Adoption and Cognitive Development: A Meta-Analytic Comparison of Adopted and Nonadopted Children's IQ and School Performance.

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Abstract
This meta-analysis of 62 studies (N=17,767 adopted children) examined whether the cognitive development of adopted children differed from that of (a) children who remained in institutional care or in the birth family and (b) their current (environmental) nonadopted siblings or peers. Adopted children scored higher on IQ tests than their nonadopted siblings or peers who stayed behind, and their school performance was better. Adopted children did not differ from their nonadopted environmental peers or siblings in IQ, but their school performance and language abilities lagged behind, and more adopted children developed learning problems. Taken together, the meta-analyses document the positive impact of adoption on the children's cognitive development and their remarkably normal cognitive competence but delayed school performance.