Abstract
Cooperation among kin is common across the natural world and can be explained in terms of inclusive fitness theory, which holds that individuals can derive indirect fitness benefits from aiding genetically related individuals. However, human kinship includes not only genetic kin but also kin by marriage: our affines (in-laws) and spouses. Can cooperation between these genetically unrelated kin be reconciled with inclusive fitness theory? Here, we argue that although affinal kin and spouses do not necessarily share genetic ancestry, they may have shared genetic interests in future reproduction and, as such, can derive indirect fitness benefits though cooperating. We use standard inclusive fitness theory to derive a coefficient of shared reproductive interest (s) that predicts altruistic investment both in genetic kin and in spouses and affines. Specifically, a behaviour that reduces the fitness of the actor by c and increases the fitness of the recipient by b will be favoured by natural selection when sb > c. We suggest that the coefficient of shared reproductive interest may provide a valuable tool for understanding not only the evolution of human kinship but also cooperation and conflict across the natural world more generally.
Highlights
Hamilton’s rule provides a framework for understanding the evolution of altruism among genetically related individuals [1,2,3]
The coefficient of shared reproductive interest defined here provides a framework for estimating the fitness benefits that individuals can derive from social interactions with spouses and affinal kin as well as genetic kin
We argue that shared reproductive interest represents a concept that is theoretically consistent with inclusive fitness theory and its subsequent derivations and expansions [12,13] and lends itself to the empirical study of social relations among genealogical and affinal kin, extending the work of Hughes [8,9]
Summary
Hamilton’s rule provides a framework for understanding the evolution of altruism among genetically related individuals [1,2,3] It states that an altruistic act will be fitness enhancing if the cost to the altruist (c) is less than the benefit to the recipient (b) multiplied by the relatedness of the altruist to the recipient (r). Hughes [8], argued that even if affinal kin are not genetically related, they may have a shared genetic interest in the generation, a hypothesis empirically tested by Burton-Chellew & Dunbar [9] We extend these previous studies, developing a general framework for estimating the degree of shared reproductive interest that individuals have in their social partners. We propose a coefficient that captures the shared reproductive interest between individuals
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