Classification of investors.

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.