Having opportunities to leave an economic relationship can be beneficial to prevent being exploited by an uncooperative counterpart. However, leaving a relationship is risky too, especially when we consider the survival of individuals depending on its dynamic state of resource. They may not be able to find a better partner after the exit or may lose the cumulative benefit from staying in the same relationship and lose their lives. The cost and benefit of leaving, and the accompanying pattern of social network rewiring, may depend on the interactions between individual decision-making, the population-level distribution of strategies, and the environmental conditions.
Previous research has suggested that human societies established in historically and environmentally harsh environments have less relational mobility because they cannot help but stay in stable relationships to survive. It has also been suggested that the cooperation level would increase in societies with higher mobility since poor cooperators would be easily left by their opponents. Here, we investigated whether individuals living in harsher environments become more tolerant of poor partners, less likely to exit, and less cooperative, even when they have the option to exit.
Using individual-based model simulations, we investigated how these factors might influence the evolution of cooperation in a public goods game. In some conditions, individuals could exit from the current relationship if their partner contributed less than their tolerance level. Individuals who lost their partner were matched randomly with another who had also lost a partner. We also assumed that agents’ survival was contingent upon accumulated resources obtained through the gameplay, with an upper limit of the storage assumed in some conditions.
In our model, conditions with an exit option, a resource storage limit, or both did not show a decline in cooperation level when the environment was less harsh, while the condition with neither an exit option nor storage limit observed a decline in cooperation level and populations were more likely to extinct. In harsher environments, exit events were less likely to occur, but the tolerance level of poor cooperators was not lower than in easier environments. The conditions with the exit option did not show decreases in the cooperation level. However, increases in the cooperation level were not observed as well, even under the less harsh environment. The exit option seems to be a safety mechanism to prevent the drop in cooperation level, but not a driving force to increase it.