Within- and between-family genetic effects on educational achievement vary across countries and ages

Polygenic score (PGS) predictions of educational achievement are sizeable at the population level. Yet, population-level PGS predictions are environmentally confounded, due to gene-environment correlations, assortative mating, and population stratification. This confounding complicates the interpretation and application of PGS predictions of educational achievement. Here, we charted the variability of PGS predictions in N = 8115 dizygotic twins from UK, US, Swedish, and German samples aged 7 to 19 years. Population-level PGS predictions of educational achievement ranged from β = 0.16 to β = 0.37 across ages and countries. Discerning within- and between-family level estimates, we found that 10 to 65% of the population-level PGS predictions were due to environmental confounding, of which 29 to 100% were accounted for by family socioeconomic status. Variability in within-family and population-level PGS predictions was largely unsystematic across countries' school systems (multi-tiered vs. comprehensive) and children's ages. Therefore, interpretations regarding the sources of environmental confounding effects on educational achievement remain, at present, speculative.