this might not hurt, but close your eyes anyways — In a much-noted laboratory experiment several years ago, described in the report "Monkeys Reject Unequal Pay," the primatologists Sarah F. Brosnan and Frans B.M. de Waal trained capuchin monkeys to perform a certain task for which they received cucumber slices. The monkeys performed just fine, until they were permitted to see others being rewarded with grapes, a higher-value payment.