Moral similarity as a cue for cooperative investment: Reciprocity norms predict helping toward friends

Purpose This research examined whether similarity in moral foundations—particularly reciprocity-related Fairness—predicts altruistic behavior toward friends and acquaintances. Drawing on multilevel selection theory and partner choice models of reciprocal altruism, we tested the hypothesis that moral similarity functions as a cue for strategic compatibility, thereby facilitating cooperative investment. We further distinguished between help provision and help receipt and examined whether effects vary by helping cost. Methods Two online studies were conducted with Japanese adults (Study 1: N = 255; Study 2: N = 243). Participants identified a same-gender friend living nearby and reported perceived similarity in moral foundations and other traits. Study 1 measured retrospective frequencies of low-, medium-, and high-cost helping. Study 2 replicated the design using multi-item Moral Foundations Questionnaire scales and assessed willingness to help via a behavioral proxy (checkbox method). Data were analyzed using ordinal logistic regression. Results Across both studies, similarity in Fairness consistently predicted provision of low-cost help and, in Study 2, high-cost help. In contrast, moral similarity did not reliably predict help received. Medium-cost helping was more strongly associated with similarity in appearance or contact frequency. Conclusions Similarity in reciprocity norms appears to guide individuals’ cooperative investment decisions rather than predict behavioral symmetry. Shared moral commitments may facilitate positive assortment among non-kin by signaling long-term strategic compatibility.