This study examined the development of trusting relationships as investments in relation-specific assets. A new experimental game called "the dilemma of variable interdependency" was created based on iterated prisoner's dilemma game, in which subjects faced a choice of increasing or decreasing the level of dependency in addition to the usual choice between cooperation and defection. Results of the experiment confirmed the hypothesis that high-trusters (those who have a strong belief in human benevolence) would take a risk of making themselves vulnerable to exploitative behavior of the partner more strongly than low-trusters. This resulted in formation of mutually highly dependent relationships among high-trusters. It was further demonstrated that the subjects' levels of trust affect their cooperation levels when they were given an option to choose the level of dependence but not in the ordinary two-person, iterated PD.