Four levers of reciprocity across human societies: concepts, analysis and predictions

The social leverage effect: Institutions transform weak reputation effects into strong incentives for cooperation

Institutions allow cooperation to persist when reciprocity and reputation provide insufficient incentives. Yet how they do so remains unclear, especially given that institutions are themselves a form of cooperation. To solve this puzzle, we develop a mathematical model of reputation-based cooperation in which two social dilemmas are nested within one another. The first dilemma, characterized by high individual costs or insufficient monitoring, cannot be solved by reputation alone. The second dilemma, an institutional collective action, involves individuals contributing to change the parameters of the first dilemma in a way that incentivizes cooperation. Our model demonstrates that this nested architecture creates a leverage effect. While insufficient on its own to incentivize cooperation in the first dilemma, reputation incentivizes contributions to the institutional collective action, which, in turn, strengthen the initially weak incentives for cooperation in the first dilemma. Just as a pulley system transforms minimal muscular strength into significant lifting capability, institutions act as cooperative pulleys, transforming initially weak reputational incentives into powerful drivers of cooperative behavior. Based on these results, we suggest that institutions have developed as social technologies, designed by humans to exploit this social leverage effect, just as material technologies are designed to exploit physical laws.

Islands of cooperation emerge by stigmergic interactions in iterated spatial games

This article focuses on the emergence of cooperation in societies of self-interested agents. In particular, it introduces a mechanism based on indirect-stigmergic-interactions between agents moving in an environment, to express the likeliness of finding cooperative partners. On the one hand, agents that find themselves cooperating with others emit pheromones in their current location, expressing the presence of agents willing to cooperate. On the other hand, agents that sense pheromones tend to move towards regions with a higher pheromone concentration. Results show that this mechanism leads to the emergence of spatial regions where cooperation can be effectively sustained, and in which agents can overall get better payoffs than those agents not taking into account pheromones in their choices.

Explaining variation in the kinship composition of mammal groups

Variation in cooperative behavior across mammals is strongly related to the kinship composition of groups. Although the factors affecting average genetic relatedness within groups have been studied, the factors that contribute to the production of different categories of kin remain underexplored. Here, I use a mathematical model to explore the factors that determine the proportion of full siblings, maternal half-siblings, paternal half-siblings, and non-siblings within mammal groups. The results suggest that the production of paternal half-siblings is increased by high male reproductive skew and a female-biased sex ratio, the production of maternal half-siblings is increased by high female reproductive skew and male-biased sex ratio, and that there are two routes to the production of full siblings: either high reproductive skew in both sexes (as seen in cooperatively breeding species) or pair-bond stability within groups of low reproductive skew (as seen in humans). These results broadly correspond to observed variation in sibling composition across mammals.

Why cultural distance can promote - or impede - group-beneficial outcomes

Quantifying the distance between cultural groups has received substantial recent interest. A key innovation, borrowed from population genetics, is the calculation of cultural FST (CFST) statistics on datasets of human culture. Measuring the variance between groups as a fraction of total variance, FST is theoretically important in additive models of cooperation. Consistent with this, recent empirical work has confirmed that high values of pairwise CFST (measuring cultural distance) strongly predict unwillingness to cooperate with strangers in coordination vignettes. As applications for CFST increase, however, there is greater need to understand its meaning in naturalistic situations beyond additive cooperation. Focusing on games with both positive and negative frequency dependence and high-diversity, mixed equilibria, we derive a simple relationship between FST and the evolution of group-beneficial traits across a broad spectrum of social interactions. Contrary to standard assumptions, this model shows why FST can have both positive and negative marginal effects on the spread of group-beneficial traits under certain realistic conditions. These results provide broader theoretical direction for empirical applications of CFST in the evolutionary study of culture.

Group Formation and the Evolution of Human Social Organization

Humans operate in groups that are oftentimes nested in multilayered collectives such as work units within departments and companies, neighborhoods within cities, and regions within nation states. With psychological science mostly focusing on proximate reasons for individuals to join existing groups and how existing groups function, we still poorly understand why groups form ex nihilo, how groups evolve into complex multilayered social structures, and what explains fission-fusion dynamics. Here we address group formation and the evolution of social organization at both the proximate and ultimate level of analysis. Building on models of fitness interdependence and cooperation, we propose that socioecologies can create positive interdependencies among strangers and pave the way for the formation of stable coalitions and groups through reciprocity and reputation-based partner selection. Such groups are marked by in-group bounded, parochial cooperation together with an array of social institutions for managing the commons, allowing groups to scale in size and complexity while avoiding the breakdown of cooperation. Our analysis reveals how distinct group cultures can endogenously emerge from reciprocal cooperation, shows that social identification and group commitment are likely consequences rather than causes of group cooperation, and explains when intergroup relations gravitate toward peaceful coexistence, integration, or conflict.