The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (48 page)

To understand equilibrium selection, we first have to understand a little about equilibria and game theory. Game theory is the study of strategic decision-making, where your payoff for

doing something depends not only on what you do, but on what other people do. A "game" is any social situation in which there are incentives to pick one's own strategy in anticipation of the strategies favored by others—but where their strategies will in turn depend on their anticipations of your own behavior. This sounds like an infinite regress: I anticipate that you anticipate that I anticipate that you anticipate ... How can game theory make any progress in predicting human behavior in such games, when games seem like hopeless muddles?

Around 1950, the economist John Nash cut through this Gordian knot by developing the idea of an "equilibrium" (now known as a Nash Equilibrium). An equilibrium is a set of strategies, one for each player, that has a simple property. The property is that no player has an incentive to switch to a different strategy, given what the other players are already doing. An equilibrium tends to keep players playing the same strategies. The idea of an equilibrium is the foundation of modern game theory, and therefore of modern economics, business strategy, and military strategy. For his insight, Nash received a share of the 1994 Nobel Prize for Economics.

Driving on the left side of the road is a good example of an equilibrium. If everybody else is already driving on the left, as in Britain, no rational individual has a good reason to start driving on the right—such rebels against convention would quickly be eliminated from the population of drivers. But driving on the right side of the road is also an equilibrium, apparently favored by some former British colonies in North America as a mark of their independence. There is a third equilibrium in the driving game, which consists of driving on the left 50 percent of the time and on the right 50 percent of the time. If everybody is already doing that, you might as well too. This randomized equilibrium seems
to
be favored in Britain's former colonies in south Asia, especially by the taxi drivers of Bangalore. Nash realized that in most realistic games there are many equilibria. We cannot necessarily predict which equilibrium will be played, but we can predict that players will coordinate their behavior on one of the equilibria. In the

driving game, different countries play different equilibria.

Equilibrium selection is the gradual process by which an equilibrium becomes established for a particular game. Imagine an anarchic country without cars that suddenly starts importing cars. People would start driving without knowing which side of the road other drivers will favor. Some would pick the left consistently (the British equilibrium), others would pick the right consistently (the American equilibrium), and still others would toss a coin every day to decide (the Bangalore equilibrium). Now we have a process of competition between three strategies that would each produce a different equilibrium. Suppose that every head-on collision kills both drivers involved. If left-driver meets left-driver, they both survive. If right meets right, they both survive. If Bangalore meets Bangalore, they both die half the time. If right-driver meets left-driver, they both die. There is no rational basis for predicting which equilibrium will become established. Every equilibrium is equally "rational" in the sense that every individual is doing as well as possible given what everyone else is already doing. Although rationality cannot select between equilibria, the contingencies of history can. We can be virtually certain that within several weeks, either the drive-left equilibrium or the drive-right equilibrium will win out. Which of them wins will be due to chance, but one of them will win. (There is only a very small chance that the Bangalore equilibrium will win.)

In this example, the equilibrium selection problem is solved not by rational logic but by historical contingency. When species evolve to play one equilibrium rather than another in the game of courtship, evolutionary contingency can play the role of historical chance. It is easy to simulate this process in a computer, as Brian Skyrms did in his wonderfully lucid 1997 book
Evolution of the Social Contract
The same equilibrium selection processes must happen all the time in real biological evolution. Most interactions between animals can be interpreted in strategic terms, and so can be modelled using game theory. But for most realistically complex games, there are vast numbers of equilibria: not just three equilibria as in the driving game, but hundreds or thousands of

possible equilibria. For realistic games with many equilibria, equilibrium selection processes become absolutely crucial to understanding and predicting behavior.

In our sports example, we considered two possible equilibria in the game of displaying athletic fitness: club-fighting and hunting. If everyone is already club-fighting, you can attract a mate only by club-fighting too, so you have no reason to do anything else, and that makes club-fighting an equilibrium. But if everyone is already hunting, you can only attract a mate by hunting well, so hunting is an equilibrium too. The mate preferences that favor good hunters or good fighters tend to be genetically and culturally conservative, and this sexual conservatism maintains the equilibrium.

Club-fighting and hunting are equally rational from the individual point of view, but hunting is the equilibrium with the higher payoff for everyone. With the Arabian babblers, we saw that altruistic behaviors such as food-sharing and alarm-calling could work as an equilibrium in the game of displaying fitness. The general point is that courtship games have many possible equilibria, and some of them will include a lot of apparently altruistic behavior. Most of them do not, because most ways of wasting energy to display one's fitness do not transfer any benefits to others. The peacock's tail simply wastes one peacock's energy to display his fitness, without transferring that energy to any other peacocks or peahens. But in some species, such as Arabian babblers and humans, our costly courtship displays actually bring some benefits to others.

Anthropologist James Boone described how equilibrium selection can favor altruistic displays in his 1998 paper "The Evolution of Magnanimity." He envisioned different groups playing different equilibria in the game of conspicuous display:

Now imagine that, in some of these groups, elites signal their power by piling up their year's agricultural surplus in the plaza and burning it up in front of their subordinates. In other groups, elites engage in status displays by staging elaborate

feasts and handing out gifts to their subjects. After several generations of intense warfare, which type of display behavior is likely to survive in the population? One might expect that the "feasters" would be much more successful at attracting supporters than the "burners."

Competition between groups would favor a magnanimous equilibrium over a wasteful equilibrium. Yet this would not be "group selection," as traditionally defined by biologists, in which individuals incur an individual cost to produce a group benefit. In this case, every individual is acting selfishly and rationally in trying to gain high status and sexual attractiveness through their costly display. The individual sexual benefits, not the group benefits, maintain the equilibrium: group competition merely picks between equilibria. Anthropologists Robert Boyd and Peter Richerson have argued that this sort of interaction between equilibrium selection and group competition is extremely important, not only in genetic evolution but in cultural history. Their ideas offer a new foundation for the comparative analysis of human cultures and social institutions, and I wish I had more space to discuss them further here.

In summary, evolution sometimes favors courtship equilibria in which animals are very generous to others. This does not mean that evolution favors truly selfless altruism, simply that the hidden benefit of generosity is reproductive rather than nepotistic or reciprocal. In principle, evolution could sustain very high levels of altruism by rewarding the altruistic with high social status and improved mating opportunities. Without sexual selection, generosity to unrelated individuals unable to reciprocate would be very unlikely to evolve. With sexual selection, such generosity can evolve easily as long as the capacity for generosity reveals the giver's fitness. In our species, the fact that we find kindness and generosity so appealing in sexual partners suggests that our ancestors converged on a rare, and wonderful equilibrium in the game of courtship.

Leadership

High status among chimpanzees and gorillas does not depend only on physical dominance. It also depends on an individual's ability to prevent fights among other group members, to mediate conflicts, to initiate reconciliations, and to punish transgressors. Frans de Waal observed that one of the chimpanzees in Arnhem Zoo named Yeroen sustained his high status late into life by being good at this sort of moral leadership. Yeroen had the social intelligence to notice when trouble was developing between group members, and the social skills to intervene in just the right way to defuse tension and maintain group harmony. He was remarkably impartial, not allowing his own social relationship and consort-ships to bias his peacekeeping. Other individual males could beat Yeroen in a fight, but his high status was maintained through popular support and respect.

Chimpanzees have apparently transformed the ancient tradition of primate dominance hierarchies into a status system based on moral leadership. We used to imagine that this was a distinctively human achievement, but it is not. If chimpanzees and gorillas respect peace-keeping and policing ability, and modern humans do too, then it is likely that our common ancestor five million years ago did as well. Status based on moral leadership is a legacy of the great apes. For at least five million years, our ancestors have been striving to attain status through their moral leadership, rather than just through their physical strength.

But what exactly does "high status" mean? In primates, it generally brings greater reproductive success, which depends on greater sexual attractiveness. Status is not a piece of territory that can be taken by force. It must be granted by others, based on their likes and their dislikes, their respect and their disrespect. "Status" is a statistical abstraction across the social and sexual preferences of the members of one's group. If our ancestors attained high status through moral leadership, that meant moral leadership was socially and sexually attractive. It was favored by social choice and sexual choice. Because sexual choices have so much more evolutionary power than social choices about friends, grooming

partners, and food-sharers, we come to this conclusion: moral leadership evolved through sexual choice in both chimpanzees and humans.

Leadership is like hunting in this respect: it provides a common good that looks purely altruistic until one considers the behavior's sexual attractiveness. Sexual selection could have favored the opposite of moral leadership, but that preference would tend to go extinct along with its tense, bickering, exhausted groups. One could imagine a primate species in which females happened to develop a runaway sexual preference for hair-trigger psychopaths who randomly pick fights. Males could obligingly evolve into violent bullies. But groups playing that psychopathic equilibrium would go extinct in competition with efficient, peaceful groups playing the good-leadership equilibrium. As with hunting versus club-fighting, this is an example of equilibrium selection. It is not an example of the discredited group selection process in which individuals pay an individual cost for a common benefit. The sexual rewards of moral leadership mean that good leaders obtain a net individual benefit from behavior that provides for the common good.

Where chimpanzees evolved moral leadership, humans evolved the more advanced capacity of moral vision, including the passionate articulation of social ideals concerning justice, freedom, and equality. Moral vision is sexually attractive, and may have been generated by sexual selection. It takes the impartiality of the peacekeeping primate to a more conscious, principled level. In discussing such an important human capacity, we must be especially careful to distinguish evolutionary function from human motivation. When Malcolm X used his verbal genius and moral charisma to forge a vision of a Muslim society free of racism, he was motivated by moral instincts, not "sexual instincts." His moral instincts happened to attract a beautiful young woman named Betty Shabazz to become his wife, as they had evolved to do through sexual selection. Likewise for Martin Luther, whose Protestant vision attracted the ex-nun Katharina von Bora to marry him and raise six children. The peacock's tail is no less

beautiful when we understand its sexual function. Nor should the validity of human moral vision be reduced when we understand its origin in sexual choice.

Why Scrooge Was Single: The Evolution of Charity

Survival of the fittest was supposed to make us act selfishly. Like Charles Dickens's Scrooge, the traditional Darwinian account depicts humans as mean and miserly, perhaps with a little nepotism toward close relatives and some prudent loan-sharking to those who might pay us back. This is a convenient myth for educators, priests, and politicians, because it presents us as badly in need of socialization through schools, churches, and prisons. Supposedly, we need these character-improving regimes as our Ghosts of Christmas Past, Present, and Future, to transform our selfish, biological, pre-Christmas Scrooge into our generous, cultured, post-Christmas Scrooge.

The Dickens story makes a poor parable for human evolution, though. Scrooge survived well enough, but he was single and childless. In Victorian London, his manifest selfishness exiled him from the mating market. No self-respecting Englishwoman would pay him the slightest notice. As far as his genes are concerned, his miserliness was not self-interested, but self-castrating.