We evaluated STEM (science, technology, engineering, and mathematics) coursework selection by women and men (representative longitudinal sample, 10,370 Australians) in senior high school and university, controlling achievement and expectancy-value variables. A near-zero total effect of gender on high school STEM enrollment reflected pathways favoring boys through achievement and expectancy-value variables, but a counteracting direct effect of gender favoring girls. In contrast, subsequent university STEM enrollment favored boys. In both high school and university, enrollments favored girls in life sciences and boys in physical sciences, but at university there was a leaky pipeline in which girls who qualified to pursue physical sciences opted for non-STEM subjects. Qualitative analysis not only supported quantitative results but also highlighted alternative mechanisms of STEM engagement/disengagement, and mostly supported gender similarities rather than differences.