Many findings of previous research have supported the predictions of the emancipation theory of trust (Yamagishi, 2011) in experimental settings. Although the key concept of the theory is social uncertainty, few studies of the concept have been conducted in natural settings. In the present study, we operationally defined social uncertainty by comparing kin and nonkin relationships. We analyzed survey data representative of the Japanese population and compared the exchange of social support between kin and nonkin. The prediction was that commitment helps respondents construct cooperative nonkin relationships more than it does kin relationships, and general trust helps only resource-rich respondents to do so. The results supported the predictions. The study found significant interactions with relationship type x commitment and relationship type x household income x general trust. The patterns of mean score for social support exchanged were consistent with the predictions. High trusters with low household income exchanged more support independent of the relationship type.