Abstract
Although research findings have revealed that both genetic and environmental factors contribute to variation in IQ at the individual level, no studies have examined how genetic differences contribute to differences in IQ within and between families. The current study uses data drawn from the National Longitudinal Study of Adolescent Health (Add Health) to examine the potential role genetic factors play in shaping differences in IQ scores within and between families. The results revealed that while genetic variation between family members did not significantly predict differences in IQ scores, family-level dopaminergic risk scores did significantly predict differences in family-level IQ. The implications of the results are elaborated upon, and suggestions for future research are discussed.
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Notes
Due to the identification of outliers, only families with a standardized IQ score greater than 60 on all IQ measures (wave I, wave III, and the composite measures) were included in the final analytic sample. To check the robustness of the findings with respect to outliers, the results were reanalyzed with all scores included in the analyses. The pattern of results was virtually identical to those reported with the outliers removed.
Only two siblings per household were examined in the current study. For households with more than one sibling, two siblings were randomly chosen. Additionally, since MZ twins are genetically identical, they were excluded from the within-family analysis, but were included in the between-family analysis.
All figures presented in the current study present genetic risk in 0.5 increments even though the distribution of the genes in individuals does not naturally occur in 0.5 increments. Rather, the dopamine risk scores presented in the figures are averages of each family members’ individual scores, making it possible for families to have a dopaminergic risk score that falls between two whole numbers.
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Acknowledgments
This research uses data from Add Health, a program project directed by Kathleen Mullan Harris and designed by J. Richard Udry, Peter S. Bearman, and Kathleen Mullan Harris at the University of North Carolina at Chapel Hill, and funded by grant P01-HD31921 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, with cooperative funding from 23 other federal agencies and foundations. Special acknowledgment is due to Ronald R. Rindfuss and Barbara Entwisle for assistance in the original design. Information on how to obtain the Add Health data files is available on the Add Health website (http://www.cpc.unc.edu/addhealth). No direct support was received from grant P01-HD31921 for this analysis.
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Schwartz, J.A., Beaver, K.M. Examining the effects of dopamine genes on verbal IQ within and between families. J Neural Transm 120, 477–486 (2013). https://doi.org/10.1007/s00702-012-0881-0
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DOI: https://doi.org/10.1007/s00702-012-0881-0