option to remain silent, or confess, which effectively betrays the other prisoner, and both prisoners cannot communicate to form a strategy. If both prisoners remain silent, both will be imprisoned for one year. If one remains silent and the other betrays, the silent prisoner gets a five-year sentence while the other goes free. If both betray each other, both prisoners get a three-year sentence. While both prisoners cooperating with each other results in both getting a fairly light sentence, the outcome for a prisoner who cooperates with a prisoner who betrays is worse than the outcome from both prisoners betraying each other, and the benefit from betraying a silent prisoner results in the best case scenario for the one who betrays. When applying this to evolution, where individuals in a species have to all use common resources, it would seem logical to assume that individuals that are selfish would do better resulting in lack of cooperation in modern species. Despite this, cooperation and mutualism have arisen between many species. Explaining how this is the case has been a vexatious problem in evolutionary studies. Darwinian context Charles Darwin's theory of how evolution works ("By Means of Natural Selection") is explicitly competitive ("survival of the fittest"), Malthusian ("struggle for existence"), and even gladiatorial ("nature, red in tooth and claw"). Species are pitted against species for shared resources, similar species with similar needs and niches even more so, and individuals within species most of all. All this comes down to one factor: out-competing all rivals and predators in producing progeny. Darwin's explanation of how preferential survival of the slightest benefits can lead to advanced forms is the most important explanatory principle in biology, and extremely powerful in many other fields. Such success has reinforced notions that life is in all respects a war of each against all, where every individual has to look out for himself, that your gain is my loss. In such a struggle for existence altruism (voluntarily yielding a benefit to a non-relative) and even cooperation (working with another for a mutual benefit) seem so antithetical to self-interest as to be the very kind of behavior that should be selected against. Yet cooperation and seemingly even altruism have evolved and persist, including even interspecific cooperation, and naturalists have been hard pressed to explain why. Social Darwinism The popularity of the evolution of cooperation – the reason it is not an