Inequity aversion in miceWhitney Swain

Advisor: Dr. Lustofin

Humans are a social species who often feel like they have been treated unfairly if they do not receive an equal reward for performing the same task as another individual. Humans also tend to react negatively when thought to be treated unfairly. It is thought that these behaviors are the driving forces for cooperation (Brosnan and de Waal, 2003, 297; Range et al., 2009, 340). Previous studies have provided evidence that species other than humans show inequity aversion (van Wolkenten et al., 2007, 18857; Brosnan and de Waal, 2003, 298; Range et al., 2009, 341-342). Inequity aversion has been defined as partners resisting inequitable outcomes (Range et al., 2009, 340). Numerous studies have been conducted on primates regarding this behavior. Brosnan and de Waal studied capuchins that were trained to exchange a token with the experimenter in order to receive a reward (2003, 298). Capuchins who were not given a reward for performing the task responded negatively, for example by rejecting the reward (Brosnan and de Waal, 2003, 298). Range et al. performed a similar experiment where dogs were required to give the paw to receive a reward (2009, 341). Dogs that were treated unfairly responded negatively, such as by hesitating (Range et al., 2009, 343). Since primates and dogs are the only organisms known to be studied for this behavior, there is skepticism that there is only a correlation between inequity aversion with primates and domesticated species. It is also thought that it only occurs in cooperative organisms. In this experiment, similar procedures were used as mentioned in the previous research. The mice were required to climb on the block in order to receive a reward. Mice were chosen since are not always considered to be domesticated and they are not known for being a cooperative species.

Twelve female mice were trained to climb on a wooden block and received peanut butter as their reward. Their food supply was removed approximately five hours before training. The training sessions would last until the mice would no longer climb on the block or eat their reward. Rings were drawn on the tails of the white mice with a sharpie marker to so they could easily be identified. Each of the eleven mice were randomly assigned to the conditions by drawing their color and number of rings out of a bag (one mouse was not included in this study due to the inability to train it). There were two control conditions. One was the isolation condition, where the mice were tested individually and received a treat for climbing on the block. The other was an equal rewards condition, where the mice were paired with a partner and both mice received rewards for climbing on the block. The experimental condition was a condition with unequal rewards, where the partners received a reward for climbing on the block and the test subject did not. Each mouse participated in each of the conditions once and were never tested more than once on the same day. The number of times the mice climbed on the block before refusing to perform the behavior was recorded. A Friedman’s test and a paired samples t-test were used to analyze the data.

The mean number of times that the mice climbed on the block varied for each condition. The mean for the isolation condition was the lowest as shown in Figure 1. Also, the means for the equal condition for the partners and the test subjects were about the same. In addition, for the unequal condition, the mean number of times the mice climbed on the block was higher for the test subjects than it was for the partners (Figure 1).

The Friedman’s test and the paired samples t-tests found that the differences between the conditions were not significant. It was expected that the t-test between the equal conditions would not be significant since the mice were treated equally in both conditions. However, for the unequal conditions it was expected that the test subjects would climb on the block significantly fewer times than their partners. It was found that the test subjects climbed on the block more times than the their partners, even though the test subjects did not receive a reward for climbing on the block. However, this difference was not significant. Based on this, the hypothesis was not supported; therefore, inequity aversion was not a behavior found in mice.

Brosnan SF, de Waal FBM. 2003. Monkeys reject unequal pay. Nature, 425: 297-299.

Brosnan SF, Schiff HC, de Waal FBM. 2005. Tolerance for inequity may increase with social closeness in chimpanzees. Proceedings of the Royal Society B, 272: 253-258.

Range F, Horn L, Viranyi Z, Huber L. 2009. The absence of reward induces inequity aversion in dogs. Proceedings of the National Academy of Science, 106: 340-345.

Van Wolkenten M, Brosnan S, de Waal FBM. 2007. Inequity responses of monkeys modified by effort. Proceedings of the National Academy of Science, 104(47): 18854-18859.

•Dr. Lustofin•Dr. Brown•Dr. Klein

Introduction

Materials and Methods

Discussion

Literature Cited

Acknowledgements

Figure 1. Mean number of times the mice climbed on the block for each condition.

The results of the Friedman’s test showed that the means between the conditions were not statistically significant (p= 0.076). A paired samples t-test for the equal condition between the partners and the test subjects was also not statistically significant with a (p= 0.939) (Table 1). In addition, a paired samples t-test also showed that the means for the unequal condition between the partners and the test subjects was not significant (p= 0.496) (Table 1). The mean difference of the number of times the behavior was performed for the partners and the test subjects in the equal and unequal conditions were determined (Table 1.).

Condition Mean Diff. Std. Dev. Sig. (2-tailed)

Equal (partner) vs. Equal (test) -0.64 27 0.939

Rewarded vs. Not rewarded -6.46 30 0.496

Results

Table 1. Paired samples t-test

Conclusions

Explanation of Results

Background

Future Studies There are many studies regarding inequity aversion that could be conducted. Experiments studying other organisms need to be performed to learn more about the origin of the behavior. For example, since inequity aversion is considered to be a driving force for cooperation, cooperative species such as birds or wolves that have not been studied for this behavior could be examined to help determine if this idea can continue to be accepted. It would also be beneficial to study domesticated species, such as horses, to see if the hypothesis that inequity aversion is only correlated with primates and domesticated species can be also continue to be accepted.

Objective The object of this experiment was to determine if mice exhibit the inequity aversion behavior.

Hypothesis It was hypothesized that mice that were treated unfairly and performed the same task as their partner would climb on the block significantly fewer times.

There are many possible explanations for the results of this study. For example, the mice that were treated unfairly could have been trying harder to get rewarded so they continued to climb on the block. Another possible explanation could be that the partners of the mice got full from eating the reward so they stopped climbing on the block. The test subjects were not getting rewarded so they would not have gotten full, and therefore, continued to climb on the block. Also, inequity aversion is a behavior that has been found in cooperative species (Brosnan and de Waal, 2003, 297; Range et al., 2009, 340) and mice are not known for their cooperation. This behavior is expected in species that are cooperative; therefore would not be found in all species (Brosnan et al., 2005, 253). In addition, previous research has mentioned in their studies that the partnered individuals witnessed their partners performing the behavior and receiving a reward (Brosnan and de Waal, 2003, 297). The mice in this study may not have reacted as expected, since they were busy running around their cages and did not pay attention to what their partners were doing.

Num

ber o

f Tim

es

•Mrs. Jarrell•My classmates

Condition