Background: Friendship and sibling relationships differ in that there is an element of choice in friendships, whilst the latter represent a fixed and given relationship. The present study set out to investigate the temperamental correlates of friendships and sibling relationships, as well as to examine their genetic and environmental components.
Method: Two hundred and twelve same-sex twin-pairs (102 MZ and 110 DZ; 104 male pairs and 108 female pairs) aged between 12 and 15, together with their parents, comprised the sample. The twins themselves reported on both negative and positive aspects of the two relationships, and all family members provided reports of the twins' temperament (mothers, fathers, and adolescent self-reports).
Results: Analyses revealed that negative aspects of both the sibling relationship and friendship were related to emotionality, while the positive aspects of these relationships were associated with sociability and activity. Genetic influences and the shared environment accounted for variance seen in the sibling relationship. In addition, nonshared environment accounted for a moderate amount of the sibling relationship variability and explained most of the variance for the friendship components. Finally, bivariate analyses of the covariance between temperament and close relationships highlighted the importance of nonshared environmental influences during adolescence.
Conclusion: Three themes emerged. First, congruent temperamental expressions were seen across these two relationships' contexts. Second, friendship and the sibling relationship have distinct etiological profiles, indicating the child-specific character of adolescent friendships versus the reciprocal nature of the sibling relationship. Finally, the common influences to temperament and relationship quality provided evidence for the validity of adolescents' unique, nonshared experiences.