Recent work in quantitative biology has shown theoretically why Fisher’s Fundamental Theorem of Natural Selection does not preclude genetic influences on fertility, sexuality, and related processes. This work takes the next step, and presents a number of successful empirical searches for such genetic influence on a broad range of processes, such as puberty, marriage, sexual behaviour, and twinning. Employing a broad range of methodological approaches, including molecular and behavioral genetics, this text weaves a new theoretical framework that shows how genes can help relate fertility planning to fertility outcome, and how puberty, sexuality, marriage, and reproduction can be conceptually linked through the genes that contribute to individual differences in the human process.