Animal behaviour Fair refusal by capuchin monkeys

We have shown that animals compare their own rewards with those of others, and accept or reject rewards according to their relative value. Our aim was not to demonstrate that capuchin monkeys make a human response to inequality, but rather to elucidate evolutionary precursors to inequity aversion. We use this term as in ref. 2 - ``people resist inequitable outcomes; that is, they are willing to give up some material pay-off to move in the direction of more equitable outcomes'' - and specifically focus on ``disadvantageous inequity aversion''. The monkeys in our experiment could not change the reward division, and hence could not actively avoid inequality, but we wanted to determine whether they would at least recognize inequality if subjected to it. We found that the capuchins reacted negatively, refusing to complete the interaction.