According to an old joke, everyone knows that men and women are different . . . except social scientists. In A Billion Years of Sex Differences, evolutionary psychologist Steve Stewart-Williams tackles some of science's most controversial questions: How do men and women differ? Where do the differences come from? And how do they shape modern life?

The result is the most up-to-date, balanced, and engaging account of human sex differences yet written, covering everything from dating and mating to aggression and parenting, from children's toy preferences and workplace gender gaps to mental health and the politics of equality. Drawing on a century of research—and a billion years of evolution—Stewart-Williams explains why many sex differences appear despite socialization, not because of it; why in our mating and parenting patterns, humans are more like the average bird than the average mammal; and why sex differences are sometimes a sign of societal health rather than injustice.

With more than two decades of research in evolutionary psychology and an international reputation in the field, Steve Stewart-Williams is uniquely positioned to challenge prevailing views in this fraught debate. Avoiding the extremes of either overstating or denying the differences, he argues that, rather than trying to make men and women identical, we should strive for a world where bias and barriers are eliminated, and where people are free to be themselves whether they conform to gender norms or defy them.