Single young women in their sexual prime—that is, their 20s and early 30s—are for the first time in history more successful, on average, than the single young men around them. They are more likely to have a college degree and, in aggregate, they make more money. What makes this remarkable development possible is not just the pill or legal abortion but the whole new landscape of sexual freedom—the ability to delay marriage and have temporary relationships that don’t derail education or career. To put it crudely, feminist progress right now largely depends on the existence of the hookup culture. And to a surprising degree, it is women—not men—who are perpetuating the culture, especially in school, cannily manipulating it to make space for their success, always keeping their own ends in mind. For college girls these days, an overly serious suitor fills the same role an accidental pregnancy did in the 19th century: a danger to be avoided at all costs, lest it get in the way of a promising future.