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.

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