Kin selection was formulated by Hamilton to explain the evolutionary logic underlying altruism: why individuals behave in ways that reduce their personal fitness and provide a fitness advantage to others [12, 13]. Kin selection theory demonstrates that organisms can evolve to perform costly actions if they benefit their relatives, who carry (and thus pass on) their genes. Although we usually think of kin selection in terms of individuals behaving altruistically towards their blood relatives, it is more straightforward to explain the theory from the perspective of the gene that confers the costly behavior. Such a gene can increase in frequency only if the costly behavior it causes in some is more than offset by the fitness benefit it confers in others. Several corollaries of kin selection theory follow more naturally from this gene-centered explanation. First, individuals would ideally direct the benefits of their costly behavior to other individuals who carry the same gene - thus, the ability to discriminate kin from non-kin, called kin discrimination, is thought to be important for the evolution of such a costly trait. Second, 'relatives' are really those who carry the same gene. They do not necessarily have to be genetically similar throughout the entire genome, although altruism that runs counter to relatedness across the genome could provoke a coevolutionary response to suppress its effects [67, 68]. In practice, knowing to whom you are related via shared ancestry may be an easy way of approximating true (gene-specific) relatedness: high average relatedness across the genome owing to a recent common ancestor also means a high probability of relatedness at any given locus, including the one responsible for the altruism. Finally, with respect to a costly ('altruism') gene becoming established in a population by natural selection, relatedness is relative to the scale at which competition between alleles occurs. Strictly speaking, the cells of the body do not behave altruistically towards one another because they are genetically similar, but because they are on average more genetically similar to one another than they are to cells from other individuals. By the same token, if competition occurs primarily among the cells within the individual rather than between individuals, then selection may no longer favor acts of altruism and instead promote the evolution of more selfish behaviors.