Despite the massive influx of women into the workforce, women have made only minor gains into top management positions. Most explanations for this asymmetry have been based on sex differences in socialization and traits. We propose that an evolutionary psychological perspective offers an alternative explanation: sex differences in power are due to differences in the way men and women use influence behaviors in small groups, and these differences were sculpted, in part, by natural selection. This produced sex differences in psychological and physiological mechanisms - principally in the neuroendocrine system - that influence motivations to use influence in groups. We review studies on sex differences in influence in small groups. For each type of influence behavior that we examine - competition, dominance, and coalition formation - we discuss ultimate and proximate causes. We conclude with implications for future research and for public and organizational policy.