This paper studies how college major choices shape earnings and fertility outcomes. Using administrative data that link students' preferences, random assignment to majors, and post-college outcomes, we estimate the causal pecuniary and non-pecuniary returns to different fields of study. We document substantial heterogeneity in these returns across majors and show that such variation helps explain gender gaps in labor market outcomes: women place greater weight on balancing career and family in their major choices, and these preference differences account for about 30% of the gender earnings gap among college graduates. Last, we use our causal estimates to evaluate the effects of counterfactual assignment rules that target representation gaps in settings with centralized assignment systems. We find that gender quotas in high-return fields can significantly reduce representation and earnings gaps with minimal impacts on efficiency and aggregate fertility.