The constitutional amendment mechanism of the Mexican Constitution of 1857 (reproduced in the Constitution of 1917) and Article V of the US Constitution are very similar in design. Both require a two-thirds majority of each of the houses of a bicameral Congress and ratification by the states (half of the state legislatures in Mexico and three-fourths in the United States). Both articles were the result of an experiment aiming at striking the right balance between rigidity and flexibility. Yet, while characterized by similar levels of formal rigidity, these experiments have had the exact opposite effect. While the US Constitution has been described as one of the world’s most rigid and has only been amended twenty-seven times, the Mexican Constitution of 1917 has gone through over 700 amendments. Why are the amendment rates so divergent? This article argues that Mexico’s amendment practice offers an opportunity to deepen our knowledge about how non-institutional factors condition the way amendment provisions work and, thus, to dispel the idea that amendment difficulty is institutionally determined. In particular, there are at least three lessons that may be drawn from the Mexican case: (i) constitutional scholarship needs to shift its attention to political parties and party systems; (ii) unwritten rules influencing the behavior of party members need further study; and (iii) we must carefully look at the agency of constitutional decision-makers, specifically regarding the choices they make among different means to advance their interests and agendas.

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宪法僵化：墨西哥实验

1857年墨西哥宪法（1917年宪法再现）的宪法修改机制与美国宪法第五条在设计上非常相似。两者都需要两院制国会的三分之二多数和各州的批准（墨西哥州议会的一半和美国的四分之三）。这两篇文章都是一项旨在在刚性和灵活性之间取得适当平衡的实验的结果。然而，虽然以相似程度的形式僵化为特征，但这些实验却产生了完全相反的效果。虽然美国宪法被描述为世界上最严格的宪法之一，并且只修改了 27 次，但 1917 年的墨西哥宪法却经历了 700 多次修改。为什么修正率如此不同？本文认为，墨西哥的修正实践提供了一个机会，可以加深我们对非制度因素如何影响修正条款的运作方式的认识，从而消除修正难度由制度决定的观点。特别是，从墨西哥的案例中至少可以得出三个教训：（i）宪法学需要将注意力转移到政党和政党制度上；（二）影响党员行为的不成文规定需要进一步研究；(iii) 我们必须仔细审视宪法决策者的能动性，特别是关于他们为推进其利益和议程而在不同手段中做出的选择。