Frustrations that arise from perceived income disparity – pay inequality – has been seen in studies involving primates, who are similarly enraged when they earn “unequal compensation” as compared to their cohorts. While human work relations are far more complicated, due in great measure to employees who often labor at different tasks than their coworkers, fair pay is not automatically the same as equal pay. Employees generally understand that some employees should get paid more; what causes disgruntlement is the perception of unfairness which may result from a lack of transparency during the process of allocating rewards based on quantifiable achievements.
See "The Fairness Factor", Bourree Lam, The Atlantic, February 25, 2016