert. Twin pairs were offered a week's accommodation and hospitality in return for co-operating in full medical and psychological examinations. The 48 Minnesota MZa pairs were around age forty at the time they were interviewed and tested; and they had typically been separated for some twenty years - from around five months after birth. The pairs' degree and timing of separation turned out to bear no relation to their correlation in overall IQ Their within-pair IQ correlation of .78 was as high as the 40-year test-retest correlation for IQ (see Chapter 1). The twins' home environments had in fact correlated modestly, at .27, for social class; but since the adoptive fathers' SES levels correlated merely at .17 with the eventual IQ's of the twins that they had reared, the similarity between the twins in home SES could not begin to explain their similarity in IQ. (Horgan (1993) is critical of this study; but he simply neglects to consider the above detailed checks for environmental influence that Bouchard et al. (1990) had been able to make from their data (see also McGue et al.,1993).)

    3. (iii) The Texas Adoption Study has furnished modern evidence of correlations for IQ between some 200 adoptees and their unseen biological parents (Loehlin et al., 1989). Even in childhood, while the adoptive child's environment is created and controlled by the adoptive parents, adoptees' Wechsler IQ correlations with the biological mother are almost double those with the adoptive mother (.23, .13); and by mid-adolescence, this difference is even greater (.26, .05) - reflecting presumably the greater influence of the adolescent child (and thus of its biological mother's genes) over the local micro-environment. In the past, adoption studies were sometimes claimed to be difficult to interpret because of restrictions of range of environments (since adoptive homes are often 'middle class'); or because of selective placement of adoptees with families having educational levels similar to those of biological parents. Such possibilities do not account for the discrepancy between the 'biological' and 'adoptive' correlations in the Texas Study. Adoptive mother's showed r's between their achieved SES and IQ of .45, indicating normal ranges; and children of brighter biological mothers did tend to go to the higher-SES homes. But home SES correlated little with adoptees' IQ's either in childhood or adolescence (.14, .11); and the biological mothers' IQs bore no relation (as a placement hypothesis would predict) to the IQ's of other children in the adoptive family. Hoffman (1991) suggests that correlations between adoptive parents and their children may be somehow 'irrelevant' as an estimate of environmental influence on children. This suggestion may pave the way to new thinking, but it must amaze genuine supporters of social-environmentalist ideas.

    4. (iv) As well as the above confirmations of the traditional views of academic hereditarians, modern research has also produced some novelties. The first of these is that, as MZ and DZ twins develop through childhood and adolescence, they seem to diverge increasingly in their degree of similarity. The correlations between MZ twins stay high, but those for DZ twins decrease (e.g. Scarr, 1992). This was how active G,ECOV (or 'transaction'), was first demonstrated in studies of intelligence. The obvious interpretation of the finding is that the genotypic differences between DZs play out in the later, less parent-dominated years of adolescence: the DZs' biological differences make for the same differences in niche-selection or milieu-creation that occur between ordinary siblings. These micro-environmental differences may in turn push the DZs further apart in measured IQ. Similarly, it turns out in modern research that the classic .25 correlation between unrelated adoptees for IQ is limited to the years of early childhood. By adulthood, once co-adoptees have had the opportunity to express their genetic differences, results from the four studies conducted show that even this slight r has reduced to -.01 (Plomin, 1989; McGue et al., 1993).

    5. (v) G, ECOV has also been found much earlier in childhood - using adult twins' memories of how their parents handled them. Even when reared apart, in homes having little correlation in social class, MZa twins are likely to have been treated more similarly than DZas; and MZs also report more similar treatment from their peer groups in childhood and adolescence. (Parental child-rearing practices were reported by the twins themselves as adults. Such memories are notoriously unreliable, so it is remarkable that any similarities of treatment for MZa's should have been detectable. For the treatment received by twins from their peer groups see Baker & Daniels,1990.) Early 'environment' is thus itself partly 'genetic': parents of MZa twins evidently adjust their demands and practices to fit in with their children's own genetically influenced proclivities and personalities. After the behaviourist generation, Bell (1968) was the first psychologist to give house-room to an idea that behaviouristic social psychologists and 'interactionist' developmental psychologists would still long resist: that, instead of parental behaviour being a cause of child behaviour, it is often an effect. As Sandra Scarr (1992, p.14) put it: children "evoke responses from others, actively select or ignore opportunities, and construct their own experiences." Bouchard (1994) observes: "Current thinking holds that each individual picks and chooses from a range of stimuli and events largely on the basis of his or her own genotype, and creates a unique set of experiences - that is, people help to create their own environments." (That is not to say that early environmental similarities, even when genetically triggered, are invariably part of a causal chain: twin researchers usually find that similar parental handling (e.g. being dressed alike) is not predictive of later psychological similarity: the typical r is around .05 (Loehlin & Nichols, 1976; Bouchard, 1995/6).)

    6. (vi) A further novelty from modern psychogenetic work is the growing awareness that adult lifespan development does not simply continue the causal processes of childhood. The Minnesota work on adult twins has been followed by a study of 65-year-olds in Finland showing just as large an MZa correlation (.78), together with a much lower DZ correlation (.22): this yields a particularly large MZ - DZ difference in similarity (.56) (Pedersen et al., 1992). Further, in the Finnish study, that the twins had been reared together (rather than apart) made no difference, by late middle age, to their degree of similarity. (Indeed, the DZts (r = .23) were actually less similar in IQ than were the DZas. For a general consideration of how DZts grow apart with age, see McCartney et al., 1990: presumably the twins' different genes gradually establish more influence over their micro-environments.) All these observations are compatible with the idea that genetic factors (epistatic and active-G, ECOV as much as additive) are overwhelmingly important in determining late-middle-age levels of intelligence. Apparently, the longer the time span across which genes operate, especially via a person's micro-environment, the greater is their effect on g . By contrast, the imposed environmental differences that are involved in different types of homes and styles of parenting seem to affect the children only in early childhood, and not beyond. (The more demanding and punitive homes of former centuries may, of course, have had greater impact - thus accounting for the widespread belief in the importance of 'early conditioning'.)

    7. (vii) Estimates can be made of the degree to which individuals' genetic similarity on one test is correlated with their genetic similarity on another: for example, it can be asked whether similarity due to genetic factors on one test yields more phenotypic similarity in other test scores than is usually found. Using data from the Colorado Adoption Project results of such calculations so far have been that "genetic influences on all specific cognitive abilities overlap to a surprising degree" and that "genetic effects on scholastic achievement overlap completely with genetic effects on general cognitive ability" (Plomin et al., 1994). In other words, instead of separate abilities being inherited separately, their heritability is principally the result of the genetic variance that yields the g factor.

    8. (viii) One last and novel type of analysis is interesting, even though results are presently inconclusive. Detterman et al. (1990) found MZ within-pair differences to be smaller, and DZ differences to be greater at below IQ 90: the heritability for IQ may thus be greater in the lower-g than in the higher-g range. This would fit with the observations (see Chapters I and II) that g is itself somewhat more 'unitary', and is more strongly correlated with 'intake speed' at lower levels of intelligence. It would especially add to the impression that more elementary aspects of intelligence are under greater genetic influence; and it would suggest that it is when g itself is already above average that environmental differences might contribute especially to whether really high levels are found of intellect, excellence and g itself.

    Why was so little done till recently to deploy the empirical methods of twin and adoption study? After all, the methods were in use on with significant numbers of cases by the1930's (Burks, 1928; Leahy, 1935; Rosanoff et al., 1937; Skodak & Skeels, 1949). Admittedly there were design limitations such as restrictions of relevant variance (especially amongst adoptive parents - who tend to be middle class): these meant that interpretation of the results remained controversial - see e.g., Kamin & Eysenck, 1980. Yet such problems were remediable - if the efforts had been made that went into the study of the laboratory rat; so other explanations are required. At first, a too ready assumption that 'nature' was all-important meant that the first twin study was only conducted (in Germany) in 1920(28); and, until the rise of behaviourism, few could believe that the case for heredity would ever need to be proved. Latterly, however, the problem has been antipathy not only to crude hereditarian and eugenic ideas but to any research that might falsify psychology's long-reigning paradigm of simplistic environmentalism - or of the latter-day interactionist replacement. It was probably the very environmentalist biases of psychologists in Britain after that led Burt to overplay his hand - temporarily most damaging to hereditarianism as his mischief proved. Today, Burt's conclusions have been entirely vindicated and this makes it hard to believe that he and his lady research assistants(29) never had any relevant data to analyse (even if twin results from other researchers were incorporated); and the continuing shortage of psychogenetic data in Britain is plainly the fault of Burt's opponents.

    Despite the experts in human genetics and development who plainly preferred the position of the ostrich, the psychogenetics of intelligence has consolidated traditional claims and broken new ground. Unfortunately, adoption and twin studies suffer from not spanning the entire range of human genotypes and environments in the data that they can collect; and it is likely that both heritable and environmental influences are somewhat obscured if (as Burt first mooted) family environmental differences interact especially with higher levels of IQ (Scarr-Salapatek, 1971). But, bearing in mind such special effects (which themselves require much more testimony from research) it is true that adoptive children have seemed generally more similar in IQ to their biological (r = .43) than to their adoptive mothers (r = .14) (e.g. Munsinger, 1975, 1978); and that MZ twins are markedly more similar than DZ twins (e.g. McGue et al., 1993b) - especially in adulthood, by which time MZ r' s are around .80 and DZ r's are around .40.(30) The last claim by environmentalists to detect really low MZ similarity other than by post hoc means is apparently that of Schwartz & Schwartz, 1974. However, this paper merely reanalyses data on child twins whose zygosity was never ascertained: instead, dissimilarity in sex was used as a (very partial) substitute for attributing dizygosity. The original author, Sandra Scarr-Salapatek (1972), said that she herself "hesitated to guess what the standard error of an estimated intraclass correlation coefficient might be." Further, she pointed out that 247 of her 1239 pairs of Philadelphia twins had been lost to the research - often when one or both were in 'special' classes: "Certainly, the low-aptitude end of the distribution was lost." Adams et al. (1976) reported nationally representative, 11-year-old MZs (41 pairs, r = .76) to be little more similar than same-sex DZ 's (55 pairs, r = .60) on a non-verbal IQ-type test; but (almost incredibly, reflecting lack of interest in twins and IQ in what was a major British longitudinal study) zygosity had not been determined by blood sample, and the test had, the authors admit, "not been cross-validated with better-known, standardised tests." In the report from Japan of the largest-ever number of MZ 's, MZr was .78 (N = 543 pairs) and DZr was .49 (N = 161 pairs) (Lynn & Hattori, 1990).

    Certainly among those psychologists who claim any expertise in intelligence testing, there is overwhelming acceptance that individual genetic inheritance contributes to human variations. Considering Mackintosh's (1975) defence of psychogenetic studies of IQ against Leon Kamin's critique, even Evans and Waites (1981, p.217) had rejected Kamin's "zero heritability hypothesis." (They rejected likewise the 100% heritability hypothesis (which no hereditarian scholar had held) and also the very effort to quantify heritability (h²).(31) While all measures have their limitations, the heritabilities of human differences are thus assigned to a select club of estimates that should never be made - a club having IQ itself as its other well-known member. ) A survey of 661 such American IQ-test professionals found them just as politically 'liberal' as the journalists and broadcasters who so often decry the idea of genetic involvement in IQ; and these experts in testing rated Sir Cyril Burt the lowest of fourteen leading writers about IQ. Nevertheless, 94% were persuaded of a genetic involvement in IQ by one or other of the types of research evidence considered in the present chapter; and the average of their estimates of the percentage of population variance in IQ attributable to genetic factors (i.e. of the 'broad heritability' of IQ, h²B) was 59.6% for whites and 57.1% for blacks (Snyderman & Rothman, 1988; Gottfredson, 1994). Kamin (1974, p.1) had written: "There exist no data which should lead a prudent man to accept the hypothesis that IQ scores are in any degree heritable." But he had not persuaded the experts. In 1994, fifty professors claiming expertise on intelligence placed a full-page advertisement in The Wall Street Journal to attest the measurability and importance of intelligence: they observed that serious empirical estimates of its heritability range between 40% and 80% (Arvey et al., 1994).

    As to the most crucial source of evidence, the simplest of the methods provides a clear result. There are just five methodologically adequate studies of MZa twins (1937, 1962, 1980, 1990, 1992 - yielding 180 such pairs for most analyses). They involve many different IQ measures in five countries and three languages. They give a weighted average within-pair correlation of .75 for IQ - scarcely less than the r of .771 for which Kamin took Burt to task; and attempts to attribute twin similarity to similar homes or to physical similarities (including 'attractiveness' - Farber, 1980; Taylor, 1980; Ford, 1993) have failed (Loehlin, 1981; Locurto, 1991; Bouchard, 1982, 1983, 1995/6). For example, when MZa pairs who are alleged to have had especially similar homes have been set aside, the remaining twins still show just as strong within-pair correlations for IQ; and objectively assessed attractiveness to others, though it correlates a little with sexual experience in adolescence (.18), has no general correlation at all with intelligence (.00) or with other personality features. Because of their adoption, only a few MZa twins have grown up in real poverty or with illiterate parents, and this will have reduced the range of environmental differences among them. At the same time, researchers' twins involve many healthy volunteers who are interested in research and have only 90% of the population range of IQ's: so, equally, they do not represent the full range of genetic differences.

    Finally, it is appreciated today that there is a powerful environmental influence that applies uniquely to MZs and must push members of twin pairs apart in ways that will not apply to singletons. Monochorionic MZs are likely to suffer 'twin transfusion syndrome' in their competition in the womb for maternal resources: in effect, within the one chorion, one twin can 'steal' the other's blood supply. This results in one twin being large and plethoric and the other smaller and anaemic at birth, and in a 7% mortality of such twins (Phillips, 1993). Pairs of monochorionic MZs are 150g lighter at birth, and the two twins differ more among themselves in birthweight than do DZ twins (who never share the maternal chorion so cannot engage in this type of competition). Congenital malformations are suffered by 3.5% of monochorionic MZs, as compared to 0.25% of dichorionic MZs. In line with this finding, Storfer (1990) noticed that MZ 's IQ differences are associated with birthweight differences, and Jensen (1995) has observed that bigger differences in IQ between MZ twins occur in those pairs where one of the twins has a particularly low IQ. Thus, assuming that resticted foetal blood supply will depress the IQ of the affected twin, monochorionic MZ 's will actually be less similar than if they had developed, as do ordinary children, in the wombs of different mothers. Because of this special environmental dissimilarity, MZ r's will underestimate heritabilities in the normal population. It has often been said that MZ 's are treated similarly (or 'placed' similarly when separated); so it has been necessary for hereditarians to point out that parental handling is often a response to a child's nature rather than an imposition by the parents, and that parental handling anyhow shows little correlation with IQ (or with other personality features). Today, however, the discovery of monochorionic hazards make it clear that MZ correlations will have underestimated the importance of genetic factors for IQ.

    Since they have all their genes (and thus all their gene-gene interaction effects, genetic dominance-recession effects and active G, ECOV ) in common, MZas provide the best estimate of the broad heritability of measured IQ differences. [Broad heritability is wider concept than is narrow heritability, which itself involves simply the additive genetic influences that principally account for genetic similarities between parents and their offspring. For IQ, narrow heritability itself is probably around 45% (Pedersen et al., 1992) - as was first concluded by Burt (see Burt & Howard, 1971): via nature and nurture together, parents pass on around 55% of their own mean difference from the population IQ of 100; but, along with the 45% from inherited genetic variations which this 55% includes, there is a 10% environmental component in this transmission - at least in childhood, as in the data of Schiff et al. (op.cit.).] Within normal demographic and environmental ranges of the mid-twentieth century West, the MZa correlation indicates a broad heritability of IQ of around 75%. Spelling this out, some 45% of population variation arises from genetic factors that can be transmitted from parents to children [i.e. from narrowly heritable factors that can be subjected to selective breeding], and a further 30% arises from genetic factors that are not conventionally [i.e. narrowly] 'inherited'. The remaining 25% of population variance in IQ occurs because of the environmental differences between homes (10%), other environmental influences (5%) and sheer test unreliability (around 10%). All such percentage estimates must be understood in conjunction with the considerations raised in this chapter - they make no range corrections or allowance for monochorionicity, for example. However, a broad heritability estimate of 75% is one way of saying that any 'zero heritability' hypothesis is totally unreasonable; and that a mere 35% estimate, say, would be quite unlikely within twentieth-century Western parameters. And this should be no surprise to common sense: for "perhaps the most dramatic evidence of genetic influence is typically overlooked" - "the sheer number of children who exhibit high levels of talent and who are found in the most unlikely environmental circumstances" (Humphreys, 1994).

    To some, however, it has seemed proper to refrain from 'insensitive' researches that could spread gloom and bitterness in society. Even if they can mount little argument against a 75% estimate, they still hope that new champions of social environmentalism will arise. To them, high heritability coefficients speak of dread determinism and likely state manipulation. Yet where is the threat in intelligence being largely inherited and influencing people's own selections of their personal environments? What is the point of a freedom that is not guided by nature - of a freedom that does not, among other things, achieve a correspondence between opportunities and abilities? What is it that can actually be done to help children, potential parents and minority groups if belief in the inheritance of g could somehow be kept at bay? The next chapter will discuss the importance of IQ differences and the proper educational response to them. But if IQ is not involved in education, its heritability can surely not matter. Alternatively, if IQ is actually a useful predictor and guide to action in education, how could the world possibly be a safer place for any insistence by new champions of environmentalism that IQ's heritability is low, unknown or unknowable?

    Since disagreeable and dangerous practices are often feared to follow from hereditarian beliefs, it is worth examining the practices that would follow naturally in the wake of environmentalism. What if the usual IQ correlation of .50 between parents and children had in fact been shown to result from nothing but the environments that are supplied by different parents? What if MZ 's had proved no more similar in phenotype than DZ 's? Would determinism have been overthrown? Would not one causal account just have been replaced with another? And would such a reversal of hereditarianism have policy implications that were strictly anodyne? After all, if maximum reductions were desired in illiteracy, criminality and unemployability in future generations, it would still be necessary to help duller parents (quite the most common suppliers of low-stimulation, low-supervision, high-punishment, high-child-abuse home environments (Herrnstein & Murray, 1994)) to restrict their family sizes. It can hardly matter to the committed social improver and utopian whether the children of lower-IQ families would have had their lower IQ's because of genetic or because of environmental influences. If children of the future are to achieve maximum intellectual and educational levels and to be more employable, there would need to be relatively fewer homes where parents and caretakers were unstimulating, drug-addicted, neglectful, or themselves of low IQ - even assuming largely environmental origins of g. When Reed & Reed (1965) collected data on the 80,000 descendants of the grandparents of 289 state colony patients having IQ < 70 (and without epilepsy), they concluded that having a family member showing what have in the 1990's been called 'learning difficulties' was the major predictor of mental retardation in a descendant. Only 88 of the original 289 patients were classified as having retardation of definitely genetic origins; yet retardation ran in families to such an extent that the overall rate of retardation would have been reduced by 50% if retarded people themselves had not had children.

    How can progress towards a reduction of low-IQ parenting be achieved? Well, the percentage of such homes in the future population could be reduced by interventions in high-rise mothercare that are expensive, draconian or both. If environmental influences are crucial to intelligence, compulsory fostering can be justified for children who fail to thrive. This is what happens currently in Britain - where youngsters of low IQ are encouraged to have babies by health and welfare officials and by charities concerned with mental handicap. However, even though whole teams of social workers are appointed to assist the mothers and their babies, these babies are the most common target of forcible removal of children from biological parents: whatever their initial personal beliefs in the viability of low-IQ parenting with 'community support', social workers gradually become aware of the humanitarian risks involved.(32) Alternatively, progress could be achieved economically and acceptably by voluntary changes in contraceptive practice and mate selection. So what is new? Persuading and assisting lower-IQ people towards selecting contraception, higher-IQ sexual partners, or both would raise the intelligence of the next generation quite regardless of 'the heritability of g '. Similarly, any would-be parent who regards intelligence as an asset to a child will try to select a relatively bright spouse - no matter whether the correlation between parental and child IQ is expected via the spouse's genes (nature) or via the spouse's child-rearing skills (nurture)! It would not be well-meaning citizens, would-be parents or even honest social engineers who would need to change their ways if they ever learned that intelligence was actually largely heritable. The parent-child IQ correlation itself shows how to make improvements for the future quite regardless of hereditarian or environmentalist interpretations of how the correlation arises.(33) A eugenic policy focussed on IQ must be attractive to any would-be improver of human happiness - whether hereditarian or environmentalist.

    As if in appreciation of the even-handed appeal of discouragement of low-IQ parenting, advocacy of extended uptake of contraception is sometimes expressly undertaken on environmentalist grounds(34), by scholars who decline to acknowledge any great importance of genetic inheritance in generating major social problems (Flynn, 1992; and see Chapter IV); and, looking further afield, classical Islamic tolerance of polygyny recognizes the value of successful males supporting and rearing larger families. Hence the reasonable anxieties aroused by hereditarian ideas can be no greater than those that would equally be aroused by environmentalist ideas if any attempt were made to derive practical reforms from environmentalist accounts of intelligence. Obscuring the role of genetic factors is usually hoped to delay the arrival of intrusive state policies for family life; but there is quite as much justification for state intervention if low-g parents transmit low g to their children mainly via the environment which they supply - assuming that adults of the future are expected to need markedly higher average intelligence than they have today. To use the resources of the state to try to improve the population is an option that is available to environmentalists just as much as to hereditarians; likewise, improvers of both kinds will need to decide, quite independently of their favoured heritability estimates, whether to use inducements, appeals to reason or punishments. In the twentieth, human rights have been trampled on by Russian Communists and Third World religious enthusiasts believing in the environmental perfectibility of mankind just as by Nazis believing in the genetic purification of their racial stocks; and today the policy of encouraging sterilization of the mentally abnormal and subnormal is evidently being pursued on a large scale in Communist China.(35) It is folly to think that hereditarian beliefs are either uniquely threatening to human rights or in any simple sense 'right-wing'. Acceptance of others' rights is what protects everyone from state manipulation of any kind; and such acceptance follows perhaps a little more easily from a belief in biologically based individual agency than from an environmentalism that stresses the power of society to shape and even 'construct' the individual. The threat of heritable IQ is not to human rights or liberties - which need defending against both genetic and social environmentalist manipulation. Rather, the threat is to ideologues whose own belief systems and particular job prospects dictate that all-round ignorance about causation is bliss. This is why talk of 'complex interaction effects' has been so popular: it keeps all causes in the dark and strengthens the priesthood of Western social experts who imply they have some mastery of the 'many complexities of development' that is superior to what published researches indicate. Virtually any knowledge of causes at all - whether genetic or environmental - challenges those Western experts in welfare and education who have presided over steadily rising levels of crime, single-parenting, illiteracy and unemployability. Far from being able to do better for children than did Sir Cyril Burt, these experts can be argued to have been engaged in such outright deception of the general public about IQ as to put Burt's own self-indulgence quite in the shade (see also Chapter IV).

    Nevertheless, though an informed environmentalism that admits its own utopianism is better than ignorance, piety and mumbo-jumbo, environmentalists do have something of a cross to bear for a while. Not only have they been largely factually wrong in their beliefs about how g differences arise, but they have commonly been self-contradictory in their own planned response. To achieve wider practice of contraception unless and until sexual partners are ready to provide a good home for children should plainly be as much a policy objective of serious, progressive environmentalists as of hereditarians; and so must the encouragement of young women to have children only by men who can be relied upon to pay the costs (e.g. from prior insurance) of a modern upbringing. The environmentalist who declines to advocate birth control and greater sexual selectivity is just as hamstrung as any hereditarian whose personal or religious principles forbid even the gentlest interference with human nature. To load the hereditarian with advocacy of extended contraceptive practice, spousal selection and state interference is a moral evasion as thoughtless as it is cowardly. It attracts cheap popularity - but at the expense of making environmentalism incoherent, pointless, timid and supportive chiefly of those who already enjoy official positions in their countries' welfare states. Environmentalists hope to despatching hereditarian truth claims by popularity-courting manoeuvres; but these serve in the end to leave hereditarianism not only with greater credibility and a slight edge on human rights but also with a monopoly of common sense.


  1. There is proper anxiety that a society which believes in the importance of genes may end up violating human rights in the pursuit of compulsory, state-orchestrated eugenics. In part for this reason, modern psychogenetic research into intelligence has been surrounded by acrimony. When Sir Cyril Burt was shown to have used (or even invented) poor-quality data on twins, fresh excuse was provided for the suspicion that London School psychologists and psychogeneticists might have unacceptable motives - even though Burt was much more interested in education than in eugenics.
  2. Since Burt's exposure, the more immediate objections to research on nature-nurture questions about intelligence have generally taken the form of denying that anything of value can be discovered. In particular, nature and nurture are said to be so 'closely interwoven' as to be 'inextricable from each other' in their 'complex interaction'.

  3. Although research into twins and adoptees, separated or together, could disclose some irresoluble conundrum, and certainly show up any specifiable 'genetic x environmental interactions', it does not in fact do so in the case of g . Instead, by adolescence (by which time parental influence is weak in the modern West), unrelated children who have grown up as adoptees in the same family show quite simply no similarity at all in their levels of g . As Neisser et al. (1995) conclude in their review for the American Psychological Association, "Severely deprived, neglectful or abusive environments must have negative effects on a great many aspects of development, including intelligence. Beyond that minimum, however, the role of family experience is now in serious dispute."
  4. Psychogenetic methods are quite able to register and analyse complexity - even though data are all too few because of limited state funding of research. For example, measures of temperament, character and psychopathology currently find: modest (additive) genetic influence; virtually no influence of the imposed ('between family') environment; and thus much remaining variance to be explained (by G x E interaction, by G x G epistasis, and by sibling rivalry). Intelligence, however, is different - even though many of the same investigators are involved. For g , there is a strong, 'additive' genetic effect (inherited from parents) and a definite, though modest effect of the imposed environment (in particular, during childhood, of SES and parental IQ). There is also evidence that other genetic effects work to some extent via a growing child's choice of micro-environment. Whether in infancy, adolescence or middle age, modern studies of g show people's 'nature' operating via the selection of their 'nurture'.
  5. (5) Children's g differences arise both from the parental supply of genes and from the parental supply of environments; and working out the balance is of great academic interest. However, for the purpose of raising average levels of intelligence in the future population, the balance of these two types of causal influence matters little. The correlation (of .55) between parents and children of itself ensures that the encouragement of planned and affordable births to intellectually adequate parents would have an improving effect on the g levels of children. Hereditarian views have been contested and psychogenetic research frustrated by ideology : there would actually be quite as much reason for discouraging low-IQ parenting if social environmentalist theories about IQ were correct. Obscuring the role of genetic factors is usually hoped to delay the arrival of intrusive state policies for family life; but there is quite as much justification for state intervention if low-g parents transmit low g to their children mainly via the environment which they supply - assuming that adults of the future are expected to need higher average intelligence than today. That environmentalists have seldom advocated discouragement of low-IQ parenting probably reflects a certain lack of faith in environmentalism.


  1. Sidney Webb (1859-1947), the founder of the London School of Economics, was himself a socialist pioneer of the welfare state and of environmental improvement; but he approved Asquith's rhetorical question as being "on the right track" (Webb, 1901). Like many, Webb was horrified by "the stunted, anaemic, demoralised denizens of the slum tenements of our great cities." While Gladstonian Liberals remained "axiomatically hostile to the state", Webb opposed such individualism and "administrative nihilism"; and he insisted that "the maximum of individual development will not be secured by allowing each unit to pursue its own ends without reference to the welfare of the whole." Such sentiments were as congenial to eugenicists as to environmentalists.

  2. The term 'eugenics' will be used here in its conventional sense, to refer to the applied science of improving the human gene pool by selectivity in procreation. However, Galton himself had defined it more broadly - see Chapter I, Footnote 4. As genetic engineering throws up new ways of improving the inborn qualities of future generations, perhaps a wider use of the term will develop.

  3. In 1934, Huxley publicly commended German legislation for the compulsory sterilization of certified mental defectives. He wrote: "If conditions remain as they are now, and if the present tendency continues unchecked, we may look forward in a century or two to a time when a quarter of the population of these islands will consist of half-wits. What a curiously squalid and humiliating conclusion to English history!....What is the remedy for the present deplorable state of affairs? It consists, obviously, in encouraging the normal and super-normal members of the population to have larger families and in preventing the sub-normal from having any families at all." (Quoted by Robinson, 1995)

  4. Nazi patience with the compulsory sterilization that was sanctioned by some of their 'race hygiene' experts ran out by 1937. Simply, euthanasia was quicker and cheaper (especially in releasing hospital beds for military use). Moreover, the Nazis had other objectives that were far from eugenic with regard to IQ. In 1937, IQ testing was banned in Germany so as to restrict awareness of the superior results of even those Jews who had not already escaped. In this flight from reality, Hitler was to be joined by other dictators - for Stalin, Mao and Iran's Ayatollah Khomeini also banned IQ-testing as ideologically unhelpful; and by many US states seeking to justify abolition of forms of educational provision that varied in their relevance to black and white children.

  5. The Chairwoman of the European Commission's 'Group of Advisers on the Ethical Implications of Biotechnology' was quoted in New Scientist (24/31 xii 1994, p.11) as wanting to "outlaw" germ-line therapy.

  6. In Britain, advocacy of private insurance replacing state welfare activities is undertaken by the Adam Smith Institute, London (Butler, 1995).

  7. To challenge Western states' permissiveness of criminality would be to touch the weak spot of modern utopianism and indiscriminate welfarism. At present, government measures against crime include spending on virtually everything except that recompense of costs to which victims would be entitled if their losses were handled under civil, instead of under criminal law.

  8. Nazi social practices had little connection with the findings of science; and Nazi beliefs in the blue-eyed Aryan race, in the mental and moral inferiority of Jews, Slavs and homosexuals, and in the value of unquestioning loyalty to the Führer all proved positively costly fictions.

  9. McDougall would go on to Harvard and be the Chairman of the Psychology Department there while Yerkes was conducting the earliest work on group testing of IQ (see Chapter I).

  10. Gould (1981) makes the interesting suggestion that, in comparison to the 'Aristotelian' Spearman, Burt could be considered somewhat more of a 'Platonic' realist in his lofty and non-materially-grounded conception of mental reality. However, there was no such overt contest between the two men; and it was Burt who took more interest in gathering data relevant to the question of whether g had some definite biological basis.

  11. There were two main eugenic proposals regarding the mentally retarded: colonization and sterilization. Compulsory sterilization of mentally defective girls was first permitted in Indiana in 1908. By 1930, some thirty states of the USA had passed similar legislation. Castration of rapists was popular in several European countries - being especially supported by women's groups; and eventually the Nazis seem to have sterilized about one woman in thirty in urban areas (if the case of Hamburg is representative). Throughout this period Britain enacted no specifically eugenic enabling legislation; but large-scale compulsory hospitalization of the mentally ill and the mentally retarded invariably segregated them to locked, single-sex wards that provided few heterosexual opportunities and thus constituted a de facto version of colonization. (Colonization was the preferred option of most IQ psychologists: for example, the keen eugenicist, H.H.Goddard (1919) held that sterilization was too far out of step with traditional sensibilities.)

  12. For differing views of Burt's responsibility for the post-War changes, see Sutherland, 1995, and Wooldridge, 1994. By 1943, Burt was sixty years old, living in Aberystwyth (to which University College London had been evacuated) and suffering from Menière's disease and hearing loss; but his influence in the 1930's had been substantial - as would be expected from his occupancy of what was then the most prestigious chair of psychology in Britain.

  13. This was at a time when places in the state-funded grammar schools of Britain were available for only about one child in ten.

  14. Graduates are relatively successful in gaining employment, but only because of their age and IQ's; and, reflecting the limited impact of today's first degrees, it is now rare for professional and administrative careers to begin until aspirants reach their late-twenties.

  15. However, the hereditarian JamesShields (1962) had discounted Burt's 1943 report (of 15 MZa pairs) as lacking adequate detail. It may have been this embarrassing dismissal of his work that spurred Burt to take the matter up once more in 1966.

  16. At very least, Burt's own data had been collected and recorded skimpily and haphazardly; and, to supplement genuine data of his own, Burt may have used without acknowledgment some of the data published by others. (The MZa data set out so fully by Burt's young rival, James Shields (1962), yielded almost exactly the same MZa correlation as Burt reported.) In fairness to Burt, he cannot be accused of fabricating elaborate details of studies that were never conducted: like most psychological research reports today (which provide little detail of testees or their motivations for participation - to disguise the fact that subjects are usually psychology students) Burt's papers are written as if making the best of rather poor data about which 'the less said, the better'. However, Burt can certainly be blamed for: (i) not admitting the loss of at least some of his data (perhaps in wartime bomb damage at University College London); (ii) implying that the minimal data that might have been reported to him by former students deserved full-scale journal articles - with or without supplementation from already published cases; and (iii) illustrating methodological points in ways that could mislead readers into thinking that real data were being discussed. [The latter problem was first noted by Jensen (1974); but a spectacular example was furnished later by Dorfman (1978). Burt had discussed the class distribution of IQ using figures that yielded an impossibly perfect bell-shaped (Gaussian) distribution. However, Burt's IQ distribution was more regular than the normal distribution of the heights of the 67,000 soldiers measured during in the American Civil War. (It was this bell-shaped distribution which had first suggested to statisticians the likely causal play of multiple, independent factors in yielding human differences.)] The scientist who has messy, real-life data faces an unenviable choice that can be called Dorfman's Fork (after Dorfman, 1995). If he publishes in a top scientific journal, Dorfman will accuse him of not providing the essential details that require exposition; and if he publishes a 845-page book, Dorfman will accuse him of not taking his data to a top scientific journal.

  17. 'Hereditarians' believe - sometimes from evidence, sometimes from prejudice - that many important human features are either inherited from parents or have some other genetic basis (as when recessive genes allow expression of a trait rarely seen in the rest of a family). By contrast, 'methodological solipsists' believe there is no way of knowing the degree to which a character is under genetic influence: one eminent professor of genetics in Scotland used to claim in the 1970's that, despite a century of genetical science, it was simply not known even whether human differences in height are heritable. In their corner, 'environmentalists' believe that observable phenotypic differences in the population arise from people having experienced different physical or social environments or events. Some environmentalists stress the involvement of culture and language in all human experiences and outcomes, and thus hope to avoid testing their (often idealistic) beliefs against any evidence at all regarding individual differences; other environmentalists just take the influences of different families to be so obvious as to need no further explicit testing. (For whichever reasons, most environmentalists prefer to criticize all methods of psychogenetic enquiry rather than conduct studies of their own - the Paris team of Schiff et al.(1986) being an honourable exception. Environmentalism thus tends to shade into methodological solipsism.) For a fourth, very popular 'interactionist' position, see later in this Chapter, p.{Draft 4: 15}. (Some versions of 'interactionism' also shade into methodological solipsism: for example, testing for 'complex interaction effects' (on the rare occasion of these being spelled out) is impossible without large data sets that interactionists show little propensity to collect.)

  18. 'Selective placement' was motivated by two considerations: that the adoptee should have adoptive parents who would be understanding; and that the adoptee should not stand out like a sore thumb in the adoptive home - e.g. by having brown eyes when the eyes of the adoptive parents were blue.

  19. 'Phenotype' is the technical term in genetics for what is observable, on-the-surface and measurable, as distinct from the hidden 'genotype' that the geneticist seeks to read.

  20. The first recorded appearance of the 'areas' argument for the 'inextricability of genes and environment' was during a sabbatical visit by Hans Eysenck to the Berkeley campus of the University of California in the 1960's. The argument is still heard today, even from academics who are sympathetic to hereditarianism: "I think the idea of nature versus nurture is silly. It's like asking, 'Is it longer to New York or by train?' Or, as one friend of mine likes to say, 'Is the area of the rectangle caused by the length or the width?" (Professor R.Masters, interviewed by K.McDonald, 1994). - The answer is 'Yes: differences of areas among a number of rectangles can be wholly explicable by reference only to their differing lengths - or widths. For a particular collection of rectangles may all be equal to each other in widths - or lengths.' Refusal to 'separate nature from nurture' and to answer questions about how individual differences most likely arise is still part of the Piagetian position (e.g. Annette Karmiloff-Smith, 1995, BBC IV UK, 18 xii, 09.45hrs).

  21. Herrnstein & Murray (1994) report for the years around1990 the same soaring rate of out-of-wedlock births among white American females that occurred among black American females around 1970.

  22. (i) A normal distribution is the bell-shaped frequency distribution that will result from a quantifiable trait being under the influence of multiple uncorrelated influences. If a coin is tossed twelve times, it will more likely come down as heads on six of the trials than on just one or on as many as twelve of the trials. Similarly, when a number of unrelated chance factors influence people's scores on a final variable, most scores will be in the middle of the final phenotypic range and few will be at the extremes. (ii) Ordinary statistical interaction - as distinct from the Piagetian variety considered in Chapter II - occurs when scores on variable Z can be predicted by weighting and multiplying together testees' scores on variables X and Y. To the extent that there are substantial positive interaction effects between several of the relevant variables, the resulting distribution will tend to lose its bell-shaped distribution and become skewed.

  23. Variables like extraversion, neuroticism/emotionality, conscientiousness, will /disagreeableness, and affection /openness [i.e. the 'big five' personality dimensions other than g - see Chapter I] all yield these effects [with perhaps lower MZ similarities for will (Brand, 1994a)]; and the tendency for 'psychotic' personality characteristics (e.g., in questionnaire measurement, suspicion and disillusionmnent) to show such a picture is specially marked. The usual interpretation is that, when the genetic packages of epistasis are crucial to trait levels, two people need an especially high proportion of their genes in common if marked phenotypic similarity is to be observed. Only at MZ levels of genotypic similarity (i.e. near 100%) are virtually all genetic 'packages' be held in common between two people; so especially high MZ correlations occurring while correlations for ordinary first-degree relatives are low will indicate that epistasis is at work. For the calculation of the 'heritability' of a trait, see below, Note III, 32.

  24. H.H.Goddard was the American popularizer of IQ and pioneer of research into children of around IQ 70. In the 1920's he was director of research at the Vineland Training School for Feeble-Minded Girls and Boys in New Jersey.

  25. By French standards of the time, the adoptive parents were 1,5 standard deviations high in socio-economic status (SES). In contrast, the homes provided by the biological mothers were, on average, 1 standard deviation low in SES.

  26. Each standard deviation of SES had thus proved worth only 3 IQ points to the average child. The gap on verbal IQ was a little bigger; and in educational attainment and school record there was a gap of a full standard deviation between the adopted and the biological-mother-reared groups. (It will be of interest to learn whether this environmentally created difference survives adolescence.)

  27. Funding came from the 'Pioneer Foundation', based in Washington - an 'old Right' body concerned with demographic issues and the quality of life in North America and usually presumed to set some store by the qualities of the Nordic peoples and to favour the exploration of eugenic social options for the USA. A notable individual initiative was that of a Hertfordshire social worker who, after experiencing the happiness he could bring about by re-uniting siblings who had been parted during the Holocaust, sought to carry on such work by re-uniting twins who had been separated by adoption. (Sometimes one twin was unaware of the other's existence; but delight was the invariable reaction to reunion.)

  28. The first discussion of an MZa pair was provided by Popenoe (1922) and followed up by Muller (1925)..

  29. After Kamin's exposure of the flaws in Burt's, it was often alleged that even his research assistants and co-authors had been an invention. However, testimony subsequently became available from academics who recalled meeting at least two of the ladies in the 1930's (Joynson, op. cit.). (Psychology degrees are not very vocationally oriented - as shown by their very different content in different universities; so students seldom become professional psychologists and often disappear without trace after graduation.) For a more recent defence of the idea that Burt may well have had data for his key claims, see Brand, 1995b.

  30. McGue et al. (1993) review evidence from published studies involving 190 MZ pairs and 178 DZ pairs; and they add 142 MZt pairs and 103 DZt pairs from their own work. These latter Minnesota twins were all ascertained from birth records as described by Lykken et al., 1990, thus reducing the likelihood of volunteer bias (the possibility that phenotypically more similar MZ 's may be over-represented because of a special interest in twin research).

  31. The most commonly used estimate of the degree to which a trait is inherited (i.e. of its heritability, h²) is to double the difference between MZ and DZ correlations: 2(rMZ - rDZ ). Such 'broad heritability' (h²B) estimates range theoretically between zero (when the two types of twin have the same correlation) to 1.00 (if a trait were entirely additively heritable and MZ twins correlated at 1.00 - twice as highly as DZ 's, in line with their having twice as many genes in common [of those genes for which there is any substantial species variation]). In the early 1980's, because of some studies that did not involve a representatively wide range of IQ's, some put the h²B for IQ as low as .55. Some psychogeneticists would perhaps be content with that today, thinking that the more modern studies mentioned in this chapter have perhaps not involved a wide enough range of social environments. (Obviously - whatever social workers may arrange today to avoid cross-racial adoptions - adoptees of the past were not sent by agencies to homes of drug addicts in inner-city high-rise urban ghettoes having today's rates of truancy, crime and welfare dependency.) However, the majority of psychogeneticists and differential psychologists today would estimate h²B for I.Q. in modern Western conditions - with colour T.V. widely available, and with none but voluntary malnutrition - as being >.70; though, due to parental control of the environment in the early years, the figure may well be lower in young children than in adults.

  32. Selection pressure will reduce the 'narrow heritability' (h2N). h2N is the degree to which a phenotypic trait is passed genetically from one generation to the next: it quantifies the genetic variance that 'breeds true', making individuals more similar as a simple linear function of the percentage of genes that they should have in common (at any level of biological relationship). However, other genetic effects, e.g. those arising from genes being dominant or recessive, or from multiplier effects between genes (epistasis), make an additional contribution to the degree to which, in the population as a whole, a trait has high h2B. Thus genes that make (narrowly, additively) for low intelligence might disappear in response to natural selection; but genes that only make for low intelligence when combined with other G or E features could survive a particular selection pressure that operates against their non-interactive manifestations.

  33. Details of two such cases were broadcast on 'Face the Facts', BBC IV UK, 15 vi 1995.

  34. I am indebted to Lloyd Humphries for the essential idea behind this paragraph - though I must take any blame for the way in which it is expressed.

  35. Discouragement of low-IQ parenting so that future children will enjoy more stimulating (etc.) environments and thus have higher IQ's themselves could actually be called 'eugenic' under Galton's original broad definition - see Chapter I, Note 4, ii. (If genes and environment both contribute to g , then an environmental improvement would be 'improving inborn quality' and 'developing it to the utmost advantage'; thus the improvement would fall under Galton's broad notion of a eugenic measure.)

  36. A major programme of encouraging the sterilization of both men and women having psychological abnormalities has been in operation in the north of China since 1990 (BBC IV UK News, 1 vi 1995, 0745a.m.). Apparently the programme is regarded as an experiment which, if successful, will be extended to the rest of China.

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