Why Chimpanzees Make Bad Suicide Bombers

The Evolution of Spite is the Evil Twin of Altruism

Someone walks into a crowded restaurant, looks about the diners calmly, and blows themselves up as well as everyone nearby. Why? This is a scenario that forces us to explain the dark side of human nature. Why do humans have a capacity for such hate that they’ll take their own lives in order to destroy others?

A study in the current issue of PNAS on chimpanzee behavior suggests that humans may be alone in this way: a dubious distinction to say the least. In a review published yesterday in the Chicago Tribune the researchers suggest:

"Spitefulness may be a peculiarly human trait," said Keith Jensen, a Canadian evolutionary biologist who has been looking to see whether human concepts like fairness and punishment are present in the social organization of another highly socialized species.

In biological terms spite is the flip side to altruism and both have posed a thorny issue for evolutionary biologists. While an altruistic act is one where the actor takes a hit in order to help someone else, a spiteful act is one where that same actor seeks to hurt someone else at a cost to themselves.

"Spite is kind of interesting, because it is altruism's evil twin," Jensen said. "Humans can care about making somebody feel better, but we also have the darker side of sometimes wanting to make somebody feel worse."

W.D. Hamilton, the British evolutionary biologist most famous for kin selection theory, proposed how altruism could evolve in a population composed of close relations. If the cost to the actor is less than the benefit to the recipient times their coefficient of genetic relatedness (0.5 for full siblings, 0.25 for nephews) than the altruistic act improves their inclusive fitness and the trait will perpetuate.

Hamilton also wrote on altruism’s evil twin in his classic 1970 Nature paper “Selfish and Spiteful Behaviour in an Evolutionary Model”. Hamilton suggested that the evolution of spiteful behavior could be selected for in cases where the recipient of the spite was less likely to be related than an average member of their population. This is because, if a spiteful organism goes out of their way to hurt someone related to them, those spiteful genes would be less likely to be passed on by both the individual and the recipient of the spite. However, if that same organism were to sacrifice themselves to hurt someone less related than the rest of the population it could benefit their inclusive fitness.

In a 2004 article in The Scientist (which is an excellent review and has been reproduced in full at The Primate Appendices) researchers have done studies with bacteria which suggest that, not only are spite and altruism related, they often rely on each other.

Many bacteria manufacture toxins called bacteriocins, which they release explosively, killing both themselves and sensitive competitors, but sparing clonal relatives that possess a resistance gene. Gardner says the spiteful credentials of such bacterial suicide bombings are reinforced by the presence of an equivalent altruistic trait, siderophore production. Siderophores are compounds that scavenge iron from the environment for absorption.

"You do better as a bacterium if you don't produce the proteins and just mop up those produced by others. So, production of the proteins is altruistic," says Gardner. In line with predictions, West's team, in collaboration with Angus Buckling at Oxford University, has shown that bacteriocin production is increasingly favored by selection, as competition between bacterial strains becomes more local, whereas siderophore output declines.

This suggests that suicide bombers could be motivated, at least in part, by inclusive fitness. The tactic of suicide bombing only exists because of a perceived threat to the bomber’s family and community (sometimes extended more widely to their "family" of faith). By targeting populations that are outside this group they aim to benefit their community and, by consequence, their inclusive fitness by hurting other, unrelated, individuals. This interpretation has recently been explored in the American Journal of Economics and Sociology.

While chimpanzees have yet to show evidence of spiteful behavior (a distinction that we should remember when holding ourselves above “mere animals”) they have been shown to be capable of altruism. Perhaps the rainforest equivalent of an Israeli restaurant is at this moment quietly munching figs as a solitary individual from a distant troop creeps through the underbrush, canines at the ready. But this is probably unlikely for one very important reason: chimps aren't religious.