Since antiquity - from the eunuchs in the royal courts of ancient China to the booming market for “elixirs of youth” in nineteenth-century Europe - humans have understood that typically masculine behaviour depends on testicles, the main source of testosterone in males. Which sex has the highest rates of physical violence, hunger for status, and desire for a high number of sex partners? Just follow the testosterone. Although we humans can study and reflect on our own behaviour, we are also animals, the products of millions of years of evolution. Fascinating research on creatures from chimpanzees to spiny lizards shows how high testosterone helps males out-reproduce their competitors. And men are no exception. While most people agree that sex differences in human behaviour exist, they disagree about the reasons. But the science is clear: testosterone is a potent force in human society, driving the bodies and behaviour of the sexes apart. But, as Hooven shows in T, it does so in concert with genes and culture to produce a vast variety of male and female behaviour. And, crucially, the fact that many sex differences are grounded in biology provides no support for restrictive gender norms or patriarchal values. In understanding testosterone, we better understand ourselves and one another - and how we might build a fairer, safer society.