Book excerpt: Offering new research and analysis on the relation between gender and evolution, this book explains conflict between the sexes and the frequent emergence and stubborn continuation of patriarchal regimes that serve to control the behavior of women in societies around the world, both past and present. Women and men are different, on average. But that does not mean they are unequal. Indeed, understanding average differences is key to the full realization of equality in health care and other dimensions of social life. Hopcroft shows that gender differences in physiology, psychology, and behavior can be traced to slight differences in evolved traits between men and women. These differences exist because of sex differences in investment in offspring, which meant that, in the environment of evolution, some adaptive problems were more important for men to solve than for women, and vice versa. For men, the most important adaptive problem to solve was that of finding a mate. Men who did not solve this problem are not our ancestors. For women, the most important adaptive problem to solve was that of successfully bearing and raising children. Women who did not solve this problem are not our ancestors. These small differences underlie all the differences described in the book, including sex differences in mate preferences, physiology, cognition, aggression, status striving, and emotional experience. It can also help explain the differential treatment of children by parents, the differential success of boys and girls in modern schools, and sex differences in style of communication.