However, chimpanzees in this study went beyond the basic tenets of the social contract and demonstrated what could be considered the foundation of social solidarity. In 95 trials chimpanzees that received a grape were significantly more likely to refuse the high-value reward when their group mate only received a carrot (p = 0.008). Even those who benefitted from inequality recognized that the situation was unfair and they refused to enjoy their own reward if it meant someone else had to suffer.
This particular behavior was not seen in the monkeys. Did a sense of fairness evolve along with cooperation among higher primates? Link
(image credit: Flickr user Owen Booth)