Marriage equality movements have been successful in achieving policy change in an increasing number of states. Hence, a growing body of scholarship has explored institutional and cultural factors that influence activists’ tactics and messaging and, in turn, contribute to marriage equality policy diffusion. Democracies with parliamentary, presidential, and semi-presidential systems, federal and unitary states with varying levels of centralization, and the presence or absence of constitutional anti-discrimination protections provide social movements with divergent political opportunity structures, contributing to dynamics in their tactical choices. In addition, the type of electoral system and party system, the presence of political parties that are movement allies, the use of conscience votes, the level of party discipline, the presence of out LGBT elected officials and straight political allies, and the degree of political will to enact policy change also impact activists’ strategic calculations. Finally, the use of personalized narratives in advocates’ messaging, the framing of marriage equality and LGBT rights as human rights norms, the adoption of family values frames to coopt opponents’ messaging, and the use of homonationalist versus homophobic discourses to justify policymaking decisions regarding same-sex marriage are explored. This article provides a comprehensive review of state-of-the-art research concerning all of the states that have legalized same-sex marriage as well as a detailed analysis of the mechanisms used to achieve policy change. After examining how different explanatory factors perform in accounting for the dynamics in marriage equality activism and policy convergence across a broad range of national contexts, new directions for future scholarship are suggested.
