Children adopted into a good familial environment from a poorer one have typically shown IQ gains in childhood relative to their biological parents, although these gains are not always present into adulthood. These IQ gains in childhood are to be expected considering that the adopted children are typically raised in a more intellectually enriching environment than their parents were as a result of the adoption. This is presumably because of longer and better education, better nutrition, a more intellectually stimulating environment, and less exposure to neurotoxins. In regard to the nature of these gains in intelligence researchers te Nijenhuis, Jongeneel-Grimen, Armstrong (2014) looks at the correlation between adoption gains in IQ and their g loading.
All tests of mental ability correlate to each other to some extent. This common variance between mental tests is referred to a g, or ‘general intelligence’ (Jensen, 1998, p. 29). The extent to which an individual test predicts this common variance between mental tests is referred to as that test’s g loading. So a test with good g loading will better predict the results of other tests than one with a poor g loading. Additionally, when a test is more g-loaded its scores are more negatively affected by inbreeding depression, it shows greater disparities between ethnic groups, and it is more related to brain size and reaction time. Likewise, on tests that are less g-loaded the effect of test practice and the Flynn Effect (Herrnstein & Murray, 1994, p. 307) are larger. So it appears that g-loading is more strongly related to biological factors.
To determine the correlation between adoption gains and g loading te Nijenhuis et al. (2014) preformed a meta-analysis of studies that measure adoption gains in IQ. To identify studies for inclusion researchers searched for published and unpublished studies, browsed content tables of major research journals, contacted several well-known researchers who’ve conducted research on adoption gains in IQ to obtain additional articles and information, and finally, checked the reference list of already included studies for further studies that may have been missed. Of the studies found, four met the criteria of the meta-analysis which were that the studies had to be well-validated, include reliable estimates of the true correlation between adoption gains and g loading, and have a minimum of seven subtests. After computing the adoption gains and subtest g loadings the researchers corrected for various statistical artifacts.
After appropriate statistical corrections the researchers estimate a true correlation of -1.00; a perfect inverse correlation between adoption gains within an IQ subtest and that subtest’s g loading. Because adoption gains in IQ are due to environmental changes the results of this study support the insights of other research that environmental differences in IQ are not g-loaded while genetic ones are. Therefore, the g loading of IQ differences seems to be a good indicator of whether or not that difference is due to genetic or environmental differences. Further research on this topic should focus on collecting a broader and more diverse sample of adoption gains in IQ and subtest g loading as only four studies were available to be included in the present meta-analysis and all used the same IQ test, the Wechsler Intelligence Scale for Children (te Nijenhuis et al., 2014).
Herrnstein, R., & Murray, C. (1994). The bell curve: Intelligence and class structure in American life (p. 307). New York, NY: Free Press.
Jensen, A. (1998). The g factor: The science of mental ability (p. 29). Westport, CT: Praeger Publishers.
te Nijenhuis, J., Jongeneel-Grimen, B., & Armstrong, E. L. (2014). Are adoption gains on the g factor? A meta-analysis. Personality and Individual Differences 73(2015) 56–60.