Last week I posted a clip in which Frans de Waal described observations of a monkey offended by getting lower pay for completing a task (a bit of cucumber rather than a grape) in comparison with another monkey. This was actually part of a 15-minute TED talk that's worth viewing in its entirety. Some of it's funny, but mostly it's thought-provoking.
At the end de Waal adds a few interesting facts about the unequal pay experiment. A similar one was done with chimpanzees in which the recipient of the better reward (the grape rather than the bit of cucumber) rejected the grape until the other chimpanzee was paid in grapes as well. Not all the chimps behaved with such a high level of integrity, but then again, neither do all humans.