A critical analysis of IQ studies of adopted children

The pattern of parent-child correlations in adoption studies has long been interpreted to suggest substantial additive genetic variance underlying variance in IQ. The studies have frequently been criticized on methodological grounds, but those criticisms have not reflected recent perspectives in genetics and developmental theory. Here we apply those perspectives to recent IQ adoption studies and show how they further question two sets of problems: first, the assumption of additive gene and environmental effects; second, the assumption that the adoption situation approximates a randomized-effects design. We show how a number of possible factors having systematic effects in breach of those assumptions can produce the received pattern of correlations without appealing to unusual amounts of additive gene variance