Forber and his co-researcher, Rory Smead of Northeastern University in Boston, wanted to understand why spite might evolve. They used a famous economics game, called the Ultimate Game, to find out.

In the Ultimatum Game, there are two players. The first is given a resource — say, $10 — and told to offer part of that resource to the second player. If player two refuses the offer, neither player gets anything. If player one accepts, both get the amounts proposed by player one.

If a person is playing to maximize his or her profits in the Ultimatum Game, the rational thing to do is to offer as little as possible to player two and accept anything offered when in player two’s shoes. That’s not how people work, however; they regularly make fair, even offers and reject unfair offers. That rejection is an example of spite, because the player refuses a reward in order to punish someone who gave an unfair offer.