Katharina Hamann1, Felix Warneken2, Julia R. Greenberg3 & Michael Tomasello1
Collaboration encourages equal sharing in children but not in chimpanzees Humans actively share resources with one another to a much greater degree than do other great apes, and much human sharing is governed by social norms of fairness and equity1–3. When in receipt of a windfall of resources, human children begin showing tendencies towards equitable distribution with others at five to seven years of age4–7. Arguably, however, the primordial situation for human sharing of resources is that which follows cooperative activities such as collaborative foraging, when several individuals must share the spoils of their joint efforts8–10. Here we show that children of around three years of age share with others much more equitably in collaborative activities than they do in either windfall or parallel-work situations. By contrast, one of humans’ two nearest primate relatives, chimpanzees (Pan troglodytes), ‘share’ (make food available to another individual) just as often whether they have collaborated with them or not. This species difference raises the possibility that humans’ tendency to distribute resources equitably may have its evolutionary roots in the sharing of spoils after collaborative efforts. Among great apes, only humans are true collaborative foragers8,9,11. Other apes forage in small parties, but they do not actively work together jointly to produce food—the only exception being chimpanzee grouphunting of monkeys12,13. In contrast, humans in all societies produce significant portions of their food through collaborative efforts, even bringing the results of their labour back to some central location to share with other group members14,15. After group-hunting, chimpanzees mostly share only under pressure of harassment by others16 or else reciprocally with coalition partners17. Human children actively share valuable resources with others to some degree from early in ontogeny. A fairly well-established pattern across cultures is that three- to four-year-old children tend to divide a windfall of resources unequally, keeping the majority for themselves4–6,18,19. As they approach school age, they begin to share more equally4,5,7,18,19. But given that humans generate many or most of their resources collaboratively, a plausible hypothesis is that children would share a resource more equitably at an earlier age if it was not provided by adults as a windfall, but if instead they had to work together to produce it20. Furthermore, we might expect this positive effect of collaboration on sharing to be confined to humans, among great apes, as only they have an evolutionary history of obligate collaborative foraging8,9,11. In the current series of experiments, therefore, we presented pairs of human children and pairs of chimpanzees with resource distribution problems in which one individual had control of more than half of the resources and could choose whether or not to share them equally with their partner. The basic variable was whether the initial unequal distribution of resources resulted from a collaborative effort in which each contributed equally, or whether it came from some non-collaborative source (for example as a windfall or as a result of each individual working on their own). In study 1, pairs of either two- or three-year-old children were in a room by themselves. In the ‘collaboration’ condition, they faced an enclosed board with a rope extruding from each end (Fig. 1a), and they knew from previous experience (from a demonstration phase) that 1
they had to pull together to bring the board towards them. On each end of the board were two rewards (small toys) that could be accessed once the board had been pulled close enough. As the children pulled, one of the toys rolled to the other end of the board such that one child ended up with three toys and the other ended up with only one. In the control,...