People who share are selfish at heart Dr Adrian Jaeggi, one of the authors of the study and an anthropologist at the University of California Santa Barbara, said: "Among chimpanzees for instance, hunting and possession of meat is biased to males, and in particular dominants, who may trade meat for coalitionary support or mating. Among human foragers, food production is more balanced across individuals, and there is a substantial risk of shortfalls even for the best producers, resulting in a need to buffer this risk through reciprocal sharing as expressed here by a stronger effect of in-kind reciprocity. The researchers examined 32 separate studies looking at how humans and primates share resources - 16 in indigenous human tribes from all over the world, eight in great apes and eight in monkeys. all Statistical analysis of the findings allowed Dr Jaeggi and his colleagues to examine the main underlying effect for sharing behaviour in individuals, included kinship, scrounging and reciprocity. They found that in humans, for example, the expectation of reciprocal sharing explained about a quarter of the food given, while the other reasons accounted for far less. Dr Jaeggi, whose study is published in the Proceedings of the Royal Society B, said more work was needed to understand how other factors like social rank and proximity may influence reciprocal behaviour. But he said: "Evidence presented here suggests that the responses of possessors may be affected by some form of score keeping. Ultimately, no matter what proximate mechanisms brought about the significant correlations between giving and receiving observed here, they yield benefits to both individuals and should thus be under positive selection.