Social behavior is often described as altruistic, spiteful, selfish, or mutually beneficial. These terms are appealing, but it has not always been clear how they are defined and what purpose they serve. Here, I show that the distinctions among them arise from the ways in which fitness is partitioned: none can be drawn when the fitness consequences of an action are wholly aggregated, but they manifest clearly when the consequences are partitioned into primary and secondary (neighbourhood) effects. I argue that the primary interaction is the principal source of adaptive design, because (i) it is this interaction that determines the fit of an adaptive and (ii) it is the actor and primary recipients whom an adaptation foremost affects. The categories of social action are thus instrumental to any account of evolved function.
- Altruism and spite are pervasive, but widely misconstrued, phenomena.
- Altruism has positive effects on recipients of the primary interaction and negative effects on the "neighborhood", whereas spite has the opposite effects.
- Only the primary interaction determines the fit of an adaptation.
- Thus, altruism and spite are best understood as pertaining to the consequences of actions on those primarily, rather than secondarily, involved.
- When appreciated properly, these phenomena help us to recognize adaptive design.