Partner selection is a vital feature of human behavior with important consequences for individuals, families, and society. Hypergamy occurs when a husband's earning capacity systematically exceeds that of his wife. We provide a theoretical framework that rationalizes hypergamy even in the absence of gender differences in the distribution of earnings capacity. Using parental earnings rank, a predetermined measure of earnings capacity that solves the simultaneity problem of matching affecting earnings outcomes, we show that hypergamy is an important feature of Norwegian mating patterns. A vignette experiment identifies gender differences in preferences that can explain the observed patterns.