A recent genome-wide-association study of educational attainment identified three single-nucleotide polymorphisms (SNPs) whose associations, despite their small effect sizes (each R² ≈ 0.02%), reached genome-wide significance ( p < 5 × 10⁻⁸) in a large discovery sample and were replicated in an independent sample ( p < .05). The study also reported associations between educational attainment and indices of SNPs called “polygenic scores.” In three studies, we evaluated the robustness of these findings. Study 1 showed that the associations with all three SNPs were replicated in another large ( N = 34,428) independent sample. We also found that the scores remained predictive ( R² ≈ 2%) in regressions with stringent controls for stratification (Study 2) and in new within-family analyses (Study 3). Our results show that large and therefore well-powered genome-wide-association studies can identify replicable genetic associations with behavioral traits. The small effect sizes of individual SNPs are likely to be a major contributing factor explaining the striking contrast between our results and the disappointing replication record of most candidate-gene studies.