Stickleback fish rely on tit-for-tat when approaching a potential predator to determine how dangerous it is. A pair of stickleback approach in short spurts, each spurt can be thought of as a round of the prisoner's dilemma. (Image by Ron Offermans)

We humans pride ourselves on our ability to be altruistic and charitable, but it turns out that we're not the only ones. Many animals also perform apparently selfless acts, including those we wouldn't normally credit with much sophistication, such as stickleback fish.

This raises a baffling question: since evolution is based on survival of the fittest, and helping someone else doesn't immediately increase your own fitness, how did altruism first evolve? Mathematics has an answer, based on a famous mathematical game called the *prisoner's dilemma*. To find out more, read *Does it pay to be nice?*

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