The study proposes a model to explain how gift interactions produce changes in interpersonal relations and, subsequently, the structure of social organizations. The authors also suggest that gift interactions are a driving force in the transition of social organization (bands, tribes, and chiefdoms). According to the model, there is a positive correlation between the frequency of gift transactions and economic and social disparities. To verify their theoretical results, the authors compare this model to statistical analysis performed on 155 societies from the Standard Cross-Cultural Sample. The results support the model, as there are positive correlations between gift interactions and social/economic disparities. As additional results, the authors show that some cultural and environmental factors positively correlate to gift interactions, including population density, surplus production, and herding societies.

This study explores the evolution of family systems in non-industrial agricultural societies based on environmental conditions. First, the authors conduct a multi-evolutionary simulation for each family system: absolute nuclear families (nuclear with unequal inheritance), egalitarian nuclear families (nuclear with equal inheritance), stem families (extended families with unequal inheritance), and community families (extended families with equal inheritance). Second, they use Spearman's rank correlation analyses to assess the relation between the 186 non-industrial societies and the types of inheritance relationships, either parent-child (nuclear or extended) or inter-sibling (strongly biased or equal). The results show that the four core family systems are related to wealth and land resources. Other relevant findings are that low polygyny is related to agricultural societies, higher poverty levels to extended families, and accelerated wealth accumulation to strongly biased inheritance.