Fehr-Schmidt inequality aversion constrains vmPFC value coding in social context: A computational fMRI study

The ventromedial prefrontal cortex (vmPFC) encodes subjective value across monetary and social outcomes, but whether it tracks individual differences in fairness preferences remains unclear. We applied computational modeling to an openly available fMRI dataset in which participants completed an Ultimatum Game with a computer, an ingroup peer, and an outgroup peer. Individual disadvantageous inequality aversion parameters (α) were estimated from behavioral data using a one-parameter Fehr-Schmidt model, and vmPFC activation was extracted trial-by-trial using a coordinate-based region of interest. Offer size predicted vmPFC activation across proposer types, consistent with a value-coding role. α moderated vmPFC responses for ingroup offers: participants with stronger inequality aversion showed a weaker vmPFC signal when receiving offers from an ingroup peer. A direct contrast of the ingroup and outgroup slopes did not reach significance, and the corresponding effect in a ventral striatum control region was null. At the subject level, α did not predict overall vmPFC engagement. These results indicate that vmPFC value signals are associated with individual fairness preferences within the ingroup context, with only weak evidence that this association differs across proposer types. The findings extend prior work on group-modulated fairness processing.