Human cooperative behavior

Summary Evolutionary theory provides the biological sciences, with a fundamental and powerful model to explain the emergence of cooperative behavior. A detailed explanation for the existence of cooperation between related individuals is provided by the theory of kin selection. When kin cooperate the helper gives the receiver an advantage and thereby increases the relative probability that copies of his own genes are present in the next generations. However, one cannot explain examples of apparent altruism through kin selection, because in these cases unrelated individuals interact. The answer for many of these examples is provided by the theory of reciprocal altruism, where individuals behave reciprocal by returning help to a previous donor. By applying these two theories it is possible to explain many, but by far not all cooperative situations. There have to be other mechanisms that lead to cooperation and sustain already established cooperation. In my dissertation I have tested empirically new models and predictions of how cooperation between unrelated humans can be established. This research is especially important because we interact in a close net of relationships, where cooperation between unrelated individuals plays one of the main roles. Modern human societies are impossible to imagine without cooperation between unrelated individuals. By identifying the circumstances under which cooperation is stable between unrelated individuals, it will be possible to understand the deciding factors in politics, economy and in our private lives. As a consequence we would be provided with intellectual tools to positively influence the deciding factors by alternating the circumstances accordingly. We are often not aware of the importance of cooperation between unrelated partners in our daily lives. Regularly people find unconsciously cooperative solutions, for instance when they try simultaneously to walk through a narrow door. Some professions depend very strongly on cooperative behavior between unrelated colleagues. To act uncooperatively in such a profession can endanger the health or even the lives of the colleagues (e.g. firemen and firewoman). Cooperative strategies for these kinds of situations have to have evolved and need to be evolutionary stable, otherwise we would hardly ever find cooperative behavior in the present and then only between related individuals. According to the evolutionary theory the cooperative strategies found today, also have to provide an advantage to the bearer. For a long time economists and biologists have been interested in the emergence and sustainability of cooperative behavior. Nevertheless, only with the introduction of game theory, a mathematical basis was established to incorporate this behavior into biological evolutionary models. From then on it was possible to make predictions with the help of theoretical models, about the circumstances under which certain behavioral patterns emerge and what underlying mechanisms possibly sustain these patterns. In my dissertation I have empirically tested predictions of circumstances that promote cooperative behavior between unrelated humans. The main results of my work are the following: (i) Humans often donate money to charity. On first sight this seems to be a disadvantageous behavioral trait. Donations to charity include costs that reduce the direct fitness of the individual. However, it has to be beneficial to the bearer, otherwise it would be eliminated from the population through evolutionary processes. The study showed that there is indeed an advantage. By donating money to charity (here to UNICEF) one builds up a good personal reputation in the own social group. Participants that donated to UNICEF received with higher probability help from other participants and were as well more likely to be voted the group representative. (ii) Reputation is known to be an important currency in indirect reciprocity games. Humans therefore should also try to establish a good reputation in other social games, when this reputation is known in future indirect reciprocity games. Humans are in general unable to sustain a public resource that everybody is free to overuse anonymously. Is it possible that humans sustain a public resource if the use of the resource is linked to the personal reputation? The experiment showed, that the risk of loosing a good reputation by overusing the public resource actually lead to sustaining it. Furthermore the public resource was not only sustained, but also turned out to be surprisingly profitable to all group members. (iii) A theoretical model supplied a new possibility to sustain a public resource and hereby make humans act cooperatively. The strikingly simple idea was to introduce the possibility not to participate in the public goods group and instead use a personal resource with a low but sure payoff. The prediction was an always recurring rise to dominance, of three strategies ((a) not to participate in the public goods group, (b) participate in the public goods group and to cooperate within the group or (c) participate and to defect within the group) within the population. This dynamic was expected because whenever most members of the population choose the same strategy, one of the other two strategies had a higher payoff. The same type as the predicted dynamic has also been found in models of the famous children game of “rock-paper-scissors”. The model predicted that the public resource is sustained by the ongoing dynamic, which is liked with a recurring rise of cooperative behavior. Is it enough to supply humans with the possibility not to participate in the public goods game to produce such recurring rise of cooperation? The dynamic was established as predicted, whereby the changing dominance of the three strategies with repeated cooperative phases could be observed and the resource was on average sustained. (iv) When humans make decisions about using a public resource, which at some times are reputation relevant and at other times are not reputation relevant, do they use this information strategically? In this study it was shown that, humans are aware when their decisions are not reputation relevant and immediately reduce their cooperation to maximize their personal profit. Once more, as soon as the decisions about using the public resource were linked to the reputation, cooperation was much higher and the resource was sustained. (v) In some potentially cooperative situations humans’ meet partners from outside the own social group. These “strangers” have a reputation that they have built in another social group. Do humans put a different value on a strangers’ reputation in comparison to the reputation of members of the own social group? It was shown that it is not relevant if the reputation was built within the own or in a foreign social group. In summary we found the following: Humans behave uncooperatively, when it is to the personal advantage. However, certain circumstances lead to cooperative behavior in humans. Reputation building is one of the most important mechanisms in this context, which enables us to cooperate even with not related strangers. However humans consciously make strategic use of situations where they do not harm their reputation by behaving uncooperatively. Nevertheless, even in completely anonymous situations it is possible to create circumstances, like introducing optional participation in the public goods situations, which promote cooperative behavior in humans.Contents Summary 7 Zusammenfassung 11 Introduction 1 Cooperation 17 2 Game Theory 18 3 General approach to testing 20 4 Thesis outline 23 Chapters I Donors to charity gain in both indirect reciprocity and political 29 reputation II Reputation helps solve the 'tragedy of the commons' 37 III Volunteering leads to rock-paper-scissors dynamics in a public 47 goods game IV A human cooperation strategy that is conditional on being 59 recognized in other situations V Reputation is valuable inside and outside the own social group 69 Conclusion 81 Danksagung 83 References 85 Glossary 91 Appendix 93 Curriculum vitae 99 Erklärung 10