Often, it's less irritating if a friend or family member receives slightly better treatment than you do, compared to getting the short end of the stick among strangers. The same holds true for chimpanzees, according to scientists at the Yerkes National Primate Research Center. From Scientific American:
"In the new work, the researchers tested the reactions of pairs of chimpanzees to exchanges of food that varied in quality. The animals received either a grape, which they coveted, or a less appealing cucumber and they could see what their partner obtained. In pairs of chimps that had lived together since birth, the individual given the cucumber was less likely to react negatively to the situation than was the short-changed member of a pair that did not know each other as well. Indeed, chimps in the short-term social groups refused to work after their partner received a better reward for the same job."
