The present study of general cognitive ability attempts to replicate and extend previous investigations of a biometric moderator, family-of-origin socioeconomic status (SES), in a sample of 2,494 pairs of adolescent twins, non-twin biological siblings, and adoptive siblings assessed with individually administered IQ tests. We hypothesized that SES would covary positively with additive-genetic variance and negatively with shared-environmental variance. Important potential confounds unaddressed in some past studies, such as twin-specific effects, assortative mating, and differential heritability by trait level, were found to be negligible. In our main analysis, we compared models by their sample-size corrected AIC, and base our statistical inference on model-averaged point estimates and standard errors. Additive-genetic variance increased with SES—an effect that was statistically significant and robust to model specification. We found no evidence that SES moderated shared-environmental influence. We attempt to explain the inconsistent replication record of these effects, and provide suggestions for future research.
Gene–environment interaction SES Multimodel inference General cognitive ability IQ Twin study Adoption study
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This research was supported in part by USPHS Grants from the National Institute on Alcohol Abuse and Alcoholism (AA09367 and AA11886), the National Institute on Drug Abuse (DA05147, DA13240, and DA024417), and the National Institute on Mental Health (MH066140). The first author (RMK) was supported by a Doctoral Dissertation Fellowship from the University of Minnesota Graduate School and by grant DA026119 from the National Institute on Drug Abuse. The authors acknowledge the assistance of Niels G. Waller and Saonli Basu, who provided helpful comments on an early draft of this paper. The first author gives his special thanks to Scott I. Vrieze and Joshua D. Isen for thought-provoking discussion of model-selection and of the main effects of SES, respectively.
Conflict of interest
Robert M. Kirkpatrick, Matt McGue, and William G. Iacono declare that they have no conflict of interest.
Human and Animal Rights and Informed Consent
The MTFS and SIBS studies were reviewed and approved by the Institutional Review Board at the University of Minnesota. Written informed assent or consent was obtained from all participants, with parents providing written consent for their minor children.
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