Several studies have demonstrated that similarity between friendship partners is higher in the West than in East Asian countries. We hypothesized that these differences could be explained by relational mobility, or the number of opportunities to form new relationships in a given society. Through two studies, we confirmed that whereas the preference for similarity did not differ, similarity between friendship partners was higher in the USA than in Japan. Furthermore, a measure of relational mobility mediated the cultural difference in similarity, supporting our hypothesis. The effectiveness of considering socio-ecological factors when interpreting cultural differences in behaviour is discussed.