Temporoparietal Junction Selectively Mediates Social Projection in Mentalizing

Mentalizing, the cognitive process of inferring others' mental states from limited social information, is often facilitated by social projection, a "self-as-proxy" strategy that engenders self-other mergence (SOM). While SOM overlaps conceptually with the simulation or theory-driven processes, its neural basis remains unresolved. Using fMRI during a dyadic task where participants estimated both their own confidence (metacognition) and a partner's confidence (mentalizing), we found that SOM critically depended on self-confidence encoded in the dorsal anterior cingulate cortex (dACC) during metacognition. The temporoparietal junction (TPJ) tracked SOM magnitude via effective connectivity with the dACC, while the partner's confidence was distinctly represented in the dorsomedial prefrontal cortex. Further, disrupting TPJ activity via transcranial magnetic stimulation causally attenuated the SOM effect. These findings disentangle social projection from competing frameworks and suggest that the TPJ plays a pivotal role in mediating social projection during mentalizing, advancing the mechanistic understanding of how the self scaffolds social cognition.