Humans (Homo sapiens) fail to show an inequity effect in an up-linkage analog of the monkey inequity test

Brosnan and de Waal (Nature 425:297-299, 2003) claimed that if a capuchin sees another capuchin receiving a superior food, she tends to reject an inferior, previously acceptable food. They related this phenomenon to human inequity aversion. This phyletic extension is down linkage, because nonhuman research is interpreted in terms of human research. The present experiment makes an up-linkage test of this claimed connection by attempting to reproduce the capuchin-inequity effect in humans. In Experiment 1's equity condition, a subject and an adjacent confederate each clicked a computer mouse to mark the number 7 from a random numbers table, earning 0.5 yen per mark. In the inequity condition, the confederate's pay rate was twice that of the subject. There was no between-condition difference in quitting times or likelihoods. Experiment 2 replicated Experiment 1 except, before beginning, the subject and confederate clicked a mouse over a rapidly switching message that said they would earn either 0.5 or 1 yen per marked seven. For the equity condition in this rigged test, subject and confederate stopped the message at 0.5 yen, while in the inequity condition, these values were 0.5 and 1 yen, respectively. Now, inequity-condition subjects quit sooner than equity-condition subjects. Experiment 1 found no inequity effect, but Experiment 2 did. These results show that: (a) a sense of control/responsibility may be critical to an inequity effect and (b) the inequity effect putatively present in capuchins cannot be reproduced in an up-linkage human analog of that research, thereby calling this linkage into question. This report exemplifies that up- and down-linkage tests are often requisite to establish commonality of psychological process between nonhuman primates and humans.