Guided by the theories of inclusive fitness (Hamilton, 1964) and reciprocal altruism (Trivers, 1971), two studies tested hypotheses related to the notion that emotions are part of an evolved psychological system that functions, in part, to regulate social exchange. Emotional experience and exchange behaviors were predicted to vary based on both the structure of the situation and the type of relationship one has with a partner. Due to an absence of inclusive fitness effects, interaction with non-kin (compared with kin) exchange partners was expected to trigger more intense emotional responses. Study 1 found that, as expected, unfair offers led to feelings of anger, but more so for non-kin partners compared to kin partners. Similarly, fair offers led to feelings of gratitude, but more so for non-kin partners. Study 2 used a 3 (emotion induction: anger, gratitude, control) by 4 (relationship partner: stranger, friend, cousin, sibling) by 2 (social dilemma task: take-some, give-some) experimental design and found evidence in support of the prediction that emotions are more likely to influence exchange behaviors with non-kin partners compared with kin partners. This research extends the social-functional approach to emotions into the context of evolutionary social psychological theory
