Cooperation through interdependence

Cooperation is fundamental to human societies and widespread among animals, yet explaining how cooperative relationships persist without one party exploiting another remains a challenge. Although it is well established that altruism can be favoured when recipients are relatives or when they reciprocate, it is increasingly being recognized that many behaviours cannot be explained in these terms. Here, I argue that many cases of apparent altruism can be explained by interdependence. I formalize the degree of interdependence by defining an individual's 'stake' in another as the dependence of its fitness on that of the other. This provides a means of valuing others, analogous to Hamilton's valuation of relatives. It also provides a conceptual synthesis of theories of altruism, in which kinship and reciprocity become special cases. Interdependence means that cooperators can benefit as a secondary consequence of helping their recipients. Altruism can then be favoured when its costs are outweighed by the altruist's stake in the recipient's benefits. Whereas the exploitation problem makes reciprocal altruism inherently unstable, cooperation through interdependence can be stable because whatever others do, it is best to cooperate. I discuss the extent to which interdependence can explain examples of cooperation that cannot be explained in terms of kinship and reciprocity.