Research has shown that men tend to emerge as leaders more frequently than women. To understand why this gender difference in leader emergence occurs, we draw on social role theory and leader categorization theory to develop a theoretical model that unifies trait and behavioral mechanisms while also clearing up theoretical ambiguity regarding communal traits (i.e., whether communion tends to have positive versus negative effects on leader emergence). Specifically, we examine a sequence of effects: from gender to agentic and communal personality traits, from these traits to behavioral participation in group activities, and ultimately from participatory behavior to leader emergence. To test our model, we updated Eagly and Karau’s (1991) estimate of the gender difference in emergent leadership, using nearly twice as many studies (δ = .23; k = 131; N = 18,130; 95% CI = [.12, .29]); and also conducted original meta-analyses of the personality and behavioral mechanisms (coding 1,575 effect sizes in total). We conclude that gender differences in leadership emergence are predominately explained by agentic traits: both directly and through the mechanism of participation in group discussions. Interestingly, communal traits had a null relationship with participatory behavior and a negative direct effect on leadership emergence. Our model contributes to both theory and practice by creating awareness of the processes responsible for the gender gap in leader emergence.