Humans are an intensely social species, frequently exhibiting cooperative behaviors that benefit others who are not closely related at their own expense. One of the enduring puzzles in the social sciences is the origin and persistence of cooperation. Environmental characteristics affect people's survival and reproductive success. Environmental harshness and uncertainty are the most fundamental environmental influences on behaviors. Under the conditions of environmental harshness and uncertainty, cooperation is generally needed for humans to survive and reproduce successfully. There is a broad consensus that people adopt a faster life-history strategy by focusing on short-term outcomes in such environments, and may exhibit a lower level of cooperation. However, Rao et al. suggest that when threatened by natural hazards, an independent individual seems to be helpless and powerless. In the struggle to survive, human beings come to each other's aid at such times. Therefore, it is unclear how cooperation varies in different ecological environments.
To examine the relationship between ecological environments and cooperation, we conducted 7 studies. First, we used data from a nationwide survey in China (N=22,652) (Study 1 .1)and used World Values Survey data from 57 countries(N=80,254) (Study 1.2). We used the Human Development Index (HDI) in both Studies 1 .1 and 1.2 and a hardship index in Study 1.2 as indicators of the ecological environments of different areas. We found that individuals living in harsher and more uncertain environments, i.e.， lower HDI or higher hardship index, exhibited greater cooperation. Second, we conducted two surveys to investigate how harshness and uncertainty correlated with cooperation separately (N=628) (Study 1 .3). We measured environmental harshness and uncertainty and responses to the dictator game and the altruistic behaviors scale. We found that environmental uncertainty was associated with greater cooperation and that harshness was associated with less cooperation. Third, to provide a causal link, we conducted two controlled laboratory experiments manipulating environmental harshness and uncertainty using modified public goods games(N=512 for Study 2.1 and N=484 for Study 2.2). We found that people exhibited greater cooperation under the more uncertain condition in both studies. In specific, the contributions in the uncertain condition decreased at a slower rate than those in the certain condition and, in the last five rounds, people contributed more endowments in the uncertain condition than in the certain condition. However, we found inconsistent results for the relation between environmental harshness and cooperation. Finally, we examined the mediating effects of life-history strategy and tolerance of uncertainty on the relationships between the ecological environment and the cooperation (N=777) (Study 3). We found that the tolerance of uncertainty mediated the relation between environmental uncertainty and cooperation. It appeared that people who experienced more uncertain environments were more intolerance of uncertainty, and accordingly, they exhibited greater cooperation.
Taken together, these findings suggest that harsher and more uncertain environment could promote cooperation and environmental uncertainty may play the most important role in facilitating cooperation. Additionally, the tolerance of uncertainty partially accounts for the positive relationship between environmental uncertainty and cooperation. The present study contributes to a better understanding of the evolutionary origins of cooperative behavior and provide theoretical foundations for promoting cooperation.
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