 GWAS identify SNPs that are statistically correlated with phenotypes of interest. After nearly a century of twin and family studies consistently demonstrating relatively high heritability of traits, there was some expectation that early GWAS would find a few genes of large effect. GWAS results in this respect have been disappointing. While twin studies indicate that roughly 50% of the total variance in cognitive ability is explained by genetic differences, individual SNPs associated with cognitive ability typically explain less than .04% total variance.