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

Abstract

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.

Appendix

Script for Experiment 1 (translated from Japanese)

Thank you for participating in my experiment. In this experiment each of you will sit in front of a computer. On the computer will be many numbers. Your job will be to use mouse clicks to cross out all instances of the number seven, wherever they are seen, on the screen. (The experimenter then illustrates how this is done on a computer.) I have decided to pay you (points at the subject) 0.5 yen for each seven you cross out with a mouse click and you (points at the confederate) 1.0 yen for each crossed-out seven. The computer will automatically tally the money each of you earns. Each of you may work on this task as long as you wish, and each of you may quit whenever you wish. Are there any questions?

Script for Experiment 2 (translated from Japanese)

Thank you for participating in my experiment. In this experiment each of you will sit in front of a computer. On the computer will be many numbers. Your job will be to use mouse clicks to cross out all instances of the number seven, wherever they are seen, on the screen. (The experimenter then illustrates how this is done on a computer.) I have decided to pay you the amount of money that is listed when you play this game of skill. (The experimenter points at the rapidly switching numbers on the screen.) Notice that the image shifts quickly between 0.5 and 1.0 yen. The money that you will be paid for each crossed-out seven will be determined by the image on the screen when you click the mouse. (After the subject clicks the mouse and the computer image reads “0.5 yen,” the experimenter points at the subject.) You will earn 0.5 yen for each 7 you cross out with a mouse click. (Next the confederate clicks the mouse and the computer image reads “1.0 yen.” Then the experimenter points at the confederate.) You will earn 1.0 yen for each crossed-out seven. The computer will automatically tally the money each of you earns. Each of you may work on this task as long as you wish and each of you may quit whenever you wish. Are there any questions?