Researchers at the University of Minnesota Twin Cities looked at adults raised by foster parents to see if their IQ is more comparable to their biological mothers or their foster parents. The research involved 486 families, including 620 parents, 415 adults who were adopted, and 347 biological offspring. The average age of the adults was 32, and each had undergone an IQ test when they were 15.
The researchers were able to take two IQ tests, conducted 17 years apart, and compare them to the IQs of foster and biological parents and siblings who were their foster parent’s biological offspring.
The research was conducted by Emily A. Willoughby, Matt McGue, William G. Iacono, and James J. Lee. All four are members of the University of Minnesota Department of Psychology. The findings were published on August 25th, 2021.
Their findings are very decisive. Even if they provide the best environment possible, Foster parents ultimately only have only a minor influence on the IQ of their foster children. “Parent-offspring correlations for educational attainment polygenic scores show no evidence of adoption placement effect.”
From the abstract:
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.