Abstract: Adaptive decision-making in reciprocal cooperation relies on individuals' memory for prior interactions, as people tend to approach previous cooperators and avoid previous cheaters. Source memory, defined as the ability to remember who performed which behavior, enables individuals to distinguish between cooperative and cheating partners and to adjust subsequent decisions accordingly. However, prior studies have reported inconsistent findings regarding whether source memory is preferentially enhanced for cooperative or cheating partners. One plausible explanation for these inconsistencies lies in individual differences in Social Value Orientation (SVO), a dispositional trait that classifies individuals as prosocial or proself and influences how they evaluate and remember social information. Against this backdrop, the present study examined whether SVO differentially modulates source memory for cooperative and cheating partners in reciprocal cooperation and whether such differences are reflected in distinct electrophysiological responses during feedback processing.
Two experiments were conducted using a sequential Prisoner's Dilemma paradigm. In Experiment 1, forty participants were included in the final analysis and classified as either prosocial (n = 20) or proself (n = 20) based on the Social Value Orientation Slider Measure. Participants engaged in repeated interactions with partners who either cooperated or cheated. A 2 (SVO: prosocial vs. proself) × 2 (partner behavior: cooperation vs. cheating) mixed design was employed, with source memory accuracy and subsequent decision behavior as dependent variables. Experiment 2 recruited 46 participants, including 23 prosocial and 23 proself individuals. During the feedback phase of each interaction, electroencephalographic (EEG) activity was recorded. The independent variables were identical to those in Experiment 1, and the dependent measures indexed early-stage feedback-related negativity (FRN), late-stage late positive potential (LPP), and theta power.
Experiment 1 revealed a significant interaction between SVO and partner behavior on source memory. Prosocial individuals demonstrated superior source memory accuracy for cooperators relative to cheaters, whereas proself individuals showed better source memory accuracy for cheaters than for cooperators. In Experiment 2, SVO significantly modulated neural responses during feedback processing. During the early stage of outcome evaluation, prosocial individuals exhibited more negative FRN amplitudes in response to cooperative feedback, whereas proself individuals showed more negative FRN amplitudes following cheating feedback. In the later stage, enhanced LPP amplitudes in response to cheating feedback were observed only among proself individuals. Time-frequency analyses further indicated that SVO modulated theta power elicited by cheating feedback, with greater theta power observed exclusively in proself individuals. Correlation analyses revealed that FRN amplitude was negatively associated with source memory accuracy, whereas theta power was positively associated with source memory accuracy.
Overall, the findings demonstrate that Social Value Orientation shapes source memory in reciprocal cooperation by modulating neural processes of outcome evaluation. Divergent patterns of FRN indicate that prosocial and proself individuals differ in early evaluative sensitivity to cooperative and cheating feedback, whereas variations in LPP and theta power reflect differences in sustained attentional and motivational engagement. These differential neural processes were associated with subsequent memory performance, resulting in preferential encoding of cooperation among prosocial individuals and of cheating among proself individuals. By integrating behavioral and electrophysiological evidence, the present study clarifies how value-based motivational orientations bias memory encoding in cooperative contexts and advances understanding of personality influences on social decision-making.

Key words: reciprocal cooperation, social value orientation (SVO), outcome evaluation, event-related potentials (ERPs), source memory