Early hominid men and women had worked out a pretty good system of food exchange, but the system was seriously threatened by the caloric requirements of the infants. One woman negotiating with one man would want more than 50% of the meat, so that she could also feed her children. From the man’s point of view, this was a rotten deal, as he lost more calories than he gained in the deal. So, to make the deal compelling, women had to tie reproduction into the deal. It wasn’t just “You share your meat with me and I’ll share my nuts and berries with you.” Now it was “You share your meat with me and my children and I’ll share my nuts and berries with you, and permit you to impregnate me. The children with whom you’ll be sharing your meat will be your own progeny.”

The deal was basically sound but it suffered from two nasty weaknesses. First, the act of impregnation was the work of a single moment, but the task of raising a child extended over a number of years. The female carried out her end of the bargain on the first night, and the male carried out his end of the bargain over the next few years. This kind of asymmetric deal was all too easy for the male to cheat on. Even today, after tens of thousands of years of struggling with this problem, humanity has yet to figure out how to prevent men from abandoning women.

But men weren’t the only cheaters on this deal. Since the act of impregnation was so quick and easily hidden, it was in the woman’s best interest to cut a food-sharing deal with the best provider and get the genes from the genetically best-endowed guy. And the fact that the men were always off hunting made it all too easy for a woman to get away with such a deception. The big risk that she took by doing so was that, if she was caught, then the food-providing husband would dump her and her kids, and the kids would starve.

Thus, there was plenty of reason for mutual suspicion between the sexes. Each had its own worry. Women ask “Will you still love me in the morning?”, meaning “If I allow you to impregnate me, will you stick around and help feed the kids?” Men ask “Is it really my child?” Indeed, male suspicion of female infidelity created a strong tendency among fathers to kill infants that didn’t look enough like the father. This selection effect led to an interesting result: modern babies consistently look more like the father than the mother. “See dad, I’m just like you, so don’t kill me, OK?”

This problem proved to be serious; if men and women proved unable to cooperate enough to procreate, the entire Homo genus would have died out. As it was, the only ones who survived were those men and women who could muster the intelligence to cooperate effectively. And as everybody knows, the key to a happy relationship is

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