Requiem for Heterosis as a Cause of the Flynn Effect: Positive Combined Effects of Numbers and Lengths of Homozygosity Runs on Offspring-Parent Differences in Educational Attainment

It has been argued that reduced inbreeding depression and associated increased heterosis (hybrid vigor), due to greater gene flow between human subpopulations, is the cause of the Flynn effect (rising IQ-test performance over time). Using genotypic data from the Wisconsin Longitudinal Study, three estimates of genome-wide runs of homozygosity (ROH) are generated for a subsample of unrelated individuals of European descent. These estimates are used in a between-generation regression model to predict offspring advantage over parents in educational attainment (EA; years of schooling). After controlling for a variety of covariates, it is found that a variable combining both the numbers and lengths of ROH is a statistically significant positive predictor of the offspring EA advantage. Maternal, rather than paternal, differences are found to drive the effect when these parental influences are examined separately. Since the heterosis hypothesis (HH) explicitly predicts that this analysis would yield the opposite finding, the result constitutes substantial evidence against the HH. By contrast, the life history model of the Flynn effect (LHM) satisfactorily explains the current findings, positing that slowing life history speed increases maternal investment (MI) into offspring exhibiting greater coefficients of genetic relatedness as a means of raising inclusive fitness. According to the LHM, the Flynn effect stems, at least in part, from MI enhancing opportunities for the cultivation of narrow cognitive abilities (e.g., through greater exposure to highly predictable environments, such as in good schools). The significant independent effect of assortative mating observed here is also consistent with the LHM, as are other patterns found in these data.