Chimpanzees do not play fair

Unlike humans, chimpanzees do not tend to make fair offers and reject unfair ones, they thus behave like selfish economists rather than social reciprocators, according to researchers from the
Max Planck Institute for Evolutionary Anthropology in Leipzig. They tested chimpanzees’ inclination to be fair with a simplified version of the so-called ‘ultimatum game’.

The experimental setup was such that one chimpanzee – the proposer – would propose an offer of raisins to another chimpanzee – the responder – by partially pulling out a tray of raisins. In
order for both of them to be able to reach the tray, the responder would then have to pull it a little further, thus accepting the offer. The setup remained the same throughout the experiment,
only the number of raisins available to either one of the chimpanzees would differ.

In each version of the game, the proposer would have to choose between an unfair spilt – eight raisins for the proposer, two for the responder – and other variants, including:
– a fair offer (five raisins each);
– a hyper-fair offer (two raisins for the proposer, eight for the responder);
– no choice (both trays contain eight raisins for the proposer and two for the responder);
– a hyper-unfair offer (ten raisins for the proposer, none for the responder).

Both animals were able to see each other at any time. Access to the baited trays, however, was clearly separated.

The experiment showed that chimpanzee responders accepted any offer apart from the hyper-unfair one, which was reliably rejected. This reaction is contrary to the behaviour humans tend to show
in this situation.

When applying the ultimatum game to humans, the proposer is given money by the experimenter. He or she can then offer part of it to the responder. If the responder accepts the offer, both can
keep the money. If rejected, none of the subjects gets anything. As the proposer has to fear losing his or her own share if the responder considers the offer to be unfair and rejects it, the
human proposer will usually offer an amount close to 50% that is likely to be accepted.

This sensitivity to unfair offers, as well as being willing to pay a cost to punish someone who has made an unfair offer, contradicts economic models of pure self-interest and is unique to
humans, researchers say.