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The Supreme Court frequently relies on state law when interpreting the U.S. Constitution. What is less understood is the degree and manner in which the Supreme Court and other federal courts look not to state law, but to local law. Although it has largely gone unnoticed, there is a robust practice of acknowledging and accounting for local law in the course of constitutional interpretation. To take an example, one area in which the Supreme Court has examined local enforcement patterns is in death penalty jurisprudence. In 2015, Justice Stephen Breyer, dissenting in Glossip v. Gross, cited to empirical data to raise an Eighth Amendment arbitrariness concern with geographic variation in local practice, where in a five-year period, “just 29 counties (fewer than 1% of counties in the country) accounted for approximately half of all death sentences imposed nationwide.” In other rulings, judges seek to minimize constitutional interpretations that might disrupt local law and practice. As is done with respect to states, judges take into account whether local practices are outlying or common. Judges also look to local law and practice to inform the development of constitutional norms. This Article analyzes and defends reliance on local law and practice in constitutional interpretation — not to advocate localism or deference to local government — but as evidence in constitutional interpretation. Using local evidence in constitutional law is particularly important at a time in which empirical research on county-level data is providing a wealth of information that can better inform constitutional law.

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