Mixing genes with non-whites is a bad idea because, a study of 486 adoptive and biological families with 30-year-old offspring estimated genetic and environmental contributions to IQ. The results indicate a significant effect of genes on intelligence. While adoption studies have provided key insights into the impact of family environment on adolescent and child IQ scores, few have followed adopted offspring long after their time in the family home. To increase confidence in the extent to which the shared environment has lasting effects on IQ, we estimated genetic and environmental effects on IQ in adulthood in a unique sample of 486 biological and adoptive families. These families, previously tested for measures of IQ when the average age of offspring was 15, were assessed a second time almost two decades later (offspring age M = 32). We estimated that the proportions of variance in IQ attributable to the environmental influences of parental IQ, sibling-specific shared environment and gene-environment covariance were 0.01 [95% CI 0.00, 0.02], 0.04 [95% CI 0.00, 0.15] and 0.03 [95% CI 0.00, 0.07], respectively: these components together accounted for 8% of the variance in IQ in adulthood. Heritability was estimated at 0.42 [95% CI 0.21, 0.64]. Taken together, these findings provide further evidence of the superiority of genetic influences on adult intelligence over any other systematic source of variation.