The survival of the kindest: a theoretical review and empirical investigation of explanations to the evolution of human altruism

Charles Darwin was concerned that his entire theory of evolution by natural selection might be negated by a phenomenon prevalent in a variety of species including humans; namely altruism. If natural selection really favored the survival of the fittest, how could a strategy so irrational as to sacrifice oneself for the well-being of unrelated others survive? A number of scientists have contributed valuable theories to elucidate the �paradox of altruism�. However, in spite of the merits of these theories, there is still dissension about the origins of some particular oddities in the altruistic tendencies of humans, namely why humans act selflessly even when they are unobserved and when they are benefiting a stranger whom they will never meet again. The present doctoral thesis sheds light on answers to the question how human altruism, with all its specific features, could evolve. In the first part, both prominent (e.g., kin selection, reciprocal altruism, etc.) and less recognized theories on the evolution of altruism (e.g., green-beard altruism, the theory of the extended phenotype, etc.) are reviewed. Based on an integrative overview, it is analyzed how much of the altruism puzzle has been solved yet and which specific phenomena are still open to conjecture. With the aim of adding new insights to the issue, the second part of this work presents three empirical studies that investigate in how far prosociality might have been favored (1) by processes of assortation, i.e. the grouping of altruists, and (2) by mating strategies. Indeed, assortation may be invoked as an explanation for the evolution of altruism, if the selfish advantage of egoistic individuals is out-competed by benefits of mutually cooperating altruists. However, to make assortation work as a driver of the evolution of altruism, two prerequisites have to be fulfilled: first, individuals have to be able to distinguish altruists from egoists, and second, altruists have to elect like-minded individuals for mutual cooperation. The first study investigates whether humans are really able to identify altruists based on first impression. To test this, judges watched 20-second silent video clips of unknown target persons and were asked to estimate the behavior of these target persons in a dictator game, which measures prosociality. Estimates were significantly better than chance indicating that humans can identify the altruistic dispositions of unknown persons. The second study investigates whether individuals, in genuine groups, can identify the altruistic tendencies of their daily interaction partners. It further examines whether prosociality influences the formation of friendships in such that individuals assort themselves along the dimension of altruism. Students of six secondary school classes played an anonymous dictator game that functioned as a measure of altruism. Afterwards and unannounced, the students had to estimate their classmates� decisions and did so better than chance. Sociometry revealed that altruists were friends with more altruistic persons than were egoists. The results thus confirm the existence of the two prerequisites for the evolution of altruism through assortation: the predictability of altruistic behavior and the association of altruists. However, although the theory of assortation may explain the evolution of altruism in general, it does not explain the occurrence of inter-individual differences in altruism. The third study deals exactly with this matter. It investigates whether different levels of prosociality might have evolved as a result of different mating strategies, namely inter-individual variations in the propensity to engage in either short-term mating or long-term mating. Specifically, it assumes that prosociality is a necessity for acquiring a long-term partner, especially if an individual has to compensate for deficits in physical attractiveness. To find out whether this idea is true, the study tested whether individuals look out for different levels of prosociality depending on whether they are searching for a short-term mate or a long-term mate. Judges watched short video-clips of target persons and received additional information on the targets� prosociality. Judges were then asked to rate each of the target persons with regard to their desirability as a short-term and long-term mate. While prosociality was a significant predictor for long-term desirability, it was irrelevant when subjects chose a short-term mate. The results suggest that although altruism is costly, at least for some individuals it might be a wretched necessity to obtain access to mates and to reproduce. In the general discussion, the results of all three studies are consolidated. Conclusions are drawn as to the consequences of these findings for the study of human altruism. Finally, directions for future research are presented.