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The evolution of spite

Spiteful dudeNobody likes to be on the receiving end of spite. Yet, there is something uniquely satisfying about being spiteful. Due to this “pleasure” the spite yields, something in our evolutionary history has trained our brains for spite. Why, exactly, remains puzzling, and philosophers Rory Smead (Northeastern University) and Patrick Forber (Tufts University) wanted to get to the bottom of it. Using statistical analysis and mathematical models, they showed how spite can evolve in a given population.

In modeling the evolution of a hypothetical population, Smead and Forber used the terms “correlation” and “anticorrelation” to describe social interactions that occur within that population. Correlation means an individual has an increased chance of social interaction with another individual of the same type, while anticorrelation means that instead there’s an increased chance of social interaction with an individual of a different type.

Next, the researchers defined “spite” as paying a cost so that another individual receives no benefit without the one who paid the cost receiving any benefit. In other words, an act of spite occurs when someone sacrifices something purely to hinder someone else. Clearly, correlation would doom a population, but the researchers believed that anticorrelation explains why evolution selected for spite.

To see this, first see why spite would be favored in small populations. The researchers argued that if the ratio of “harm to others” to “cost to spite” is greater than the population size, then evolution will select for spite. In other words, if one can inflict a large amount of harm with minimal cost, then anticorrelation kicks in because both the perpetrator and victim of spite will seek individuals of different types. Of course, in a small and static population, this will quickly lead to everyone hating each other and ultimately the collapse of the society. However, given dynamic population growth, spite can spread as the population of people to spite never quite gets exhausted. On the other hand, in a very large population, anticorrelation decreases due to greater homogeneity of individuals. In other words, spite is evolutionarily stable in small populations with high reproductive rates.

The researchers specified that spite evolves in these small populations with dynamic growth rates under two conditions. First, spite as a “strategy,” that is, as a method of social interaction, must “work” or produce the desired effects within the population. Put another way, spite must be stable within a population. Secondly, and quite obviously, the population cannot go extinct. A non-existent population cannot be spiteful.

It should be emphasized that the above is not mere speculation, but is supported by mathematical models. For better or for worse, without a time machine, evolutionary theorists must rely on mathematical models and computer simulations in order to test hypotheses. Mathematical models may not be “brute fact,” but they also are not baseless.

The researchers put forward a highly plausible way in which spite evolved. Through anticorrelation in a dynamically growing population, spite can spread quite probably. While evolution can grant organisms wonderful abilities like sight, touch, and locomotion, it also seems capable of giving us less than desirable abilities, like spite.

For more, see “The Evolutionary Dynamics of Spite in Finite Populations” in the journal Evolution.


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