Objective: Social capital generated by relationships within families has a greater impact on inter-generational mobility in the United States than other forms, though community capital plays a secondary role, and student achievement and school and adult friendships serve as moderators. Background: Social capital is human capital produced by the "trustworthiness of the social environment, information-flow capability of the social structure, and norms accompanied by sanctions (Coleman, 1998, p. S119)." Social capital produced by families (Coleman, 1988), communities (Putnam & Campbell, 2012), political trust and engagement (Rupasingha et al., 2006), and cross-class friendships (Chetty et. al., 2022) contributes to inter-generational mobility within the United States. Researchers disagree as to the relative importance of each form of social capital. Method: County-level mobility rates are regressed on county-level indexes of the four forms of social capital, student achievement, demographic controls, and state fixed effects. Estimates based upon unweighted observations from 1333 to 1818 counties, depending on model. Results: Intergenerational mobility rates are largely a function of county's density of family capital (marriage rates and two-parent households), followed by community capital (community organizations, religious congregations, and volunteering). Student achievement in grades 3-8, together with cross-class friendships in school and adulthood, moderate relationships. Conclusion: To enhance mobility, public policy needs to enhance the lives of disadvantaged young people at home, in school, and in communities, not simply by modifying friendship patterns.