Social capital, as measured by social trust, pro-social values, or proxies capturing civic engagement, is a strong determinant of an area’s income, economic growth, and political institutions (Guiso, Sapienza and Zingales 2011). But in the US, it has declined significantly over the latter half of the twentieth century, with researchers and policymakers puzzling over why, and how it might be restored. In this dissertation, I propose and test a novel explanation for the decline in social capital—namely, the decline in unionization. Over much of the same period as the social capital decrease, union density decreased by over half, and workers lost “an important locus of social solidarity, a mechanism for mutual assistance and shared expertise” (Putnam 2000). To test this hypothesis, I estimate the effect of unionization on social capital, in two ways. The first is to combine data on NLRB unionization elections with data on commonly used proxies for social capital, and, in a modified RD-DD design, compare the change in social capital in areas that saw a close unionization victory to the change in areas that saw a close unionization loss. The second method is to use the PSID to estimate the effect of becoming unionized on an individual worker’s propensity for charitable donation. The results of the county-level analysis suggest that in the long term, unionization significantly increases voter turnout. However, the estimated effects on membership organizations are statistically insignificant. In an analysis of heterogeneous effects, I further find that the effect of unionization on total organizations is decreasing in a county’s per-capita income. Finally, the results of the individual-level analysis imply strong positive effects of becoming a union member on one’s family’s probability of donating to charity.