While most people agree that sex differences in human behavior exist, they disagree about the reasons. But the science is clear: testosterone is a potent force in human society, driving the bodies and behavior of the sexes apart. As Carole Hooven shows in her book T: The Story of Testosterone, the Hormone that Dominates and Divides Us (Henry Holt & Company, 2021), it does so in concert with genes and culture to produce a vast variety of male and female behavior. And, crucially, the fact that many sex differences are grounded in biology provides no support for restrictive gender norms or patriarchal values. In conversation with Harvard psychologist Daniel Gilbert, Hooven will discuss why an understanding of testosterone can lead to a better understanding of ourselves and one another—and can help build a fairer, safer society.