Durable Lessons on Intelligence from Twin and Adoption Research

Matt McGue's research has made intelligence a model case for the explanatory reach of behavioral genetics. His early studies, conducted with Thomas Bouchard and other collaborators, on twins and other kinships demonstrated that cognitive ability is substantially heritable, and later analyses showed that this influence increases from childhood to adulthood and remains appreciable in late life, while shared environmental effects diminish. McGue and colleagues also extended twin and adoption methods to the study of educational attainment and social mobility, demonstrating that cognitive ability, noncognitive traits, and family background all contribute, and that higher education seems to benefit individuals across the ability spectrum. Because intelligence is uniquely scrutinized, the credibility of these findings depends on the soundness of the adoption design. A persistent critique is that adoption agencies engage in various forms of selection, leading to restriction of range in environmental quality and the confounding of genetic and environmental influences. McGue and colleagues addressed some of these concerns in their Minnesota studies, most notably showing that a correction for restriction of range did not appreciably alter the correlation between adoptive siblings in IQ. The present paper reviews McGue's accomplishments and reports new results about selective placement based on the correlations between parental phenotypes and the polygenic scores of their adopted offspring.