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Edited by Richard E. Nisbett, University of Michigan, Ann Arbor, MI, and approved February 4, 2015 (received for review September 4, 2014)
Significance
Individual differences in cognitive ability result from a complex admixture of genetic and environmental influences. Adopted children are one way to estimate the degree of malleability of cognitive ability in response to environmental change in the context of a scientific design that can control for genetic differences among individuals. Sibling pairs in which one member is adopted away and the other reared by biological parents are a particularly powerful research design. In a large population-based sample of separated siblings from Sweden, we demonstrate that adoption into improved socioeconomic circumstances is associated with a significant advantage in IQ at age 18. We replicate the finding in a parallel sample of half-siblings.
Abstract
Cognitive ability strongly aggregates in families, and prior twin and adoption studies have suggested that this is the result of both genetic and environmental factors. In this study, we used a powerful design—home-reared and adopted-away cosibling controls—to investigate the role of the rearing environment in cognitive ability. We identified, from a complete national Swedish sample of male–male siblings, 436 full-sibships in which at least one member was reared by one or more biological parents and the other by adoptive parents. IQ was measured at age 18–20 as part of the Swedish military service conscription examination. Parental educational level was rated on a 5-point scale. Controlling for clustering of offspring within biological families, the adopted siblings had an IQ 4.41 (SE = 0.75) points higher than their nonadopted siblings. Each additional unit of rearing parental education was associated with 1.71 (SE = 0.44) units of IQ. We replicated these results in 2,341 male–male half-sibships, in which, controlling for clustering within families, adoption was associated with a gain of IQ of 3.18 (SE = 0.34) points. Each additional unit of rearing parental education was associated with 1.94 (SE = 0.18) IQ units. Using full- and half-sibling sets matched for genetic background, we found replicated evidence that (i) rearing environment affects IQ measured in late adolescence, and (ii) a portion of the IQ of adopted siblings could be explained by the educational level of their adoptive parents.
Footnotes
↵1K.S.K. and E.T. contributed equally to this work.
- ↵2To whom correspondence may be addressed. Email: ent3c{at}virginia.edu or kendler{at}vcu.edu.
Author contributions: K.S.K., E.T., H.O., J.S., and K.S. designed research; K.S.K., E.T., J.S., and K.S. performed research; E.T. contributed new reagents/analytic tools; E.T. and H.O. analyzed data; and K.S.K., E.T., H.O., J.S., and K.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Family environment and cognitive ability
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