Coalitional Formidability and Kinship Across Three Cultures

Purpose: This study investigates how kinship composition shapes perceptions of coalitional formidability across three countries with different histories of kinship-based social organization. Inclusive fitness theory predicts that kin-based coalitions will be perceived as more formidable, at least on the dimensions closest to genuine altruism (Chapais, 2006). Reciprocal altruism theory (Trivers, 1971) predicts instead that familiar partners can sustain most coalitional cooperation regardless of relatedness, confining any kinship advantage to the costliest, least reciprocable commitments. We test these competing predictions. Methods: We conducted a multilevel experimental study using vignettes depicting groups in conflict, sampling 613 participants from China (n = 204), the United States (n = 189), and India (n = 220). Participants rated coalitions described as kin-related or unrelated on seven operationalizations of coalitional formidability, which our framework sorts into four competence-dependent capability assessments (e.g., fighting skill, physical strength), two moderate-cost, reciprocable commitments (coordinated team play, standing one’s ground), and one high-cost commitment, intra-group aid. Multilevel models estimated kinship effects while controlling for country and gender. Results: The pre-registered omnibus test returned no overall kinship effect (b = − 0.024, p = .459; negative coefficients indicate higher ratings for kin-related coalitions). In exploratory dimension-specific models, kinship increased perceived willingness to provide intra-group aid (b = − 0.097, p = .005), the sole effect to survive Bonferroni correction. The four capability assessments showed no detectable kinship effects, and neither did the two commitment items on which inclusive-fitness reasoning had also predicted effects. Baseline ratings differed across samples, but the kinship effect did not detectably vary by country or gender. Conclusion: The selective effect on costly aid, together with null effects on every reciprocable dimension, favors a division of labor in which reciprocal altruism sustains most coalitional cooperation among familiar partners while kinship becomes decisive where reciprocation cannot be expected. Because the samples are three national panels and the observed effect is modest, we regard the cross-cultural consistency of this pattern as provisional; testing it in kinship-intensive small-scale societies is the natural next step.