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Obviously, if a woman gauges her genetic interests by the number of children whom she can rear to maturity, that’s a function of how much food she can provide them, so she is best off marrying a provider. But she is further well served by having show-offs as neighbors, with whom she can trade occasional adulterous sex for extra meat supplies for herself and her kids. The whole tribe also likes a show-off because of the occasional bonanzas that he brings home for sharing.

As for how a man can best advance his own genetic interests, the show-off enjoys advantages as well as disadvantages. One advantage is the extra kids he sires adulterously. The show-off also gains some advantages apart from adultery, such as prestige in his tribe’s eyes. Others in the tribe want him as a neighbor because of his gifts of meat, and they may reward him with their daughters as mates. For the same reason, the tribe is likely to give favored treatment to the show-off’s children. Among the disadvantages to the show-off are that he brings home on the average less food to his own wife and kids; this means that fewer of his legitimate children may survive to maturity. His wife may also philander while he is doing so, with the result that a lower percentage of her children are actually his. Is the show-off better off giving up the provider’s certainty of paternity of a few kids, in return for the possibility of paternity of many kids?

The answer depends on several numbers, such as how many extra legitimate kids a provider’s wife can rear, the percentage of a provider’s wife’s kids that are illegitimate, and how much a show-off’s kids find their chances of survival increased by their favored status. The values of these numbers must differ among tribes, depending on the local ecology. When Hawkes estimated the values for the Aché, she concluded that, over a wide range of likely conditions, show-offs can expect to pass on their genes to more surviving children than can providers. This purpose, rather than the traditionally accepted purpose of bringing home the bacon to wife and kids, may be the real reason behind big-game hunting. Aché men thereby do good for themselves rather than for their families.

Thus, it is not the case that men hunters and women gatherers constitute a division of labor whereby the nuclear family as a unit most effectively promotes its joint interests, and whereby the work force is selectively deployed for the good of the group. Instead, the hunter-gatherer lifestyle involves a classic conflict of interest. As I discussed in chapter 2, what’s best for a man’s genetic interests isn’t necessarily best for a woman’s, and vice versa. Spouses share interests, but they also have divergent interests. A woman is best off married to a provider, but a man is not best off being a provider.

Biological studies of recent decades have demonstrated numerous such conflicts of interest in animals and humans—not only conflicts between husbands and wives (or between mated animals), but also between parents and children, between a pregnant woman and her fetus, and between siblings. Parents share genes with their offspring, and siblings share genes with each other. However, siblings are also potentially each other’s closest competitors, and parents and offspring also potentially compete. Many animal studies have shown that rearing offspring reduces the parent’s life expectancy because of the energy drain and risks that the parent incurs. To a parent, an offspring represents one opportunity to pass on genes, but the parent may have other such opportunities. The parent’s interests may be better served by abandoning one offspring and devoting resources to other offspring, whereas the offspring’s interests may be best served by surviving at the expense of its parents. In the animal world as in the human world, such conflicts not infrequently lead to infanticide, parricide (the murder of parents by an offspring), and siblicide (the murder of one sibling by another). While biologists explain the conflicts by theoretical calculations based