This chapter tackles one of the most controversial topics in psychological science: sex differences. On average, men tend to be more interested than women in casual sex, more inclined toward violence, and more likely to take life-threatening risks. Women, in contrast, tend to be more selective about their sexual partners, more involved in childcare, and somewhat longer lived. Where do these differences come from? According to many, the answer is culture. In this chapter, however, I argue that the differences come primarily from natural selection. Specifically, they trace back to the fact that our female ancestors invested more into offspring than did our male ancestors. In support of the evolutionary account of human sex differences, I look, among other things, at evidence for the cross-cultural universality of these differences, evidence that the differences often emerge despite culture not because of it, and evidence that similar differences are found in other animals – animals that resemble us in evolutionarily relevant ways. I also argue, though, that the sex differences found in our species are relatively modest compared to those in many others, and that this is a result of the fact that men often help care for their offspring.