The costs of family and friends: an 18-month longitudinal study of relationship maintenance and decay

Evolutionary theory predicts that kin relations will be distinct from friendships, but recent studies have suggested a degree of similarity between these two types of relationships. In this longitudinal study, we examined the influence of kinship on the maintenance costs of social relationships. We followed 25 students over an 18-month period as they made the transition from school to university and examined the association between kinship, relationship maintenance and decay. The emotional intensity of friendships, in comparison to kin relations, was more sensitive to decreases in contact frequency and more sensitive to decreases in the number of activities done together. These results demonstrate that important differences between kin relations and friendships emerge when the relationships are considered longitudinally and suggest that the costs of maintaining friendships are much higher than the costs of maintaining kin relations.