Genetic and Environmental Contributions to IQ in Adoptive and Biological Families with 30-Year-Old Offspring

DOI:https://doi.org/10.1016/j.intell.2021.101579

Commentary

Studies to date have provided important insights into the effects of family environment on IQ scores in adolescents and children, but no study has followed adopted offspring far beyond the time they lived in the family home.

To increase confidence in the extent to which this shared environment has a lasting impact on IQ, the study was designed to estimate genetic and environmental influences on IQ in adulthood in a unique sample of 486 biological and adoptive families.

Participants' families were previously tested on measures of IQ when the average age of the offspring was 15 years old and were assessed a second time 20 years later (age of M offspring = 32 years).

Based on these results, the proportion of variance in IQ attributable to environmental mediation effects of parental IQ, sibling-specific shared environment, and genetic environment covariance was estimated to be 0.01 [95% CI 0.00, 0.02], 0.04 [95% CI 0.00, 0.15

These factors jointly accounted for 8% of the IQ variance in adulthood and were estimated to have a heritability of 0.42 [95% CI 0.21, 0.64].

These findings provide further evidence for the predominance of genetic influences on adult intelligence over other sources of systematic variation.