Why have humans evolved to help other humans? The answer to this question may seem somewhat trivial because each of us cooperates daily with other humans and repeatedly reaps the emotional and material rewards of such cooperation. Because cooperation is unmysterious to us personally, it appears to lack the explanatory urgency that enshrouds more puzzling forms of human behavior, such as unusual courtship rituals or religious practices. However, at an evolutionary-theoretic level, the answer to why we cooperate is not obvious and even elusive. Evolutionary biologists are keenly aware that cooperation (outside of mating) is infrequent in the animal kingdom-an observation that is broadly consistent with the principle that natural selection should tend to favor selfish, uncooperative phenotypes. Such an argument implies that special mechanisms of natural selection are required to explain the observed cases of cooperation in nature. Darwin understood this, but he was, at best, only partially successful in delineating how such selective mechanisms might work.