The Fourth Amendment protects reasonable expectations of privacy, but the Supreme Court has refused to provide a consistent explanation for what makes an expectation of privacy “reasonable.” The Court's refusal has disappointed scholars and frustrated students for four decades. This Article explains why the Supreme Court cannot provide an answer: no one test can accurately and consistently distinguish less troublesome police practices that do not require Fourth Amendment oversight from more troublesome police practices that are reasonable only if the police have a warrant or compelling circumstances. Instead of endorsing one approach, the Supreme Court has recognized four coexisting approaches. There are four models of Fourth Amendment protection: a probabilistic model, a private facts model, a positive law model, and a policy model. Using multiple models has a major advantage over using one model. It allows the courts to use different approaches in different contexts depending on which approach most accurately and consistently identifies practices that need Fourth Amendment regulation. Explicit recognition of the four models would advance this function, resulting in more accurate and consistent Fourth Amendment rules.


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