I first heard this wisecrack as a graduate student in psychology, and it instantly rang true. In everyday life, most people recognize that the sexes differ. We see it at school; we see it at work; we see it in our kids and in ourselves. To start with, we know that men and women have different bodies and reproductive equipment, that men are generally larger and stronger, and that women generally live longer. But we also know that the differences are not just physical. We know that men watch more sports and more porn, whereas women watch more rom-coms and read more. We know that men are more inclined toward violence and more likely to end up in prison, whereas women are more likely to take sensible precautions. We know that men are more interested in things and machines, whereas women are more interested in people. And we know that men are more likely to go into “nerdy” professions such as math or engineering, whereas women are more likely to go into the caring professions and to spend more time looking after children.