Fourth Amendment Article 135 Harv. L. Rev. 1790

The Aftermath of Carpenter: An Empirical Study of Fourth Amendment Law, 2018-2021


Fourth Amendment law is in flux. The Supreme Court recently established, in the landmark case Carpenter v. United States, that individuals can retain Fourth Amendment rights in information they disclose to a third party. In the internet era, this ruling has the potential to extend privacy protections to a huge variety of sensitive digital information. But Carpenter is also notoriously vague. Scholars and lower courts have tried to guess at what the law of Fourth Amendment searches will be going forward — and have reached different, contradictory conclusions.

This Article reports the results of a large-scale empirical study of the impact of a transformative Supreme Court decision in federal and state courts. It analyzes all 857 federal and state judgments applying Carpenter from its publication in June 2018 through March 2021. Relying on this unique, hand-coded database, this Article illuminates both the present and future of Fourth Amendment law.

In doing so, it identifies the factors that drive modern Fourth Amendment search decisions — and those that fail to drive them. It examines disagreements among lower courts about the scope and breadth of Carpenter, as some courts apply its concepts expansively while others attempt to narrow Carpenter from below. It assesses how state courts apply federal constitutional law, blending federal and state interests in unique ways. And it analyzes the enormous practical impact of the “good faith exception” to the exclusionary rule, which permits the government to use unconstitutionally obtained evidence to convict defendants if such evidence was collected in reliance on prior law. Based on these findings, this Article explores alternative directions that courts may take as they continue to refine Fourth Amendment law and address novel surveillance technologies. In addition to its many contributions to the Fourth Amendment literature, this Article is the most comprehensive empirical study to date of the jurisprudential impact of a Supreme Court case in the years following its publication.

† The Harvard Law Review has not independently reviewed the data and analyses described herein.
* Professor of Law, University of Utah S.J. Quinney College of Law. Thanks to Shima Baradaran Baughman, Barton Beebe, Michael Gentithes, Thomas Haley, Cathy Hwang, Ross McPhail, Ric Simmons, Megan Stevenson, Robert Williams, and all participants in the Privacy Law Scholars Conference, Rocky Mountain Junior Scholars Conference, University of Akron School of Law Faculty Workshop, and SEALS Conference for helpful comments and advice. Special thanks to Carolyn Howe, Nathan Hart Jackson, George LaBonty, and Matthew Nepute for excellent research assistance. All statistical analyses and charts were created by the author using R software, and any errors are his. All figures were initially created in R and modified in Excel.