Testosterone. The hormone that divides and rules

Observing the castrated males of domesticated animals, eunuchs in the imperial harems of ancient China, and castrated singers in 18th century Italy, people realized that typically male features of physiology and behavior are determined by the presence of testicles, otherwise known as testes. Over time, science discovered that these glands are the primary source of testosterone, the main male hormone. Who has a larger Adam’s apple, a deeper voice, and hairier chest? Who has higher rates of physical aggression, a sharper drive for privileged status, and an insatiable desire to have multiple sexual partners? Just keep an eye on the testosterone levels. Although we humans are capable of studying and assessing our behavior, we are also animals, products of evolution spanning millions of years. Astonishing studies of animals from iguanas to chimpanzees show how high testosterone levels help males produce offspring, outpacing their competitors in this. And men are no exception. While most of us agree that gender differences in human behavior objectively exist, we diverge in opinions regarding their causes. But science is unyielding here: testosterone is a powerful force acting both in the human body and in human society to separate the male sex from the female. However, as Harvard evolutionary biologist Carol Hooven demonstrates in her book, it does this in conjunction with genetics and culture, creating a vast diversity of male and female behavior. And, importantly, the fact that many gender differences are rooted in biology does not justify either restrictive gender norms or patriarchal values. By understanding the mechanics of testosterone, we will better understand ourselves and each other, as well as how to build a more equitable and safe society.