General cognitive ability (GCA) is a highly heritable trait, with correlates in numerous other domains. This dissertation reports the results of three studies of the genetics of GCA, conducted with participants from the Minnesota Center for Twin & Family Research (MCTFR). Study #1 (N = 7,100) is a genome-wide association study plus other analyses that exploit genome-wide single-nucleotide polymorphism (SNP) data. Study #2 (N = 6,439) is an association study of a different class of genetic polymorphism, the copy-number variant (CNV). In this study, we detect CNVs from genome-wide SNP-allele-probe intensity data. We aggregate them into genome-wide mutational burden scores and also carry out genome-wide association scans for specific CNVs. Study #3 is a biometric moderation study in a sample of 2,494 pairs of twins, full siblings, and adoptive siblings. We compared models by their sample-size-corrected AIC, and based our parametric inference on model-averaged point estimates and standard errors. Taken as a whole, these three studies demonstrate that GCA is substantially heritable and massively polygenic, but it is also influenced by environmental factors, and its heritability can be moderated by contextual variables like age and family-of-origin socioeconomic status (SES).