From 2019... Some apes don't seem to have a sense of procedural (un)fairness, not getting upset when they have to spend greater effort for the same reward than their peers

From 2019... The Biology of Fairness. Sarah F. Brosnan. Chapter 2 in Social Psychology and Justice. Edited By E. Allan Lind. November 2019. https://www.taylorfrancis.com/books/e/9781003002291/chapters/10.4324/9781003002291-2

Rolf Degen's take: https://twitter.com/DegenRolf/status/1325861779474354182

Abstract: Humans are not the only species that is sensitive to outcome inequality. Other species, too, respond negatively to getting less than a social partner, and this breadth suggests a common biological foundation. Across species, negative responses to inequity are linked to the degree to which those species cooperate, and within cooperative tasks, subjects are sensitive to inequity, refusing to work with partners who do not equitably share rewards. Together, these results suggest that inequity aversion evolved because it provides a mechanism by which individuals can judge the relative value of their cooperative partners. In addition, humans and chimpanzees respond negatively to getting more than a partner, suggesting that they can inhibit this short term benefit for the long term gain of maintaining cooperative relationships. The comparative approach has been extremely useful in highlighting the roots of inequity responses and, thereby, helping us to better understand our own sense of fairness.