Genetic and environmental influences on adult intelligence and special mental abilities

Human Biology, Apr 1998 by Bouchard, Thomas J Jr

THOMAS J. BOUCHARD JR.1

Abstract I review representative biometric studies of adult twins and adoptees that have been used to generate estimates of genetic and environmental influence on intelligence (IQ) and special mental abilities. The various studies converge on a heritability estimate between 0.60 and 0.80 for IQ. Estimates of common environmental influence from the same studies are near zero. Studies of twins reared together and studies of unrelated individuals reared together yield sizable estimates of common family environmental influence in childhood but also demonstrate that this influence dissipates with age and approaches zero in adulthood. Twin studies of the major special mental abilities (verbal, spatial, perceptual speed and accuracy, memory) yield heritability estimates of about 0.50 and modest estimates of common environmental influence.

KEY WORDS: MENTAL ABILITY, IQ, INTELLIGENCE

The quantitative study of genetic and environmental influences on the enormous variation in human mental abilities begins with Sir Francis Galton, who recognized that observed relationships within biological families confounded genetic and environmental determinants of human traits. Searching for a method to disentangle these two sources of influence, Galton introduced the use of both twins and adoptees (Galton 1876, 1883). He also invented the method of correlation (Galton 1888; Stigler 1989), variants of which underlie most twin research (Neale and Cardon 1992). Galton can thus be fairly called the founder of quantitative behavioral genetics.

Theories of Mental Abilities

Since Galton's time, enormous progress has been made in both our understanding of the nature of human mental abilities and methods for disentangling the various genetic and environmental sources of variance in these abilities (Bouchard 1996). Psychometricians agree that it is both theoretically and practically useful to conceptualize the domain of cognitive abilities as a hierarchy of abilities (Carroll 1993; Gustafsson 1984; Gustafsson and Un&eim 1996; Undheim and Gustafsson 1987) rather than as a taxonomy (Guilford 1967). There are views on the nature of intelligence other than the psychometric hierarchical one (Gardner et al. 1996), but because only psychometric theories have generated data relevant to the issue of genetic influence on intelligence, I limit this review to studies in this tradition.

Hierarchical theorists have a number of disagreements. The principal one is whether the hierarchy is truncated, thus having the appearance of a mountain range (a number of higher-order abilities with no general factor spanning them), or whether it is a true hierarchical structure with a general factor (Spearman's g) spanning the higher-order factors (Carroll 1996). The mountain range model is actually the current version of the theory of fluidcrystallized intelligence first put forward by Cattell in the 1930s and 1940s (Cattell 1933, 1943, 1963; Hakstian and Cattell 1978) and championed by John Horn since the mid-1960s (Horn 1976, 1994, 1998; Horn and Cattell 1967). The peaked model is the enduring theory of g originally proposed by Charles Spearman (Spearman 1904, 1927) and currently championed by a number of investigators (Brand 1996a; Carroll 1993, 1996; Herrnstein and Murray 1994; Jensen 1992, 1993, 1994). This controversy remains to be fully resolved.

A number of recent studies, however, strongly favor the g view. Bickley et al. (1995) demonstrated that comparable measures (16 ability tests) administered to 8 age groups (6, 8, 10, 13, 16, 30-39, 50-59, 70-79 years) have the same organizational structure across all age groups, and that structure is a three-stratum structure that requires a g or general factor at the top. Bickley specifically tested and rejected the hypothesis that g was identical to any of the second stratum factors. Carretta and Ree (1995), using a different set of ability measure and a large military sample-269,969 applicants for US Air Force commissions between the ages of 18 and 27 years-showed that the structure of cognitive abilities is virtually identical across males and females and across ethnic groups (white, black, Hispanic, and Native American). These studies confirm Carroll's (1993) conclusion, based on a meta-analysis of the world literature, that "there is abundant evidence for a factor of general intelligence, . . . found at the highest order. . of analysis for a given data set . . . that dominates factors or variables that emphasize the level of difficulty that can be mastered in performing induction, reasoning, visualization, and language comprehension tasks" (p. 624). Consequently, I examine genetic and environmental influences on g (the third-order factor), special mental ability factors (second-order factors), and specific tests.

The controversy generated by the publication of The Bell Curve (Herrnstein and Murray 1994), which strongly championed the g view and the argument that g was heavily influenced by genetic factors (Bouchard 1995; Dorfman 1995), should make it clear that both the issue of genetic influence on mental abilities and the construct validity and utility of mental ability tests continue to be widely debated (Fischer et al. 1996; Gottfredson 1997a; Gould 1996; Rushton 1997a). Contrary to the picture portrayed in the media, however, most knowledgeable behavioral geneticists believe that mental abilities are to a considerable degree under genetic influence (Gottfredson 1997b; Plomin and Petrill 1997; Snyderman and Rothman 1988), and most experts believe that mental ability tests have substantial validity and utility (Baker et al. 1993; Gottfredson 1996, 1997c; Hunt 1995; Lubinski and Humphreys 1997; Neisser et al. 1996; Snow 1995). Some dissenting views can be found in the special issue of the American Psychologist devoted to intelligence (Sternberg 1997). A striking example of how controversial the topic of g continues to be can be found in the de-publication (censorship) of a recent book titled The g Factor, by Christopher Brand (1996b).