Morality encompasses a set of norms that originate from the group's consensus and cultural evolution. Accordingly, the mere presence of another person is very well capable of shaping moral judgment and decision-making in a way that benefits group coherence. Here, we measured justice sensitivity (JSI), implicit moral attitudes (mIAT), and neural activity during mental simulation of interpersonal morally-laden behaviors (helping and harming) when participants were either alone or when they thought a confederate was present. Both JSI and mIAT, as well as various brain networks, were modulated and, further, synchronized by the presence of a confederate. Participants with lower scores on the mIAT and JSI enhanced their moral attitudes when they were in the presence of a confederate. This change was driven by increased signal in the amygdala and anterior insula when the low mIAT participants mentally simulated harming another person, but was effected by decreased activity in the dorsomedial- and dorsolateral-prefrontal cortex in the high JSI participants. The presence of another significantly impacts moral attitudes as well as neural correlates underlying moral behavior. Together, the results support the view that both individual dispositions and social influence shape and synchronize people's moral computations, and fits with the theoretical perspective that morality has evolved to promote group fitness.
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