Constitutions obsolesce rapidly and must be updated over time to reflect changes in the polity's circumstances and citizens' values. What institution or process should be entrusted with the authority to do the updating? If periodic wholesale replacement of the constitution is infeasible, the realistic choices are a constitutional amendment process involving legislatures, flexible interpretation by judges under the banner of constitutional common law, or some mix of the two. Here I explore the question by comparing the relative merits of formal amendments and the constitutional common law as means of constitutional updating. I attempt to dispel some prominent arguments that unjustifiably privilege constitutional common law over the amendment process and also attempt to sketch the empirical conditions under which either process proves superior to the other.

To structure the discussion, I advance two subsidiary theses about constitutional amendment. The first is that constitutional amendments can and do change constitutional law, including the law in action as well as the formal constitutional text. The second is that there is no good general reason to prefer common-law updating to the amendment process; to the contrary, an evenhanded institutional comparison suggests that each process shows to best advantage under particular conditions, and in particular domains.

These theses sound banal, but each has been denied in important recent work. As to the first thesis, David Strauss has argued that constitutional amendments are “irrelevant” because an amendment is neither a necessary nor a sufficient condition for legal change.