Testosterone is a familiar villain, a ready culprit for everything from stock market crashes to the overrepresentation of men in prisons. But your testosterone level doesn’t actually predict your appetite for risk, sex drive, or athletic prowess. It isn’t the biological essence of manliness-in fact, it isn’t even a male sex hormone. So what is it, and how did we come to endow it with such superhuman powers?T’s story begins when scientists first went looking for the chemical essence of masculinity. Over time, it provided a handy rationale for countless behaviors-from the boorish to the enviable. Testosterone focuses on what T does in six domains: reproduction, aggression, risk-taking, power, sports, and parenting, addressing heated debates like whether high-testosterone athletes have a natural advantage as well as disagreements over what it means to be a man or woman.