A) One player (“investor”) is endowed with $20 at the beginning of each round. The investor chooses any fraction I of the $20 to send to the other player (“trustee”). The investment is tripled to 3I en-route to the trustee. The trustee chooses a fraction R of the tripled amount (3I) to repay. Subjects play the same partner for ten consecutive rounds. B) Using the observed exchanges between the players, investors are classified according to their estimated inequality aversion and their depth-of-thought (strategic level) in the game (see main text for a description of the generative model). All 195 pairs included in this classification; this included 55 pairs where the trustee was diagnosed with Borderline Personality Disorder. C) First and second order interpersonal prediction errors are sought in the investors' brain responses separately for each depth-of-though category. The 1^st order interpersonal prediction error is taken as the difference between actual repayment ratio R and expected amount due to the investor's model of the trustee's repayment. The 2^nd order prediction error is taken as the difference between the investment ratio I and the investor's model of the trustee's model of what the investor will send; hence, the term second order error.

A) The distribution of depth-of-thought levels in all 195 investors. About half of the investors are classified as having depth-of-thought level 0. The remaining half is almost equally divided into having depth-of-thought level 1 and 2. B) Investment ratios by rounds from all three levels of depth-of-thought investors, level 0 (n = 102), level 1 (n = 49), level 2 (n = 44). C) Total monetary points earned at the end of the game in all three levels of investors. Both level 1 and level 2 investors made significantly more points than level 0 investors (Tukey HSD test, P<10⁻⁶ and P<10⁻⁵, respectively). No significant difference in total earnings was found between level 1 and level 2 investors (P>0.1). Error bars represent standard errors (SE).

A) Contrast analysis between rounds with high (>60%) and low (≤40%) 1^st order interpersonal prediction errors when repayments were revealed. Level 0 investors (n = 102) had robust activations in bilateral striatal regions (whole-brain FDR corrected at P<0.05; peak MNI coordinates: caudate (8, 12, 0), t = 4.49; putamen (24, 4, 0),t = 4.02). These striatal activations were not observed in investors with level 1 (n = 49) or level 2 (n = 44) depth-of-thought. B) Group contrast analysis on the 1^st order interpersonal prediction errors. Left, level 0 investors had higher caudate activation than level 1 investors (P<0.001, uncorrected; peak MNI coordinates: (4, 16, 0), t = 4.04, FWE corrected at P<0.05 with small volume correction applying the anatomical mask of bilateral caudate). Right, level 2 investors had higher right temporal-parietal junction (TPJ) activation than level 0 investors associated with the 1^st order interpersonal prediction errors (whole-brain FDR corrected at P<0.05; peak MNI coordinates: (52, −48, 28), t = 4.70, 7 voxels). C) Contrast analysis between rounds with high (>60%) and low (≤40%) 2^nd order interpersonal prediction errors when investments were submitted. Level 2 investors had significant activations in bilateral putamen (whole-brain FDR corrected at P<0.05; peak MNI coordinates: putamen (24, 8, −4), t = 3.79). We did not observe any striatal activations in level 0 and level 1 investors for the 2^nd order prediction errors. D) Group contrast analysis on the 2^nd order interpersonal prediction errors. Level 2 investors had higher ventral striatal activation than level 0 investors when computing the 2^nd order interpersonal prediction errors (P<0.005 uncorrected; peak MNI coordinates (12, 8, −12), t = 3.41, FWE corrected at P<0.05 with small volume correction applying the anatomical mask of bilateral caudate). Color bars display t scores.

Average 1st order A) and 2nd order B) inter-personal prediction errors: low (bottom two quintiles), high (top two quintiles). The differences between the high and low 1^st order interpersonal prediction errors were as follows: level 0 investors (mean = 10.05, SE = 0.38), level 1 investors (mean = 15.97, SE = 0.55), level 2 investors (mean = 14.30, SE = 0.58). The differences between the high and low 2^nd order interpersonal prediction errors were: level 0 investors (mean = 9.76, SE = 0.22), level 1 investors (mean = 10.62, SE = 0.31), level 2 investors (mean = 11.72, SE = 0.33).

A) Anonymous trustees (n = 48) remain anonymous to their investor partner for the entire game (and visa versa). B) Borderline personality disorder trustees were identified through an extensive set of formal interview procedures (see King-Casas et al., 2008). On Fisher's exact test, the borderline personality disorder-induced investor depth-of-though distribution was significantly different from investors playing anonymous trustees (panel A; p = 1.68×10⁻⁶).