Each trial begins with a picture of the opponent (i.e., a photograph of the human opponent or of a computer). The participant’s card is revealed and they then decide whether to bet (B) or fold (F). Buttons then appear – with left/right position randomized across trials – and the participant indicates their response. If the participant chooses to fold, the trial ends. If the participant chooses to bet on the card, control passes to the opponent who has 3 s to decide whether or not to match the participant’s bet or fold. Predictions are made using fMRI data associated with the time period highlighted in green.

(A) Whole-brain fMRI data was segmented into 55 bilateral anatomical regions, as described in Supplementary Material. (B) UCP for a human opponent was regressed against UCP for a computer opponent, across regions (green dashed line, slope = 1.4). For the range reported here, negative values of UCP correspond to a return to chance levels and not negatively predictive models. (C) The slope of the regression line in panel B is consistent with a general task effect (upper graph), such that social contexts tend to engage task-related regions across the brain more than similar non-social contexts. A region that was highly engaged by the task might exhibit activation in a standard fMRI contrast (triangle), even if that region carried no independent task-related information. A systematic shift away from the regression line (‘x’) would be necessary to show an independent effect in a region. (D) Social bias was defined by the residual difference (social minus non-social) in predictive power from the whole-brain regression line. (E) The magnitude of social bias in each region (normalized by the variance estimates for each segment’s UCP), sorted by UCP against the human opponent. Only one region, the TPJ, contributed significantly more UCP against a human opponent than against a computer opponent, independent of the overall task effect.

(A) The social bias observed in the TPJ was found only for those participants who considered the human to be a more informative opponent. Other putative social cognition regions do not show the same relationship with behavioral relevance (MPFC: FP-frontal pole, pCG-paracingulate gyrus, and ACG-anterior cingulate gyrus; MPC: PCC-posterior cingulate cortex and PCu-precuneus). (B) Social bias in all regions (normalized by the variance estimates for each segment’s UCP), sorted by UCP against the human opponent.