IQ: Biological And A Predictor Of Many Things
The study of human variation in intelligence appears controversial from the outside but there is little controversy in the field itself. IQ tests predict a host of characteristics of individuals including educational attainment, job performance, income, health and other non-obvious characteristics like susceptibility to Alzheimer’s disease. In general the search for social and nutritional causes of IQ differences has not led to any convincing results and most workers now regard IQ as a biological rather than a social variable. It is highly heritable: correlations between identical twins reared apart are 0.7–0.8. Genetic manipulation can raise intelligence in mice (Routtenberg et al., 2000) [...]
The basis of intelligence testing is that when people are given a battery of cognitive ability tests broadly defined – anything from general knowledge to vocabulary to digit memory to tasks requiring mental rotation of three-dimensional objects – those who do well on one of these tend to do well on all of them and people who do poorly do poorly on all of them. Different cognitive abilities are highly correlated with each other, and the underlying ability responsible for the correlations is called g. While the development of theory about general intelligence or g was entirely based on tests and correlations, it has recently become apparent that there is a neurobiological basis for variation in g, reflected for example in correlations between intelligence and brain volume, volumes of specific brain regions, current density tomography, reaction times, brain glucose utilization rate, and so on (Gray & Thompson, 2004).
There is variation in the way different subtests contribute to IQ. Males, for example, usually outperform females on spatial and quantitative tasks while females do better on tasks related to language. There are also group differences: Ashkenazim do not show any marked advantage on spatial tasks while they excel at linguistic and arithmetic tasks. North-east Asians have high spatial scores for a given overall IQ.
Whatever the reality or nature of g, for our purposes the important observation is that IQ test scores work in the sense that they are the best available predictor of academic success and job performance, especially for complex jobs (Herrnstein & Murray, 1994; Gottfredson, 1997, 2002). They are also one of the best predictors available of family stability, criminality, health and lifespan (Gottfredson, 2004).
IQ test scores are highly heritable, being almost always greater than 0.5 when adult scores are studied. Lower heritability estimates are found for children’s IQ: the IQ of children does seem to reflect in part environmental influences like the social class of the home in which the child is reared, but these influences disappear as the child matures and are essentially gone in adulthood. In the same way enrichment programmes like Head Start cause a transient elevation in IQ scores of children but these effects disappear as the child matures. The phenomenon of heritability increasing with age is characteristic of many quantitative traits in mammals (Falconer, 1981).
The heritability of IQ is probably lower than 0.80 in most human populations, and it may be as low as 0.50, so there are apparently some environmental effects on IQ. Since siblings and twins raised apart are as similar as those raised together, it has become commonplace to speak of ‘non-shared environment’, which means that siblings are exposed to different environments even when raised together. It is important to realize that so-called environmental effects include non-additive gene interactions like dominance and epistasis as well as testing error. The correlation between one IQ test score and another taken later may be as low of 0.8 or so.
There is an apparent secular trend in many countries in IQ scores: people today score much higher on old IQ tests than people did at the time. The gain varies among countries but on average is about five points per decade. This ‘Flynn effect’ (Flynn, 1987) may include some real increases in IQ that reflect improvements in biological well-being: better nutrition, vaccination, antibiotics for childhood disease, etc. A perhaps more likely explanation is the increase in school attendance, leading to familiarity with the format of short-answer timed tests. Whatever its basis, the gain has occurred uniformly across ethnic groups and social classes so relative group differences have remained unchanged. It also seems to have stopped in recent decades (Jensen, 1998).
Some would suggest that, even though IQ scores are heritable, there are no biological differences in mean IQ between populations and that the well-known differences among ethnic groups in North America are the result of racism or deprivation or some other social cause. Similarly one might argue that high Ashkenazi scores are the result of home environments that encourage scholarship. There is scarcely any support in the literature for social effects like home environment on IQ (Rowe, 1993). A standard textbook on IQ, after reviewing environmental effects, concludes that ‘. . . it is all too easy to throw up one’s hands in despair’ (Mackintosh, 1998): despair presumably because there is a widespread desire to find environmental effects that can be manipulated. So far, after intensive searching, no one has found any, and the current consensus is that variation in IQ reflects variation in the underlying biology rather than in the social environment. This parallels the current consensus that mental illness is a biological phenomenon and that the folk beliefs of half a century ago about causes – harsh toilet training, aloof fathers, etc. – have no empirical basis (Haier, 2003).
Estimates of the narrow-sense heritability of IQ vary, but generally range between 0.3 and 0.5 in children (Devlin et al., 1997) up to 0.7 or higher when measured in adults (Jensen, 1998). Heritability must vary between populations since it is sensitive to demographic phenomena like assortative mating, the extent to which spouses are similar to each other with respect to IQ, and inbreeding. Assortative mating increases IQ heritability. Inbreeding lowers offspring IQ and so could contribute extra variance to the IQ distribution, lowering heritability.
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Falconer, D. (1981) Introduction to Quantitative Genetics, 2nd edition. Longman, London.
Flynn, J. (1987) Massive gains in 14 nations: what IQ tests really measure. Psychological Bulletin 101, 171–191.
Gray, J. & Thompson, P. (2004) Neurobiology of intelligence: science and ethics. Nature Reviews Neuroscience 5, 1–13.
Gottfredson, L. (2004) Intelligence: is it the epidemiologists’ elusive ‘fundamental cause’ of social class inequalities in health? Journal of Personality and Social Psychology 86, 174–199.
Gottfredson, L. S. (1997) Why g matters: The complexity of everyday life. Intelligence 24, 79–132.
Gottfredson, L. S. (2002) g: highly general and highly practical. In Sternberg, R. J. & Grigorenko, E. L. (eds.) The General Factor of Intelligence: How General Is It? Erlbaum, Mahwah, NJ, pp. 331–380.
Haier, R. J. (2003) Positron emission tomography studies of intelligence: from psychometrics to neurobiology. In Nyborg, H. (ed.) The Scientific Study of General Intelligence. Elsevier Science, Oxford, pp. 41–51.
Herrnstein, R. J. & Murray, C. (1994) The Bell Curve: Intelligence and Class Structure in American Life. The Free Press, New York.
Jensen, A. (1998) The g Factor: The Science of Mental Ability. Praeger, Westport, CT.
Mackintosh, N. (1998) IQ and Human Intelligence. Oxford University Press, Oxford.
Routtenberg, A., Cantallops, I., Zaffuto, S., Serrano, P. & Namgung, U. (2000) Enhanced learning after genetic overexpression of a brain growth protein. Proceedings of the National Academy of Sciences of the USA 97, 7657–7662.
Rowe, D. (1993) The Limits of Family Influence: Genes, Environment, and Behavior. Guilford, New York.
Quoted on Sun May 20th, 2012