<p>Recent work incorporating gender into sex differences research has complicated the interpretation of both physiological and behavioral differences between women and men, which are traditionally interpreted as driven by “biological” sex-related variables. Alternative conceptualizations have been proposed to capture the complex relationship between sex and gender, such as sex/gender entanglement. Blurring the boundaries between gender and sex, the organism and its environments, this perspective has created a need for theoretical and methodological approaches that can be used to investigate the emergence of structures and behaviors in complex systems. Here, we review the foundational concepts of developmental and dynamic systems theories, which can serve as frameworks within which to investigate the emergence of complex phenomena. We then provide examples of empirical research programs that demonstrate the benefit of these frameworks in the context of sex/gender research.</p>