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We report successful diachronic replication of two major sets of prior findings in the social biogeography of human life history (LH) strategy: (1) the constructive replication of the diachronic changes in the latent hierarchical structure of intelligence in Britannic populations, but as presently applied to the latent hierarchical structure of human LH strategy, now cross-validated in both Britannic and Gallic populations; and (2) the diachronic replication in both Britannic and Gallic populations of the structural relations found synchronically among human LH strategy, between-group competition, and economic productivity in cross-sectional data on contemporary samples of both national and subnational polities. In addition, a supplementary methodological objective was: (3) the convergent validation of diachronic lexicographic measures of LH strategy with respect to more traditional non-lexicographic indicators of LH strategy, such as infant mortality rates, total fertility rates, and life expectancies. We obtained complete configural invariance across Britannic and Gallic biocultural groups, meaning that the same model predictors were statistically significant, but incomplete metric invariance, meaning that most but not all model parameter estimates were statistically equivalent in magnitude and direction. All new results obtained from diachronic data in Britannic populations were replicated almost perfectly in Gallic populations.

The Scarr–Rowe effect is a gene × environment interaction, which is characterized by a negative association between exposure to low socioeconomic status (SES) environments and the additive heritability of cognitive ability. Utilizing a polygenic score for educational attainment (EA3), it was found that the two-way interaction between EA3 and parental educational attainment (EA; used as a proxy for parental SES) was a significant positive predictor of participants’ composite cognitive ability (IQ) score (β = .018, SE = .008, p = .028) after controlling hierarchically for the direct effects of (population-stratification-controlled) EA3, parental EA, and 20 distinct interaction terms (10 involving the interactions between the principal components [PCs] and EA3, and 10 involving the interaction between the PCs and parental EA). The presence of this interaction is consistent with the Scarr–Rowe effect, as the expressivity of EA3 on cognitive ability increases with increasing parental EA. No statistically significant sex differences in the effect magnitudes were found, although the effect was significantly present in the female but not male sample.

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