Why are some constitutions amended more frequently than others? Despite the importance of this question to political science and legal theory, there is little consensus regarding the forces that shape constitutional amendments. Some scholars only focus on institutional factors, while others emphasize variations in culture. This paper makes a contribution to both literatures by examining how social capital reduces the transaction costs imposed by amendment rules. We conduct cross-sectional analyses of amendment rates for democratic constitutions globally and time-series analyses of efforts to amend the U.S. Constitution. The results indicate amendment frequency is a product of amendment rules, group membership, civic activism, and levels of social and political trust, but these effects vary across contexts based on the corresponding transaction costs. Our findings suggest social capital can have beneficial effects on social movements that demand constitutional amendments and the political elites and voters who supply them.