Darwin was the first to offer an evolutionary explanation for widespread sex differences in mating strategies across animal taxa. However, initial applications of evolutionary approaches to the study of human behavior were met with resistance. In response, social scientists began systematically applying rigorous evaluations of evolutionarily informed hypotheses by way of testable predictions. Mate choice research was a key area for this early work, focused on demonstrating sex differences in mating behavior. Subsequent research has tempered some earlier conclusions, increasingly relating mating strategies to aspects of socioecology. In particular, the adult sex ratio has emerged as an essential demographic variable contributing to behavioral variability within and between the sexes, leading to frequency-dependent rethinking for the patterning of mating competition and parental investment. Empirical examination of the association between adult sex ratio and aggression, family formation, partnership stability, and risk-taking is a growing field of research cross-cutting various literatures. In this chapter, we first describe early theory motivating research on sex differentiated behavior, followed by recent frequencydependent reexaminations of sexual selection. Second, we introduce the adult sex ratio as a central determinant of fitness payoffs to differing reproductive strategies. Third, we review empirical studies on the consequences of sex ratio variation, highlighting outcomes related to sociosexuality, family formation, and violence. Fourth, we detail the maturation of the field, and where theoretical and empirical work has unveiled complexities regarding reproductive strategies. Finally, we reflect on future directions, where theoretical refinement is under way or needed, and highlight promising and novel research approaches.