Adoptive & biological families with 30-year-old offspring: Proportion of variance in IQ attributable to environmentally mediated effects of parental IQs was estimated at .01; heritability was estimated to be 0.42

Genetic and environmental contributions to IQ in adoptive and biological families with 30-year-old offspring. Emily A. Willoughby, Matt McGue, William G. Iacono, James J. Lee. Intelligence, Volume 88, September–October 2021, 101579. https://doi.org/10.1016/j.intell.2021.101579

Highlights

• Genetic and environmental sources of variance in IQ were estimated from 486 adoptive and biological families

• Families include 419 mothers, 201 fathers, 415 adopted and 347 biological fully-adult offspring (M age = 31.8 years; SD = 2.7)

• Proportion of variance in IQ attributable to environmentally mediated effects of parental IQs was estimated at .01 [95% CI 0.00, 0.02]

• Heritability was estimated to be 0.42 [95% CI 0.21, 0.64]

• Parent-offspring correlations for educational attainment polygenic scores show no evidence of adoption placement effect

Abstract: While adoption studies have provided key insights into the influence of the familial environment on IQ scores of adolescents and children, few have followed adopted offspring long past the time spent living in the family home. To improve confidence about the extent to which shared environment exerts enduring effects on IQ, we estimated genetic and environmental effects on adulthood IQ in a unique sample of 486 biological and adoptive families. These families, tested previously on measures of IQ when offspring averaged age 15, were assessed a second time nearly two decades later (M offspring age = 32 years). We estimated the proportions of the variance in IQ attributable to environmentally mediated effects of parental IQs, sibling-specific shared environment, and gene-environment covariance to be 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 jointly accounted for 8% of the IQ variance in adulthood. The heritability was estimated to be 0.42 [95% CI 0.21, 0.64]. Together, these findings provide further evidence for the predominance of genetic influences on adult intelligence over any other systematic source of variation.

Keywords: IntelligenceAdoptionHeritabilityVocabularyPolygenic scores