;To investigate how the interfering factors affect people’s fairness perception, we experimentally manipulated the source of monetary reward that is determined by a luck-based or effort-based task and examined whether the allocation of the monetary reward would modulate participants’ satisfaction of monetary distribution. We conducted three experiments in total. Specifically, in the first stage of all the experiments, college students were classified as “winners” or “losers”, each of which was against one opponent, based on pure luck (i.e., random drawing) or real efforts (i.e., a number-line dissection task). In Experiment 1, we also included a math-ability task (i.e., ten arithmetic math questions). Winners across all conditions received all and the same amount of monetary reward, while losers received no reward. In the second stage of every experiment, all participants were asked to make satisfaction ratings for the distribution of an additional amount of monetary reward between themselves and the opponent.
Experiment 1 showed similar behavioral patterns of preferring the self-more distribution (i.e., participants received more money than the opponent) in the luck, the math, and the effort conditions for both winners and losers. Participants were not affected by the inequality in the first stage, possibly because the two experimental stages were perceived as independent. Therefore, in Experiment 2, we modified the total amount of the monetary reward of the distributions and emphasized the inequality between winners and losers in the second stage. The results showed that both in the luck and the effort conditions, winners preferred equal over unequal distribution of which they received more additional monetary reward than losers (i.e., the self-more distribution). Losers, in contrast, in both the luck and the effort conditions tended to prefer self-more over equal or self-less distribution of which participants received less money than the opponent. We did not observe the effect from the source of the reward in Experiment 2.
In Experiment 3, we adopted the design of Experiment 2 and changed the original between-subject design into within-subject design, that is, all participants experienced the luck and the effort conditions in combination with simultaneous fMRI recording. Behaviorally, the effect of the manipulations in the allocation of reward was weaker than that in Experiment 2, as both winners and losers preferred the self-more distribution. We did not find strong evidence for the effect of the source of the monetary reward. However, when examining the variance of rating for each distribution, we observed a larger individual difference among winners than among losers in Experiment 3. Some winners in Experiment 3 gave a positive rating even under the self-less distribution. This indicated that some winners still took fairness into consideration, yet the number of people was not as many as in Experiment 2. From our neural data, we did not find the brain areas that were identified to be involved in processing subjective value in previous literature, such as the ventral striatum and the ventromedial prefrontal cortex (vmPFC), to associate with participants’ ratings of various distributions. However, when we contrasted the process underlying participants’ rating decisions with that underlying their button pressing, we found that the activations of the parietal cortex and the insula were significantly larger in the former than in the latter condition. The activation of these brain area indicated that the participants were indeed processing the value, no matter the value was larger or smaller than the opponent’s. In addition, we also found that losers showed a stronger brain activation in association with the subjective rating in the insula than losers. This indicated that losers paid more attention and were emotionally more engaged in the second stage than winners.
In summary, we found that perception of fairness can be affected by the allocation of monetary reward. However, even when adequate attention is placed on the modulating factors that give rise to unequal monetary reward, individual difference might play a dominant role in fairness perception which is reflected in preference and satisfaction of monetary distribution.