“Genetic and Environmental Contributions to IQ in Adoptive and Biological Families With 30-Year-Old Offspring”, Emily A. Willoughby, Matt McGue, William Iacono, James J. Lee2021-09-01 (, ; similar)⁠:

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 an 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 2 decades later (Moffspring 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: intelligence, adoption, heritability, vocabulary, polygenic scores] [cf.: Wilson effect/fadeout.]

…The question of persistence is perhaps the most important in considering the effects of the rearing environment on IQ, especially since other types of studies have documented a “fadeout” of environmental improvements over time (eg. Protzko2015). Kendler and colleagues have used a cosibling control design to examine the effect of the rearing environment on IQ in a sample of 436 adoptive-biological sibships (Kendler et al 2015). These male, 18–20 year old adopted Swedish conscripts showed a mean gain in 4.41 IQ points relative to their biological siblings, who were raised by the original biological family. This finding, which they replicated in a larger sample of half-sibs (with a mean gain of 3.18 IQ points associated with adoption), is a strong indicator that IQ can be, to some extent, affected up to late adolescence by the family environment. These results are consistent with those from the classic cross-fostering study of 14-year-old French children by Capron & Duyme1989.

Studies such as these do suggest that although this effect is small relative to the genetic effects on IQ, it is not zero; however, the size of this effect diminishes substantially after adolescence. Sandra Scarr, a pioneer of modern IQ adoption studies, was perhaps the first to note this fadeout phenomenon (eg. Scarr & Weinberg1978). With respect to the tapering correlations in IQ between children and their adoptive parents as the child matured, observed in the Minnesota Adolescent Adoption Study, Scarr & Weinberg remarked:

“We interpret the results to mean that younger children, regardless of their genetic relatedness, resemble each other intellectually because they share a similar rearing environment. Older adolescents, on the other hand, resemble one another only if they share genes. Our interpretation is that older children escape the influences of the family and are freer to select their own environments. Parental influences are diluted by the more varied mix of adolescent experiences.” (Scarr & Weinberg1983, p. 264).

Figure 2: Scatter plots and associated regression lines for measures of cognitive ability g taken at intake and follow-up 3 for both biological (left panel) and adopted (right panel) offspring and their rearing parents. Intake measure of g is full-scale Wechsler IQ score, and follow-up 3 measure is ICAR-16 score. All parent-offspring pairs are included, which means that the data points are not independent. All values are standardized.
Table 3: Decomposition of variance [95% CI] for each measure and subtest of cognitive ability. Note: 95% CIs are computed from each parameter’s 200 bootstrap iterations (Efron & Tibshirani1993) for each scale. Non-shared environment is computed by subtracting the heritability, parental environment, sibling environment, and gene-environment (G-E) covariance from 1. For full parameter estimates, see SI Table S12. Column values add up to 1, total phenotypic variance.

…We tested for placement effects using polygenic scores for educational attainment (PGSEA) derived from the largest genome-wide association study to date (Lee et al 2018)…In an aggregated sample consisting of all white participants (offspring and both parents), an R2 of 0.154 [95% CI 0.113, 0.195] in our prediction of Verbal IQ with a PGS surpasses all previous benchmarks known to us (Allegrini et al 2019; Lee et al 2018; Rietveld et al 2014; Savage et al 2018; Selzam et al 2016; Sniekers et al 2017), and an R2 of 0.114 [95% CI 0.077, 0.151] for Total IQ is near the upper end of previous predictions (SI Table S13). However, we acknowledge that our sample is not large by the standards of PGS validation…These scores also enable an unique test for the so-called “placement effect”, wherein adoptees (typically twins reared apart) are thought by some skeptics to resemble their adoptive parents prior to placement, thus biasing biometrical estimates. By demonstrating a total lack of evidence (p = 0.514) for a correlation between parents and adoptive offspring in polygenic scores, we provide support for the validity of at least some adoption studies in establishing causal inference…the similarity of these correlations to their theoretically predicted values provides evidence that the placement of adoptees in their homes was not strongly purposive or selective, implying that the adoption process may somewhat approximate a true experiment.

…IQ has been subject to a large number of twin and adoption studies, many of which have found a small but statistically-significant effect of parental transmission in adoptive samples up until late adolescence…Our biometric decomposition of variance is consistent with this figure: the parental environment contributing 4% of the variance in fullscale IQs at age ~15 (Table 3), with a standard deviation of 14.2 for full-scale IQ in adopted offspring, indicates that a 1-SD increase in the quality of the parental environment would increase IQ by ~2.83 points (ie. 14.2 × √0.04; Burks1938).

The evidence for parenting effects on Wechsler IQ subtests is more equivocal, and biometric decomposition reveals a moderate but statistically-significant effect of gene-environment covariance on Vocabulary in childhood. While a similarly-sized G-E covariance is observed for childhood Total IQ, this effect has completely disappeared in adulthood; the same cannot be said unambiguously for Vocabulary, which retains weak evidence in adulthood for a persistent parenting effect.

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