During the first three years of life, children acquire knowledge about their own gender and the gendered nature of their environment. At the same time, sex-related behavioral differences emerge. How are we to understand the processes by which bodily differentiation, behavioral differentiation and gendered knowledge intertwine to produce male and female, masculine and feminine? In this talk I will describe four central developmental systems concepts that are applied to the study of early human development. The general theoretical approach to understanding the emergence bodily/behavioral difference has broad applicability for the health sciences and for the study of gender disparities.