VU Research Portal A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries Haworth, C.M.A.; Wright, M.J.; Martin, N.W.; Martin, N.G.; Boomsma, D.I.; Bartels, M.; Posthuma, D.; Davis, O.S.P.; Brant, A.M.; Corley, R.P.; Hewitt, J.K.; Iacono, W.G.; McGue, M.; Thompson, L.A.; Hart, S.A.; Petrill, S.A.; Lubinski, D.; Plomin, R. published in Behavior Genetics 2009 DOI (link to publisher) 10.1007/s10519-009-9262-3 Link to publication in VU Research Portal citation for published version (APA) Haworth, C. M. A., Wright, M. J., Martin, N. W., Martin, N. G., Boomsma, D. I., Bartels, M., Posthuma, D., Davis, O. S. P., Brant, A. M., Corley, R. P., Hewitt, J. K., Iacono, W. G., McGue, M., Thompson, L. A., Hart, S. A., Petrill, S. A., Lubinski, D., & Plomin, R. (2009). A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries. 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Sep. 2021 Behav Genet (2009) 39:359–370 DOI 10.1007/s10519-009-9262-3 ORIGINAL RESEARCH A Twin Study of the Genetics of High Cognitive Ability Selected from 11,000 Twin Pairs in Six Studies from Four Countries Claire M. A. Haworth Æ Margaret J. Wright Æ Nicolas W. Martin Æ Nicholas G. Martin Æ Dorret I. Boomsma Æ Meike Bartels Æ Danielle Posthuma Æ Oliver S. P. Davis Æ Angela M. Brant Æ Robin P. Corley Æ John K. Hewitt Æ William G. Iacono Æ Matthew McGue Æ Lee A. Thompson Æ Sara A. Hart Æ Stephen A. Petrill Æ David Lubinski Æ Robert Plomin Received: 3 December 2008 / Accepted: 3 March 2009 / Published online: 21 April 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Although much genetic research has addressed high g in Australia, the Netherlands, the United Kingdom normal variation in intelligence, little is known about the and the United States. etiology of high cognitive abilities. Using data from 11,000 twin pairs (age range = 6–71 years) from the genetics of Keywords Genetics Á High cognitive ability Á Twins Á high cognitive abilities consortium, we investigated the Intelligence Á Talent genetic and environmental etiologies of high general cog- nitive ability (g). Age-appropriate psychometric cognitive tests were administered to the twins and used to create g Introduction scores standardized within each study. Liability-threshold model fitting was used to estimate genetic and environ- A substantial body of genetic research using the classical mental parameters for the top 15% of the distribution of g. twin design has demonstrated the important role of genetics Genetic influence for high g was substantial (0.50, with a as a risk factor in the development of cognitive disabilities 95% confidence interval of 0.41–0.60). Shared environ- (Plomin and Kovas 2005). In contrast, very little is known mental influences were moderate (0.28, 0.19–0.37). We about the other end of the normal distribution—the genetic conclude that genetic variation contributes substantially to and environmental origins of high cognitive abilities— Edited by Dick Rose. A. M. Brant Á R. P. Corley Á J. K. Hewitt Institute for Behavioral Genetics, University of Colorado C. M. A. Haworth Á O. S. P. Davis Á R. Plomin (&) at Boulder, Boulder, CO, USA Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King’s College London, De Crespigny W. G. Iacono Á M. McGue Park, Box P080, London SE5 8AF, UK Department of Psychology, University of Minnesota, e-mail: [email protected] Minneapolis, MN, USA M. J. Wright Á N. W. Martin Á N. G. Martin L. A. Thompson Queensland Institute of Medical Research, Brisbane, Department of Psychology, Case Western Reserve University, QLD, Australia Cleveland, OH, USA D. I. Boomsma Á M. Bartels Á D. Posthuma S. A. Hart Á S. A. Petrill Department of Biological Psychology, Faculty of Psychology Human Development and Family Science, Ohio State and Education, VU University, Amsterdam, The Netherlands University, Columbus, OH, USA D. Posthuma D. Lubinski Section Medical Genomics, VU Medical Centre, Amsterdam, Department of Psychology and Human Development, Vanderbilt The Netherlands University, Nashville, TN, USA D. Posthuma Section Functional Genomics, Faculty Earth and Life Science, VU University, Amsterdam, The Netherlands 123 360 Behav Genet (2009) 39:359–370 despite the societal importance of exceptional talent and small sample sizes, genetic influence was modest (0.20) the well-documented extraordinary creative potential of and shared environment was substantial (0.70) for high this group (Lubinski and Benbow 2006; Lubinski et al. ability as well as for the rest of the distribution, although 2006). It cannot be assumed that the etiology of high these results may be due to the method of assessment. cognitive ability is the same as cognitive disability or the Other studies have investigated the etiology of individual same as the normal distribution of cognitive ability. For differences within high-g groups, with mixed results example, an extreme version of epistasis called emergen- (Thompson et al. 1993); however, such studies ask why esis has been suggested in which rare combinations of high-ability individuals differ from each other in their g alleles are responsible for exceptional cognitive ability scores rather than asking why high-ability individuals as a (Lykken 1982, 2006). Such a genetic model would predict group have so much higher g scores than the rest of the high correlations for identical twins and relatively low population. correlations in first-degree relatives. On the other hand, if In 2007, we formed the genetics of high cognitive exceptional cognitive ability requires an especially favor- abilities (GHCA) consortium with the goal of combining able environment, we might expect to see greater envi- cognitive ability test scores from six twin studies in four ronmental influence. countries in order to identify sufficient numbers of twins In 1869, Francis Galton raised the topic of the etiology with high g scores to conduct adequately powered analyses of high ability in one of the first books in behavioral of the genetic and environmental etiology of high g. genetics, Hereditary genius: An enquiry into its laws and Although these studies included different measures of consequences (Galton 1869). His conclusion that ‘‘there is cognitive ability, diverse cognitive tests can be used to no escape from the conclusion that nature prevails enor- create a g score that correlates highly with g scores derived mously over nurture’’ (Galton 1883, p. 241) was not from other tests (Johnson et al. 2008), which Charles warranted because his research involved family studies Spearman (1927) referred to as the indifference of the which cannot unambiguously disentangle the effects of indicator. Thus, we created g scores standardized within nature and nurture and he used reputation as an index of each study and also corrected scores for age within each ability. In contrast, more than a century later, others have study because the twins in the six studies varied in average argued that ‘‘differences in early experiences, preferences, age from 6 to 18 years (age range = 6–71 years). In opportunities, habits, training, and practice are the real another paper, we report results for analyses of individual determinants of excellence’’ (Howe et al. 1998). However, differences in g for the combined sample of 11,000 twin these authors note that ‘‘relatively little is known about pairs (Haworth et al. 2009). Heritability was estimated as the genetic origins of high-level ability’’ (Howe et al. 0.56 and shared environment accounted for 0.21 of the 1998, p. 403). In contrast, for general cognitive ability in variance. Significant heterogeneity was found across the the normal range the substantial heritability of g has been studies, but this heterogeneity is explained by the age documented in dozens of family, twin and adoption differences among the samples. When the 11,000 twin studies (Bouchard and McGue 1981; Deary et al. 2006; pairs were sorted by three age groups, heritability increased Plomin and Spinath 2004). significantly across age: 0.41 in childhood (average age of Although much research on high ability considers ath- 9 years), 0.55 in adolescence (12 years), and 0.66 in young letic and artistic ability, our focus is on general cognitive adulthood (17 years). Shared environmental influence ability (g), often referred to as intelligence (Jensen 1998). declined significantly from childhood (0.33) to adolescence The normal range of variation in g is the target of more (0.18) but no further significant decline emerged in young genetic research than any other behavioral trait other than adulthood (0.16). self-reported personality (Bouchard and McGue 1981; In the present paper, we investigated the etiology of high Deary et al.