Altruism, defined as an action that benefits the receiver but comes at some cost to the performer, is one of the four types of social interactions that can occur between animals of the same species. Figure 1 summarizes these four interactions. Cooperation, where both actor and receiver benefit, and selfishness, where the action benefits the actor at the expense of the receiver, are by far the most common of the four interactions in nature. Spite, where both actor and receiver are harmed, and altruism are very rare.

The prevalence of cooperation and selfishness over altruism and spite is explained by the rules of natural selection. The currency of natural selection is offspring. Any anatomical, physiological, or behavioral trait that enhances an individual's ability to produce more offspring will be favored, and the trait will be selected regardless of the effects on others. For example, seagulls sometimes steal food from...