Males and females in other mammalian species differ in the same ways. Biologists have developed theories about how those differences might lead to differences in their sexual behavior. For example, a male who has an innate propensity to mate with many females during their fertile period may have many offspring who survive and inherit the same propensity to mate like crazy. Females don't increase the number of their offspring by increasing their number of mates during any given fertile period. So fema les with the same propensity won't leave behind a disproportionate number of grandchildren who mate like crazy.
Biologists have good evidence that supports that theory.
Of course, their observations of animals in the wild forced them to refine the theor y a lot. He summarizes other researchers' findings, too. Here I'll focus on one theoretical proposal he makes and the evidence he offers to support it. Buss thinks that ancient evolutionary pressures have left modern women with an innate psychological propensity to seek out a husband who is a good provider. The theory is impeccable.