Building on gene--environment interaction (G x E) research, this study investigates how a genetic susceptibility index interacts with culture to influence expressive tendencies in a gene--culture interaction. Previous studies have identified specific genetic variants associated with greater susceptibility to environmental influences. Taking culture as a form of environment, individuals with more susceptible variants are expected to exhibit behavior more in line with cultural norms. We assessed susceptibility using a genetic susceptibility index, which was composed of multiple polymorphisms previously identified in gene---culture interaction studies of susceptibility genotypes. American and Korean participants completed assessments of expressive behavior, including value of expression, emotional suppression, and cognitive reappraisal tendencies, and were genotyped for OXTR, 5HTR1A, SERT, and DRD4. Increased expressive values and behaviors are normative in an American cultural context, but not in an East Asian cultural context like Korea. Comparing between Americans and Koreans, we found the predicted interaction on expression: Individuals with higher genetic susceptibility to environmental influence were more likely to exhibit expressive tendencies in accordance with cultural norms. Specifically, among Americans, increased genetic susceptibility was associated with greater value of expression and lower emotional suppression, while Koreans showed the opposite pattern. For cognitive reappraisal, which does not differ between the cultures, we found no interactive effect between genetic susceptibility and culture. Both cultural groups showed an association between increased genetic susceptibility and use of cognitive reappraisal. These findings suggest cultural factors moderate the influence of genetic susceptibility across multiple genes on the manifestation of expressive tendencies.