This chapter focuses on why sex segregation has been so resistant to change. The future of segregation depends on precisely how extensive such essentialism is. The sources of vertical segregation are, like the sources of essentialist segregation, rooted in both supply-side and demand-side processes. The two-step strategy behind our analysis entails scaling detailed occupations in terms of three forms of essentialism and two vertical dimensions, and then modeling cross-occupational differences in the extent of segregation as a function of essentialism and vertical segregation. The effects of interactional essentialism, especially sociability, are also quite strong, a result that again has not been properly incorporated into prior quantitative segregation models. If contemporary segregation were fully essentialist in form, the tendency of men to secure more desirable occupations would arise entirely because such occupations were ones requiring skills that men are understood as holding.