When rainfall does not meet crop water requirements, supplemental irrigation is needed to maintain productivity. On-farm ponds can prevent excessive groundwater exploitation-to the benefit of the whole community-but they reduce the cultivated area and require investments by each farmer. Thus, choosing the source of water for irrigation (groundwater vs on-farm pond) is a problem of collective action. An agent-based model is developed to simulate a smallholder farming system; the farmers' long-/ short-view orientation determines the choice of the water source. We identify the most beneficial water source for economic gain and its stability, and how it can change across communities and under future climate scenarios. By using on-farm ponds, long-view-oriented farmers provide collective advantages but have individual advantages only under extreme climates; a tragedy of the commons is always possible. Changes in farmers' attitudes (and hence sources of water) based on previous experiences can worsen the economic outcome.
Trust is an important concept that intersects a number of different disciplines, including economics, sociology, and political science, and maintains some meaning even in the natural sciences. Any situation where non-simultaneous exchanges between living organisms take place involves a problem of trust. We used computer simulations to study the evolution of trust in non-simultaneous exchange situations formalized by means of a Trust game. We found that trust and reciprocity-based cooperation are likely to emerge only when agents have the possibility of building trustworthy reputations and when the information regarding agents' past behaviors is sufficiently spread in the system. Both direct and indirect reciprocity play a role in fostering cooperation. However, the strength of the latter is greater under most of the examined conditions. In general, our findings are consistent with theories arguing for a positive feedback relationship between trust, reputation, and reciprocity, leading together to higher levels of cooperation.
The debate on the causes of conflict in human societies has deep roots. In particular, the extent of conflict in hunter-gatherer groups remains unclear. Some authors suggest that large-scale violence only arose with the spreading of agriculture and the building of complex societies. To shed light on this issue, we developed a model based on operatorial techniques simulating population-resource dynamics within a two-dimensional lattice, with humans and natural resources interacting in each cell of the lattice. The model outcomes under different conditions were compared with recently available demographic data for prehistoric South America. Only under conditions that include migration among cells and conflict was the model able to consistently reproduce the empirical data at a continental scale. We argue that the interplay between resource competition, migration, and conflict drove the population dynamics of South America after the colonization phase and before the introduction of agriculture. The relation between population and resources indeed emerged as a key factor leading to migration and conflict once the carrying capacity of the environment has been reached.
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