Recent cross-cultural research shows that North Americans feel greater intimacy toward their relationship partner than East Asians. The purpose of the current study was to: 1) attempt to replicate this difference; and 2) investigate whether the difference could be explained by a socio-ecological factor, namely relational mobility. In particular, we assumed societies high in relational mobility, where competition abounds in interpersonal relationships, intimacy should be adaptive, acting as a commitment device to strengthen relationships. To examine this hypothesis, we asked Japanese and Canadian participants about the level of intimacy they feel toward their best friend, romantic partner, and closest family member. We also asked them to rate the level of relational mobility in their surrounding social environment. The results indicated that, consistent with previous studies, Canadians reported feeling greater intimacy toward their best friend and romantic partner than Japanese. Moreover, this ­cultural difference in the level of intimacy toward a best friend was mediated by relational mobility.

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