# Efficiency Recalibrates Social‐Emotional Trade‐Offs Behind Partner Choice in Direct Reciprocity through Intention‐Specific Neural Bases

## TL;DR

This study explores how people choose whom to reciprocate when helped by multiple people, showing that efficiency shifts brain activity linked to social emotions.

## Contribution

The study identifies intention-specific neural mechanisms that recalibrate social-emotional trade-offs in partner choice during direct reciprocity.

## Key findings

- Higher efficiency for altruistic benefactors activates brain regions linked to communal concerns like gratitude and guilt.
- Efficiency for strategic benefactors activates brain regions associated with obligation.
- General efficiency and its integration involve multiple brain regions, showing a complex neural basis for partner choice.

## 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.

Bridging the gap between two crucial mechanisms of cooperation, namely direct reciprocity and partner choice, this study reveals the neurocognitive bases underlying how individuals prioritize reciprocal partner when receiving help from multiple benefactors with diverse intentions and reciprocal efficiency. The efficiency‐driven reciprocal decisions are not purely rational but balance intention‐specific neurocognitive trade‐offs between social‐emotional motives of communal concern and obligation.