Gender, kinship, and the structure of friendship networks among Tsimane horticulturalists in lowland Bolivia

Friendship is a central feature of human social organisation, yet most empirical evidence on friendship networks comes from children and adolescents in industrialised societies. Here, we examine adult friendship networks among Tsimane horticulturalists of lowland Bolivia to test predictions about the structure of women's and men's friendship networks and the role of age and kinship in friend choice. Using sociocentric network data from adults in a Tsimane community (N=148), we apply a stochastic actor-oriented model (SAOM) for multi-mode data to analyse same-gender and cross-gender friendship nominations. Men’s and women’s friendship networks were structurally similar, with slightly higher average degree, reciprocity, and transitivity in the women's network and slightly higher inter-individual variation in the men's network. For men, the probability of friendship peaked by age 40 and decline marginally thereafter, and similar-aged men were more likely to be friends. Age effects were weaker among women. Kinship shaped friendship in both genders, though friendship with more distant consanguineal and affinal kin was more consistent among men. Cross-gender friendships were rarer, unreciprocated, and strongly patterned by kinship. Taken together, we find overall gender similarities in friendship network structure, while highlighting some minor gender and age differences, which we compare to studies from industrialised societies. Our study also contributes methodological innovation to the calculation of affinal kinship and to application of SAOMs to multi-mode, interdependent networks.