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The temptation to cheat appears to be a universal fact of life. Regrettably too common among human beings, cheating has also been found among many other animals, including monkeys, birds and fishes. In a series of laboratory experiments, evolutionary biologist Paul Turner at Yale University has now discovered cheaters among viruses. Turner applied game theory, a branch of mathematics that analyzes competitive interactions, to the experimental evolution of a virus that preys on bacteria, the phi-6 bacteriophage. His analysis revealed that some of the viruses had evolved to replicate more quickly, apparently by usurping the resources supplied by other viruses—in other words, by cheating. The study is the first to demonstrate the evolution of irrational, selfish behavior in a biological system.