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Couples planning to adopt often want to know how their own IQ and education will affect the child. These investigators addressed the issue with a genetically informed study design. From national Swedish registries, they identified pairs of boys in which the biological parents reared one boy, and adoptive parents raised the co-sibling. Median age at adoption was 6 months. Participants were 436 pairs of full siblings and 2341 pairs of half-siblings. IQ was measured as part of military conscription at ages 18 to 20.
Adopted-away siblings and half-siblings had IQs 4.4 and 3.2 points, respectively, higher than their biologically reared co-siblings. Analyses also examined the effect of parents' education, measured on a 5-point scale. At the extrem…