Same-sex marriage policy is often codified in state constitutions in the form or constitutional prohibitions. Some studies have examined the diffusion of same-sex marriage bans or legalizations across the U.S. states, but no study considers the process differences between state constitutional amendments and standard state legislation. This study develops a theoretical framework of state same-sex policy diffusion that compares the diffusion of constitutional prohibitions and standard legislation that legalizes same-sex marriage across the U.S. State constitutional amendment requirements vary across the states, but in every state amendment approval requirements are significantly higher than standard state legislation.  These requirements are designed to provide stability to the legal framework of the state itself.  Thirty-one states banned same-sex unions through the constitutional amendment process. In states with constitutional bans legalization of same-sex marriage is impossible without a subsequent amendment. As of January 2013 nine states allow same sex marriage, but no state has approved same-sex marriage through a constitutional amendment.

This study compares the diffusion of the thirty-one constitutional bans and the nine legalizations paying particular attention to the process differences. I utilize survival analysis on all states from 1990-2013 to examine the internal state characteristics and external diffusion that explains the approval of same-sex marriage amendments and standard legislation.  The models explicitly exclude states that are not at risk of diffusion.  For example, the models only consider states at risk of approving a same-sex marriage constitutional prohibition in general election years if the state constitution requires the amendment process occurs simultaneously with a general election.  The models also only consider states at risk of general legislation allowing same-sex marriage if there is no constitutional prohibition. 

The models suggest that both constitutional bans and standard legalization diffuse across the U.S. states.  Constitutional bans of same-sex marriage differ from standard legislation because the structure of the state constitution itself influences the rate of diffusion and adoption across the states.  Since constitutional amendments require citizen approval, proposing an amendment during a presidential election also positively influences adoption. Both constitutional bans of same-sex marriage and legalization legislation are a function of the social characteristics of the state including the religiosity and political ideology of the citizens, although in opposite directions. 

These findings suggest that state constitutional amendments diffuse across the states similar to standard policy, but that amendments are a function of constitutional structure and election timing.  Implications to other state policies are discussed.