O. Why do men go to war?
A. Because the women are watching.
This classic exchange may not be the last word on human aggression, but sociobiologists would admire the insight into male psychology. In their view, male displays and bravado—from antlers in deer and feather ruffling in birds to chest thumping in apes and humans—evolved as a reproductive strategy to impress females. Machismo is biologically based and says in effect: "I have good genes, let me mate."
But such male bluster works only if females allow it to work. Among many monogamous birds, a female will mate only with a male willing to build nests before copulating. Presumably, human females have much more power to breed machismo out of the population. At cocktail parties, women often ask Anthropologist Irven DeVore when men will give up machismo. His immodest—but sociobiologically correct—reply: when women like you stop selecting high-success, strutting men like me. "Males," says DeVore, "are a vast breeding experiment run by females."
In evolutionary terms, sex is the central game in life, and the aim of all players is to get as many genes as possible into the next generation, at the lowest cost. Some of the best low-cost players are female fish, which deposit their unfertilized eggs in front of a chosen male. Then, while he is inseminating the eggs, they flee, leaving the poor male to protect his genetic investment by nurturing the young himself.
But in most species, including humans, the female has no such advantage. Men have far more chances to play the reproduction game: each male can start thousands of pregnancies; each female can start a maximum of about 30. More important, the female must invest far more heavily in each pregnancy—nine months of time, energy and eating for two. The male must invest only sperm, and promiscuity may pay off for him as a workable reproductive strategy. If he spreads his genes widely and refuses to nurture at all, he can still reasonably assume that some of his offspring will be likely to survive. Sociobiologists say this is why promiscuity is more popular among men, and why the urge to nest is stronger among women.
Why, in the vast majority of species, does the male aggressively pursue the female, instead of the other way round? According to Sociobiologist Robert Trivers, the sex that invests more is a "limiting resource." In other words, because women do most of the work to bring children into the world, they are in the position of sellers in a scarce market, and men must line up to buy. This principle explains the natural evolution of what DeVore and his colleague Joseph Popp have called "prostitution behavior" in higher species. A female chimp in estrus will use a sexual come-on to get more than her share of food. Even a very dominant male cannot afford to alienate the most precious of all resources—a willing female. Sociobiology also explains why, in most human societies, men are older than their mates: older men are more likely to control resources of value to a reproducing female. Males go along with the system because it is to their reproductive advantage to pick young females with many childbearing years left.