Trust - the killer app of civilization

From What behaviour in economic games tells us about the evolution of non-human species' economic decision-making behaviour by Sarah F. Brosnan.  On the behaviors required for chimpanzees to conduct trade among themselves.

What is notably absent, however, is the transfer of these barter relationships with humans to trade with one another. We tested this among three highly trained chimpanzees at Georgia State's Language Research Center (the same chimpanzees that showed evidence of strategy use in the Assurance game). These chimpanzees had been taught a symbol language that allowed us to communicate with more specificity than is typically possible. We made tokens representing specific foods (labelled by their symbol) that they could exchange back to an experimenter for food if—and only if—that food was present in their personal bin. In a series of tasks, we then explored whether the chimpanzees would learn to trade tokens that were of no value to them (because the food was not in their bin) to a partner to whom the tokens were valuable (because the food was in the partner's bin) so as to maximize both chimpanzees' benefits.

To cut a long story short, the chimpanzees learned to do so effectively as long as a human experimenter mediated the interactions such that neither chimpanzee could exchange a token with the experimenter for food until they had reciprocated any trades from the partner (we did not restrict which token they had to trade to a partner, just that they traded something). Despite having previously maximized their rewards, within one session of us removing experimenter-mediated quid pro quo, the chimpanzees ceased trading any tokens with their partners. Since they demonstrated all of the necessary cognitive abilities to understand trade, we hypothesized that the issue holding them back was an inability to trust their partners and a lack of third party enforcement mechanisms to make doing so worthwhile.