Games Primates Play

The evolution and economics of human relationships.

The Evolution of Spiteful Behavior

The difference between selfish and spiteful behavior.

I have seen congested traffic and aggressive drivers in many cities around the world. More than once I have noticed that if two cars arrive simultaneously at an intersection, one driver will honk his horn and try to engage the intersection before the other does. By honking his horn, the aggressive driver momentarily disorients or intimidates the other, thus gaining the few seconds necessary to allow his car to get ahead. From an evolutionary perspective, honking is a selfish act: it benefits the individual who performs it and imposes a cost on the other. Selfish behavior is no mystery for evolutionary biologists: it can easily evolve by natural selection or the analogous process of cultural evolution.

To curb drivers’ selfish tendencies and eliminate the noise, some countries such as the United States have made honking illegal, unless it’s justified by an emergency. Giving a ticket raises the cost of honking and since this cost is generally higher than the small benefit gained from it, honking has become rare. With one exception: when I drive on the streets of California and I ignore a stop sign or a red light other drivers honk at me even though they are not directly affected by my actions. These drivers are willing to risk getting a ticket for honking to increase the chance that I will get a ticket for violating another traffic rule. Evolutionarily, behavior that carries a cost to the actor and a cost to the recipient is not selfish: it’s spiteful.

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Spiteful social behavior can evolve by natural selection if the costs imposed to the recipient are greater than the costs suffered by the actor. If my worst enemy and I each have three children, and murdering his three children entails sacrificing two of mine, a gene responsible for such spiteful behavior could evolve because my surviving child could reproduce and transfer that gene to his children, while none of my enemy’s genes would be transferred to the next generation. By the same token, if the ticket I get for ignoring a stop sign is more expensive than the ticket another driver gets for honking at me then spiteful honking behavior could thrive. But that’s only part of the story. The drivers who honk at me also want to give me a bad reputation as someone who does not play by the rules. A bad reputation for being a cheater can be far more damaging than a ticket for ignoring a stop sign. Honking at drivers who break the rules is an example of what evolutionary biologist Robert Trivers called moralistic aggression.

Respecting traffic rules is a cooperative social contract between people, not dissimilar from the social contracts that make people pay taxes to the government, not steal from their employers, or not cheat on their spouses. Agreeing to these contracts requires curbing one’s selfish tendencies, but this “sacrifice” is generally offset by the benefits gained if everyone plays by the rules. Any individual, however, who cheats and takes advantage of the system can benefit a lot while making the cost of the sacrifice much higher for everyone else. The cooperators must immediately expose the cheater, give him a bad reputation, and try to exclude him from the social contract.

Given the magnitude of the damage a cheater can cause to a community of cooperators, people who engage in moralistic aggression are willing to do so at a significant cost to themselves, hence the evolution of seemingly spiteful behavior. To give just one example, some wives who have been cheated on by their husbands have paid exorbitant sums of money to put their cheating husband’s name on billboards displayed in busy urban areas to give him a bad reputation and make sure no other women will pair up with him in the future.

The higher the percentage of people who subscribe to a social contract in a particular community, the higher the intensity of moralistic aggression against transgressors. In places where almost everyone is a cooperator (for example, on the streets of California), the social contract is very valuable and must be protected at all costs. In communities in which the contract is weak, so is moralistic aggression. When I drive on the busy streets of Rome, where I grew up, nobody honks at me if I ignore a stop sign. Other drivers do it too. A Roman knows that rules exist but thinks that they are for other people. (A slightly different version of this post has appeared in Wired UK.)

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Dario Maestripieri, Ph.D., is a professor of comparative human development, evolutionary biology, and neurobiology at the University of Chicago.

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