Competitiveness pervades life: plants compete for sunlight and water, animals for territory and food, and humans for mates and income. Herein we investigate human competitiveness with a natural experiment and a set of behavioral experiments. We compare competitiveness in traditional fishing societies where local natural forces determine whether fishermen work in isolation or in collectives. We find sharp evidence that fishermen from individualistic societies are far more competitive than fishermen from collectivistic societies, and that this difference emerges with work experience. These findings suggest that humans can evolve traits to specific needs, support the idea that socio-ecological factors play a decisive role for individual competitiveness, and provide evidence how individualistic and collectivistic societies shape economic behavior.endogeneous preferences | social learning | field experiment I ndividuals frequently face a decision that can affect their wellbeing and even survival: to compete or not to compete. Natural and social scientists argue that competitions and the right dose of competitiveness significantly determine not only the future of the individual but even the evolution of the whole species (1, 2). However, behavioral experiments with humans show that there are large differences in competitiveness between individuals that cannot be readily explained by genetic endowments, abilities, or risk attitudes (3-9).A possible explanation of the large variations in human competitiveness is based on learning theories. Observational learning describes individuals' tendency to adapt by imitating successful behavior. Social or cultural learning models attribute an important role to individual experiences in the social and physical environments for the formation of traits and norms (10-13). Thus, individual variations in competitiveness may be the result of exposure to different environments and pressures.In this study we investigate how local natural forces cause human competitiveness to change. We compare competitiveness in geographically proximate individualistic and collectivistic fishing societies with experiments. Our key exogenous variation is whether fishermen spend their lives at a lake or at the sea. The main difference between these societies is that the sea ecology favors fishermen to work in collectives, whereas the lake ecology guides them to fish in isolation. As a result, the output of the fishermen in the individualistic lake societies should depend on their willingness to compete with other fishermen for the best fishing spots, the best sales, and the most beneficial trade relations, whereas such individual competitiveness is unnecessary in the collectivistic sea societies. We hypothesize that these differences result in changes in individual competitiveness and that lake fishermen become more competitive than sea fishermen with exposure to these local pressures.The experiments we used in the field facilitated comparisons and control of causal factors (14). Fishermen at the sea and at...