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Life-history theory is concerned with tradeoffs in fitness: between the benefits of muscle and fat, between immune function and reproductive effort, between quantity and quality of offspring, and between reproduction now and reproduction later. Inclusive-fitness tradeoffs may involve the personal fitness of different individuals. Social tradeoffs exist between eating food now or bringing it back to the camp to be shared, between being a dad or a cad, between sponging on mother’s kin or father’s kin, and between helping one’s mother raise sibs or having a child of one’s own. Psychology is concerned with tradeoffs in mental function: between immediate and delayed gratification, between empathizing and systemizing, between focused and diffuse attention, and between impulsiveness and executive control. A key challenge for a synthesis of these fields will be to understand how psychological tradeoffs mediate life-history tradeoffs.

Our species’ innate taxonomy of kin is defined by evolved dispositions that are directed toward some individuals but not others based on environmental cues that are correlated with degree of relatedness. An innate disposition defines the membership of a category, and the membership defines the coefficient of relatedness associated with the category. All individuals who evoke a disposition belong to the category, and all members of the category determine the relatedness associated with fitness consequences of the disposition. Thus, innate kin categories need not correspond exactly to genealogical categories, and, given enough time and genetic variation, the cultural categorization of kin can shape our innate dispositions.

An unresolved issue is the richness of our innate categorization of kin both in terms of the number of different kinds of kin for whom we have distinct dispositions and in terms of the complexity of dispositions toward each particular category. No one would seriously argue that innate structure is absent in our interactions with mothers, but there is no similar consensus over whether we innately distinguish fullsibs from halfsibs, let alone mother’s brother’s daughters from father’s sister’s daughters.

When tradeoffs exist between the individual fitnesses of relatives, inclusive fitness assigns relative values to effects on different categories of kin based on each category’s degree of relatedness to an actor. For most categories of kin, relatedness differs for genes of maternal and paternal origin. The inclusive fitness of maternal and paternal alleles will be maximized by different allocations of fitness among kin, creating the potential for conflicting goals within individual organisms and a deep-seated biological ambivalence in relations among kin.

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