Although women are underrepresented in the most mathematically intensive fields, the gender gap in these fields has narrowed over the past 2 decades. In my E. L. Thorndike address I summarized the temporal trends in sex differences for 8 fields and considered factors that drive both the underrepresentation of women and its recent narrowing. I reviewed evidence concerning sex differences in mathematical and spatial aptitude, biases in hiring, funding, publishing, remuneration, and promotion, and gendered preferences. I conclude that the most important causes of underrepresentation appear to occur before women matriculate in college and are concerned with ability-related beliefs, stereotypes, and preferences starting in early elementary school, which by the end of high school have reduced the size of the potential pool. By the time women reach graduate school, there is evidence that they are as successful as their male counterparts in being interviewed and hired for tenure-track positions, funded, and published.