Sometimes, he switched the food with a less desirable one without the monkey’s knowledge. In these cases, the monkeys responded by becoming upset and refusing the less valuable foods. This demonstrated that these monkeys had expectations about what they should have received. What Tinklepaugh did not do was investigate how they responded to a partner getting a better reward. Would the monkeys then form expectations based on what their partner received, and respond negatively if they got a less good reward than their partner? Our group has now performed this experiment.

Our studies have investigated whether primates would be willing to complete an exchange interaction (return a token and accept a food reward) if their partners got a better food for the same “ price, ” an exchange (see Figure 19.4 for a sketch of the test set-up). We tested capuchin monkeys and chimpanzees in pairs, with members of their social groups as their partners. In each case, one member of the pair, the partner, received a good reward (in this case a grape) III. SOCIAL DECISION MAKING, NEUROECONOMICS, AND EMOTION 293 de Waal, 2005 ). Thus, both the ape and the monkey appear to be responding negatively to receiving less. However, a few possibilities must be eliminated before this can be considered a reaction to inequity. For instance, it may just be “ greed, ” or a desire for a better food that is available, rather than “ envy, ” or a desire for a better food that another individual currently possesses ( Brosnan and de Waal, 2004a ; Wynne, 2004 ). To take this in to account we did an additional control, in which subjects were shown a grape, following which they exchanged and received only a cucumber piece. With the capuchins, this was done in two different ways.

In the first, the subject was isolated, rather than being tested as a member of a pair, and grapes were placed in the partner’s side of the testing chamber prior to each exchange. In this way, subjects saw the grapes pile up while they continued to receive only cucumber ( Brosnan and de Waal, 2003 ). Subjects discriminated between this and the situation in which a partner received the grape, becoming more likely to accept the cucumber over time when no other monkey received a grape ( Figure 19.6a ).

However, in this situation subjects were tested alone, while in all other conditions subjects were tested with a social partner. Although it seems minor, for a gregarious species such a change in the social environment may have large effects.

Much Holding