Borderline personality disorder (BPD) is characterized by instability in interpersonal relationships, self-image, and affect. Dysregulated negative emotional processing involving prefrontal and limbic circuits is considered a neural basis of BPD. However, it remains unclear how prefrontal modulation of social decision-making in BPD differs from non-psychiatric controls.

To investigate social decision-making in response to unfairness, we conducted an fMRI study involving adults with a diagnosis of BPD (n=77) and healthy controls (HC; n=60). Using an inequality aversion model, we derived parameters of social norm adaptation and inequality sensitivity from behavioral data during a modified ultimatum game designed to measure responses to offer norm shifts. Valence and salience signal-processing models isolated prefrontal activations related specifically to social norm prediction error (NPE).

Cumulative rejection rates indicated that individuals diagnosed with BPD exhibited consistent differences in overall offer rejection rates but similar adaptation to HC when responding to norm shifts. Preservation of normative social decision-making in BPD (no significant difference vs. HC) was evident in regression analyses of rejection rates and in reinforcement learning models, with no group differences observed in Rescorla–Wagner parameters. Furthermore, we detected no significant neural activation differences between groups, although ventral regions of the medial prefrontal cortex were preferentially involved in valence-related rather than salience-related polynomial modulation.

Contrary to our hypotheses, neither behavioral nor neural responses to economic norm violations differed significantly between BPD and HC groups across one-shot games involving unknown partners. Future research could explore whether more personally relevant or higher-stress social contexts elicit differences not observed here.