Abstract
Direct reciprocity, in which beneficiaries return favors to benefactors, is a cornerstone of human cooperation. Previous empirical work addresses partner control-how individuals decide whether and how much to reciprocate-whereas the equally critical dimension of partner choice, deciding whom to reciprocate when aided by multiple benefactors, remains understudied. This gap is addressed by testing two determinants: social-emotional motivations and reciprocal efficiency (the efficiency of reinforcing future relational capital). Combining an interpersonal task with fMRI multivariate neural expressions and representational similarity analysis, it is demonstrated that efficiency recalibrates choices between altruistic and strategic benefactors by shifting the neural balance between distinct social-emotional concerns. When reciprocating to altruistic benefactors yielded higher efficiency, participants prioritized communal concerns (gratitude/guilt) represented in the ventromedial prefrontal cortex, whereas higher efficiency for strategic benefactors led them to prioritize obligation represented in the ventral striatum. General efficiency engaged the putamen, dorsomedial prefrontal, and orbitofrontal cortices; the inferior parietal lobe integrated efficiency-driven recalibration. These findings suggest that efficiency does not merely optimize material outcomes but adaptively reweights social-emotional concerns behind reciprocal partner choices, bridging economic models of rational choice with psychological theories of social emotions, offering insights into human cooperation and related practical applications.