I'll make you an offer you can refuse: How socially sensitive is the MFN?

Medial frontal negativity (MFN) is an event-related potential thought to originate in the anterior cingulate cortex. It is evoked by outcomes being worse than expected, such as when presented with unfair economic proposals during the Ultimatum Game (UG). This could mean the MFN indexes a social-emotional response, as commonly suggested in accounts that relate it to a violation of a social norm of fairness. To examine the link between MFN and norm violation, we designed an EEG experiment with participants acting as representatives in an UG. Participants responded either as themselves, or as representatives of two charities. Of these, a norm-compatible charity conformed to the participant's values, while the norm-incompatible charity contrasted to them. The behavioral results showed that norm-incompatible representation reversed behavior, with almost all fair offers being declined. The MFN, however, was unaffected by the norm representation, with unfair offers consistently evoking MFNs across conditions. We furthermore replicated the curious finding that unexpectedly generous offers evoke as much MFN as unfair offers. Thus, the MFN is not nearly as sensitive to higher-order social-emotional processes as commonly assumed. Instead, the perceived inequality that drives the MFN likely reflects a rational, probabilistic process.