Altruism is the characteristic of benefitting others without the expectation of reward.
In biology, altruism is defined as a behavior that lowers the direct fitness of the donor while raising the direct fitness of the recipient.  Since, as Theodosius Dobzhansky stated, "nothing in biology makes sense except in the light of evolution", one may ask: how can a trait that lowers the fitness of an individual survive evolutionary selection? Mathematical modellings of population genetics show that, although altruists are less fit then nonaltruists, on an individual basis, groups of altruists tend to be fitter than groups of nonaltruists.
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