Implications of the restricted range of family environments for estimates of heritability and nonshared environment in behavior-genetic adoption studies.

Group and individual-difference adoption designs lead to opposite conclusions concerning the importance of shared environment (SE) for the child outcomes of IQ and antisocial behavior. This paradox could be due to the range restriction (RR) of family environments (FE) that goes with adoption studies. Measures of FE from 2 of the most recent adoption studies indicate that RR is substantial, about 67%, which corresponds to the top half of a normal FE distribution. RR of 67% cuts effect sizes and R2 statistics by factors of 3 and 2-2.5, respectively. Because selection into an adoption study in inherently a between-family process and assuming that comparable restriction of genetic (G) influences are absent, estimates of SE, G, and nonshared influences will be substantially biased, respectively, down, up, and up by RR. Corrections for RR applied to adoption studies indicate that SE could account for as much as 50% of the variance in IQ.