In 2009, Daphna Joel, a neuroscientist at Tel Aviv University, decided to teach a course on the psychology of gender. As a feminist, she had long been interested in questions of sex and gender, but as a scientist her research had been mostly on the neural underpinnings of obsessive-compulsive behavior. To prepare for the class, Joel spent a year reviewing much of the extensive and polarized literature on sex differences in the brain. The hundreds of papers covered everything from variations in the size of specific anatomical structures in rats to the possible roots of male aggression and female empathy in humans. At the outset, Joel shared a popularly held assumption: Just as sex differences nearly always produce two different reproductive systems, they would also produce two different forms of brains—one female, the other male.