S. F. Brosnan and F. B. M. de Waal, “Evolution of Responses to (un)Fairness,” Sci 346 (2014): 1251776; S. F. Brosnan et al., “Mechanisms Underlying Responses to Inequitable Outcomes in Chimpanzees, Pan troglodytes,” Animal Behav 79 (2010): 1229; M. Wolkenten et al., “Inequity Responses of Monkeys Modified by Effort,” PNAS 104 (2007): 18854.