General intelligence drives variation in cognitive abilities and life outcomes like income and lifespan. Twin studies show high heritability of intelligence. GWAS on 3511 adults with 549k SNPs estimates 40% crystallized IQ variance from SNP LD with causal variants. GWAS estimates 51% fluid IQ variance from SNP LD with causal variants. These are lower bounds for narrow-sense heritability. Longer chromosomes account for more IQ variation. SNP data predicts 1% of IQ variance in independent sample. Results confirm genetics cause substantial individual IQ differences. Many small-effect genes underlie additive genetic influences on IQ.