GeneEnvironment Interaction in Adults’ IQ Scores

Authors: Sophie van der Sluis Gonneke Willemsen Eco J C de Geus Dorret I Boomsma Danielle Posthuma

Publish Date: 2008/06/06

Volume: 38, Issue: 4, Pages: 348-360

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Abstract

Geneenvironment interaction was studied in a sample of young mean age 26 years N = 385 and older mean age 49 years N = 370 adult males and females Full scale IQ scores FSIQ were analyzed using biometric models in which additive genetic A common environmental C and unique environmental E effects were allowed to depend on environmental measures Moderators under study were parental and partner educational level as well as urbanization level and mean real estate price of the participants’ residential area Mean effects were observed for parental education partner education and urbanization level On average FSIQ scores were roughly 5 points higher in participants with highly educated parents compared to participants whose parents were less well educated In older participants IQ scores were about 2 points higher when their partners were highly educated In younger males higher urbanization levels were associated with slightly higher FSIQ scores Our analyses also showed increased common environmental variation in older males whose parents were more highly educated and increased unique environmental effects in older males living in more affluent areas Contrary to studies in children however the variance attributable to additive genetic effects was stable across all levels of the moderators under study Most results were replicated for VIQ and PIQHeritability of cognitive ability is at present no longer in dispute many behavior genetics studies have shown that additive genetic influences A explain large parts of the observed variation in cognitive functioning in both children and adults eg Bouchard and McGue 1981 McCartney et al 1990 Bratko 1996 Devlin et al 1997 Rijsdijk et al 2002 Alarcón et al 1998 Posthuma et al 2000 and that these influences tend to increase with age while shared environmental influences C decrease eg Bartels et al 2002 McCartney et al 1990 McGue et al 1993 Plomin et al 1997 Boomsma et al 2002 Polderman et al 2006 Bergen et al 2007The broad range heritability estimates h2 reported in these studies vary roughly between 26 and 85 and concern samplebased estimates the assumption being that the heritability is equal for different subgroups and stable across environmental conditions Several studies however suggest that in children the heritability estimates for cognitive ability depend on characteristics of the childhood homeenvironment such as parental income Harden et al 2007 parental socioeconomic status SES Turkheimer et al 2003 parental educational level Rowe et al 1999 quality of parentchild communication and degree of chaos in the homeenvironment Asbury et al 2005 These interactions between measured environmental variables and unmeasured ie latent genetic influences often referred to as G × E are not always replicated eg Nagoshi and Johnson 2005 van den Oord and Rowe 1997 and neither is the ‘direction’ of the interaction consistent across studies Moreover most studies on G × E interaction in cognitive ability have been conducted in samples of children or adolescents age 17 or below while studies conducted in adult samples are sparse The aim of the current study is to investigate whether characteristics of the childhood homeenvironment influence heritability estimates of cognitive ability in adulthood In addition we investigate whether characteristics of the present adulthood living environment moderate the heritability of cognitive ability in adultsSome G × E results observed in children support the diathesisstress model Gottesman 1991 Paris 1999 The diathesisstress model is based on the assumption that sensitivity to environmental risk factors is larger in individuals who are at genetic risk for a disorder compared to those who are not at genetic risk eg Plomin and Rutter 1998 The diathesisstress model predicts genetic influences to be larger in less advantageous environmental circumstances Support for this model in the context of cognitive ability was reported by Asbury et al 2005 who studied variation in genetic influences on verbal and nonverbal ability as a function of 10 environmental variables in 4yearold samesex twins For verbal ability interactions involving measures of family chaos and instructive and informal parentchild communication proved significant with heritability being higher in less favourable circumstances Interactions with SES maternal depression harsh parenting and negative parental feelings however were not significant and no interactions were observed for nonverbal abilityThe alternative socalled bioecological model formulated by Bronfenbrenner and Ceci 1994 predicts that genetic potential will be actualized to a larger degree when socalled ‘proximal processes’ are strong Here proximal processes are defined as those processes that enhance effective developmental functioning When proximal processes are strong environmental differences in developmental outcome are reduced and the individual differences induced by genetic effects increase Bronfenbrenner and Ceci 1994 pp 572–574 Following this model heritability is largest in advantageous stable environments In line with this model Turkheimer et al 2003 reported increased heritability of WISCscores in 7yearold children from more affluent families More specifically in children from a high SESgroup with SES defined as a linear combination of parental income education and occupational status the proportion of variance attributable to genetic influences was larger and shared environmental effects smaller h2 = 072 c2 = 015 compared to a low SESgroup h2 = 010 c2 = 058 In this study the large common environmental effect c2 reported for the low SES group which included a high percentage of impoverished families is somewhat surprising as one would not expect much variation in familial circumstances in this group A similar albeit weaker interaction was reported by Harden et al 2007 for 17yearold children They observed higher heritability for general cognitive ability and smaller shared environmental effects in children from higher income families h2 = 055 c2 = 035 compared to children from families with lower income h2 = 039 c2 = 045 With respect to the moderation effect of parental educational attainment level Rowe et al 1999 reported higher heritability of vocabulary level in 16yearold children from more highly educated families h2 = 074 c2 = 000 than in children from less welleducated families h2 = 026 c2 = 023 This interaction with parental educational attainment was replicated by Kremen et al 2005 in a sample of adult males mean age about 40 years where heritability of word recognition ability was found to be higher in males from higher educated families h2 = 069 c2 = 000 compared to males from less welleducated families h2 = 021 c2 = 052Because many ‘environmental’ moderators may themselves be under genetic influence eg Plomin et al 2001 the possibility exists that the environmental moderator under study and the trait under study have additive genetic influences in common In that case the moderator is correlated to the genetic effects of the trait rGE rather than modifying the genetic effects of the trait G × E Purcell 2002 Since rGE can appear as G × E in statistical analyses one should either model rGE explicitly or correct for the presence of rGE by including moderator effects on the mean of the trait in the model Purcell 2002 Several of the studies which report significant G × E results explicitly accounted for the possible presence of rGE by including moderator effects on the mean of the trait Kremen et al 2005 Turkheimer et al 2003 Harden et al 2007 ensuring that the reported G × E effects are not artefacts of rGEAt present the study by Kremen et al 2005 is the only study on G × E interaction in adulthood cognitive ability In this adult male sample the higher heritability of the specific skill of ‘word recognition’ in subjects whose parents were highly educated was due to a decrease in shared environmental effects rather than an absolute increase in the effects of genetic factors Yet this study illustrates that variables like parental educational attainment level which characterizes the childhood homeenvironment may have longlasting effects on the variance decomposition of word recognition ability The exact nature of these longlasting effects is yet unknown but possibly parental educational attainment level influences the variance decomposition of word recognition ability in childhood which then remains stable over time Alternatively parental educational attainment level could also be a proxy of one’s own adulthood educational attainment level as such indirectly moderating the variance decomposition of cognitive ability in adulthood Other mechanisms are still conceivable Irrespective of the exact nature of the moderating mechanism the present study set out to test whether similar moderation effects of parental educational attainment level can be detected for adulthood psychometric IQ scores as has been observed for childhood IQ

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