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For over fifty years, the Fourth Amendment’s scope has been largely dictated by the Katz test, which applies the Amendment’s protections only when the government has violated a person’s “reasonable expectation of privacy.” This vague standard is one of the most criticized doctrines in all of American law, and its lack of coherence has made Fourth Amendment search law notoriously confusing. Things have become even more complex following the Supreme Court’s landmark decision in Carpenter v. United States, which has spawned its own alternative test for determining the Fourth Amendment’s scope. The emerging Carpenter test looks to the revealing nature of the data at issue, the amount of data collected, and whether the data was voluntarily disclosed to others.

This Essay examines the uneasy state of current Fourth Amendment law, in which the Katz and Carpenter paradigms overlap and compete in the lower courts. It describes the many ways that courts have attempted to integrate these two frameworks. It also assesses several potential metaprinciples that might be used to determine when each test should be applied.

Based on this analysis, this Essay contends that the Carpenter test should be the primary test for Fourth Amendment searches going forward. Carpenter creates a coherent, multi-factor test that lower courts have already successfully applied in numerous cases. Its conceptual reach is universal, capable of addressing any Fourth Amendment scenario. And the test focuses arguments and produces clear answers, offering far more predictability than its predecessor. This Essay identifies the theoretical and jurisprudential foundations of the Carpenter test, tracing its origins to longstanding Supreme Court precedents and evaluating its application in contemporary cases. Ultimately, the Carpenter test can clarify when individuals will be protected against government surveillance and provide courts with meaningful guidance and direction.