As evidence that social factors are less important than biological ones, Summers cited the fact that increasingly that it's not until people are in their 20s that the gap between men and women opens. If socialization were the cause, he argued, its effects would have to be seen at a much earlier age. But the early mid-20s is precisely when young women are actively grappling with what it means to be an adult, and when long-held notions about femininity, family life, and sexuality have the most material impact on decision-making. You can be a math major and still consider yourself a typical girl—though it may be hard to—but it's considerably more difficult to reconcile being an engineer with our still-prevailing assumptions about gender, family, and marriage.