Social learning dynamically shapes moral decision-making by biasing subjective valuation.

Observing immoral behavior increases one's dishonesty by social influence and learning processes. The neurocomputational mechanisms underlying such moral contagion remain unclear. We tested different mechanistic hypotheses to account for moral contagion. We used model-based fMRI and a new cheating game in which participants were sequentially placed in honest and dishonest social norm contexts. Participants' cheating behavior increased in the dishonest norm context but was unchanged in the honest one. The best model to account for behavior indicated that participants' valuation was dynamically biased by learning that others had cheated. At the group level, this valuation bias was not encoded by any specific brain region. Instead, this neural signal depended on individual differences in conformity, and engaged the bilateral lateral prefrontal cortex. During learning, simulation of others' cheating behavior was encoded in the posterior superior temporal sulcus. Together, these findings provide a mechanistic understanding of how learning about others' dishonesty biases individuals' valuation of cheating but does not alter one's established preferences. Observing others' immoral behavior can increase one's own dishonesty, but the neurocomputational mechanisms driving this moral contagion remain unclear. This study shows that cheating increases in dishonest contexts via learned valuation biases involving lateral prefrontal cortex and posterior superior temporal sulcus encoding others' cheating behavior.