Anna Rotkirch in Evolutionary Psychology:
Although gratitude is a key prosocial emotion reinforcing reciprocal altruism, it has been largely ignored in the empirical literature. We examined feelings of gratitude and the importance of reciprocity in same-sex peer relations. Participants were 772 individuals (189 men; mean age = 28.80) who completed an online survey using a vignette design. We investigated (i) differences in reported gratitude and the importance of reciprocity among same-sex siblings and same-sex friends, and (ii) how relationship closeness moderates these associations. Based on the theory of kin altruism, we expect that people would feel more grateful towards friends than towards their siblings, and that lack of gratitude or failure to pay back a loan would bother more with friends than with siblings, irrespective of emotional closeness. Results showed that levels of gratitude and expectations of reciprocity were higher towards friends compared to siblings. This was the case also after controlling for emotional closeness. Being close generally made participants feel more grateful and expect lower displays of gratitude in the other. Closeness was also strongly associated with emotional gratitude among siblings compared to friends. We conclude that feelings and displays of gratitude have a special role in friendships. Although a close sibling may elicit as much gratitude as a friend does, even a very close friend is not exempt from the logic of reciprocity in the same way that a sibling is.