This study tried to replicate the design of a somewhat famous study by Brosnan and de Waal (2003) with capuchin monkeys in which they are videotaped showing a rejection of a less preferred reward when they observed their partner receiving a more preferred reward for the same level of work (trading a rock for a treat). The authors argued that the evidence showed that capuchins recognize a social inequity -- their partner getting more than them -- and react to this injustice by refusing to play or refusing to accept the lesser reward. We conducted a similarly designed study and included a condition in which the preferred food was just presented in a bowl with a clear plastic lid, so rather than the social inequity observed, the monkeys saw their preferred food was available, at least visually, but they were being given a less preferred reward. Rather than a socially-measured and mediated behavior, this kind of rejection would be considered a response to a violation of expected reward.
We found a clear demonstration of both reactions, increased rejection to play based on social inequity and increased rejection to play based on a violation of expected reward. Still the rejection to a social inequity became more severe across trials, especially when work was involved. This suggested that the monkeys typically rejected the task out of hand when they were not given what they could see was an available better reward, but they also rejected playing at an increasing rate if that preferred reward was being given to a cage mate and they were doing the same work as the cage mate. So they were not insensitive to the social inequity played out in the game, when it was.
Read the published article on social inequity (2009).
CITATION: Neiworth, J. J., Johnson, E. T., Whillock, K., Greenberg, J., & Brown, V. (2009). Is a sense of inequity an ancestral primate trait? Testing social inequity in cotton top tamarins (Saguinus oedipus). Journal of comparative psychology (Washington, D.C. : 1983), 123(1), 10–17. https://doi.org/10.1037/a0012662
View the poster on social inequity presented at the Psychonomic Society (2006).