Yamaguchi, M.（山口真奈）, Smith, A.（スミス アダム）, & Ohtsubo, Y.（大坪庸介） (2015). Commitment signals in friendship and romantic relationships. 友人・恋人間でのコミットメントシグナル Evolution and Human Behavior. doi: 10.1016/j.evolhumbehav.2015.05.002
Due to the ever-present allure of potentially more appealing or attractive partners, people in mutually committed relationships face a commitment problem (i.e., uncertainty about partner fidelity). This problem exists for both friendship and romantic relationships. In an exploratory pilot study, participants described real-life commitment-confirming incidents in either friendship or romantic relationships. The results revealed that the same types of pro-relationship acts (e.g., throwing a surprise party) were used to communicate commitment to one’s partner in both types of relationship. Using signaling theory, we predicted that costly commitment signals would be more effective than non-costly commitment signals (Hypothesis 1). Also, we predicted that failure to engage in such behaviors would communicate non-commitment, and that such failures would have a more detrimental effect on romantic relationships than friendship (Hypothesis 2). Two scenario experiments (study 1 in Japan and study 2 in the U.S.)were conducted to test these hypotheses. The results showed that costly commitment signals were more effective than non-costly commitment signals in both Japan and the U.S. In addition, the absence of situationally appropriate commitment signals (e.g., forgetting a special occasion) was substantially more damaging to romantic relationships than to friendship.