 Let us now turn to the question of norm emergence. Here we can see three classes of models: first, a purely biological approach, second, a more cognitive approach, and third, a structured interactions approach. The most famous of the biological approaches to norms seek to explain cooperative behavior. The simplest models are kin selection models (Hamilton 1964). These models seek to explain altruistic tendencies in animals by claiming that, as selection acts on genes, those genes have an incentive to promote the reproductive success of other identical sets of genes found in other animals. This mode of explanation can provide an account of why we see cooperative behaviors within families, but being gene-centered, cannot explain cooperative behavior toward strangers (as strangers should not be sufficiently genetically related to merit altruistic behavior).