Why is there so much occupational sex segregation in the 21st century? The authors cast light on this question by using the O*NET archive of occupation traits to operationalize the concepts of essentialism and vertical inequality more exhaustively than in past research. When the new model is applied to recent U.S. Census data, the results show that much vertical segregation remains even after the physical, analytic, and interactional forms of essentialism are controlled; that essentialism nonetheless accounts for much more of total segregation than does vertical inequality; that the physical and interactional forms of segregation are especially strong; that the physical form of essentialism is one of the few examples of female-advantaging segregation; and that essentialism takes on a fractal structure that generates much finely detailed segregation at detailed occupational levels. The authors conclude by discussing how essentialist processes partly account for the intransigence of occupational sex segregation.