Family environment and the malleability of cognitive ability a Swedish national home-reared and adopted-away cosibling control study

Kenneth S. Kendler, EricTurkheimer, Henrik Ohlsson, Jan Sundquist, Kristina Sundquist

Research output: Contribution to journalArticlepeer-review

49 Scopus citations

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.

Original languageEnglish
Pages (from-to)4612-4617
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume112
Issue number15
DOIs
StatePublished - 14 Apr 2015
Externally publishedYes

Keywords

  • Adoption
  • Cognitive ability
  • Cosibling control
  • Environment
  • Rearing

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