The Politics of Constitutional Rigidity.  For decades, the field of comparative constitutional law has been focused on the study of high courts as political actors and the scope and limits of judicial review to contribute to processes of democratic consolidation and rights protection. In recent years, however, there has been an increased interest in formal constitutional change. This is mainly because political leaders across the world have been using constitutional amendments to undermine democracy from within, sometimes neutralizing the influence of courts. This global phenomenon has raised many questions, including the extent to which constitutions can serve as effective limits to the exercise of power. Legal scholarship has largely taken for granted that constitutions are effective limits to the exercise of power because amendment rules are designed to make them difficult to change. Perhaps the most well-known case of “extreme” constitutional rigidity is that of the United States, where the idea that Article V makes constitutional amendments close to impossible is taken for granted. Mexico’s constitutional amendment reality challenges this traditional view of constitutional rigidity. Like the U.S., Mexico is a federal system with a rigid and old constitution that contains an amendment rule almost identical to Article V –and, yet, the constitution has gone through over 700 amendments since 1917. The Mexican case suggests that amendment rules often do very little to prevent frequent amendments. In fact, recent studies have found that across the world, there is a poor correlation between design of amendment rules and the pace of formal change. The literature has just begun to attempt to identify some of the “extra-constitutional” determinants of constitutional rigidity, but there is no theory currently able to explain why formal hurdles hardly impact amendment difficulty. My project will fill that void by providing an empirically supported theory of formal constitutional change. Building on an in-depth study of the dynamics of constitutional change in Mexico, the aim of this project is to develop a theoretical framework that identifies the non-institutional factors that affect constitutional change in general and amendment difficulty in particular. I expect that the lessons learned from the Mexican case will then allow me to extend the study of constitutional rigidity to other federal countries such as the United States and Germany.