The Mexican Constitution has been amended an extremely high number of times, challenging all theoretical expectations including the one in this book. Closer analysis indicates three reasons for this discrepancy: first, the length and inconsistency of the Mexican Constitution second, the institutional requirements for amendments (the stringent rules of Article 135 requiring two-thirds of both chambers and a majority of states combines with the loose requirements of Article 63 specifying that the presence of only half of the members is required), and third, the political consensus prevailing in the Mexican political scene (crucial instances, such as the onset of multipartyism with concertacesión and pivotal reforms to Articles 27 and 73, offer deeper context) all make amendments feasible. A comprehensive analysis of the sixty-eight constitutional amendments from 2000 to 2013 corroborates these expectations concerning institutional thresholds, coalition dynamics, and amendment significance.

This chapter presents the tools for the comparative institutional analysis of amendments. It defines the core of a constitution as the provisions that cannot be altered given the amendment rules and the preferences of the relevant actors. The larger the core, the fewer and less significant the amendments. This simple rule is used to calculate an institutional index of rigidity of each constitution. Given that the preferences of the actors are not known, the index is based only on the amendment provisions and provides a necessary but not sufficient condition for amendments: When the constitution has high rigidity, amendments will be rare and/or insignificant, but when the constitution has low rigidity, there may or may not be frequent and/or significant amendments.

This chapter presents the complicated amendment rules in Italy and Chile and calculates their constitutional cores. In both countries, the constitutional amendments failed because of the size of these cores. In Italy, the rejection was by referendum; in Chile, the next step was the design of a new constitution which needed to be approved by a referendum (procedure followed and failed twice). The analysis of these referendums as well as the cases of failed referendums in the EU provide the basis for an explanation of failures of referendums as well as a proposal of a procedure to reduce the probability of these failures based on improvement of information to the public.