Is the bad apple or the bad basket to be blamed? Neural substrate of self-interest and conformity motivation during the contagion of dishonesty.

Learning from others' dishonest behavior usually involves the motivation of self-interest and conformity, which predict different responses during the spread of dishonesty. However, these motivations are often intertwined and rarely distinguished, limiting our understanding of how dishonesty propagates. In this study, we used fMRI and a novel learning task to explore how these motivations impact participants' dishonest behaviors. We adapted the dice-rolling paradigm and included a group norm which evolved from vague to dishonest over time. Contrary to social learning theory predictions, we found that overall, participants reduced conformity when the group norm became clearer yet less informative for the rule of reward. With computational modeling, we found that self-interest was the primary driver of participants' behaviors of overreporting, while conformity motivation increased dishonesty primarily in those with higher self-interest motivation. fMRI results showed that self-interested participants had weaker learning signals in regions associated with social norm inference when the dishonest group norm became more predictable, which include the bilateral superior parietal lobes, posterior cingulate cortex, and precuneus and temporoparietal junctions. Conversely, participants driven by conformity motivation showed increased learning signals in regions related to norm compliance when the group norm became clearer, including the mid-cingulate cortex, superior frontal gyrus, and right anterior insular. In addition, they showed a decreased signal in the anterior medial prefrontal cortex which was implicated in self-reference process during moral evaluation. In summary, our findings showed self-interest motivated norm violation during the contagion of dishonesty, with conformity acting as a nudge, particularly in those with higher self-interest motivation.