Many people believe that, at its core, biological sex is a fundamental, diverging force in human development. According to this overly familiar story, differences between the sexes are shaped by past evolutionary pressures—women are more cautious and parenting-focused, while men seek status to attract more mates. In each succeeding generation, sex hormones and male and female brains are thought to continue to reinforce these unbreachable distinctions, making for entrenched inequalities in modern society. In Testosterone Rex, psychologist Cordelia Fine wittily explains why past and present sex roles are only serving suggestions for the future, revealing a much more dynamic situation through an entertaining and well-documented exploration of the latest research that draws on evolutionary science, psychology, neuroscience, endocrinology, and philosophy. She uses stories from daily life, scientific research, and common sense to break through the din of cultural assumptions. Testosterone, for instance, is not the potent hormonal essence of masculinity; the presumed, built-in preferences of each sex, from toys to financial risk taking, are turned on their heads. Moving beyond the old “nature versus nurture” debates, Testosterone Rex disproves ingrained myths and calls for a more equal society based on both sexes’ full, human potential.