Actual Expectations of Privacy, Fourth Amendment Doctrine, and the Mosaic Theory

The mosaic theory of the Fourth Amendment holds that, when it comes to people’s reasonable expectations of privacy, the whole is greater than the sum of its parts. More precisely, it suggests that the government can learn more from a given slice of information if it can put that information in the context of a broader pattern, a mosaic. This insight, that the incremental privacy threat posed by the government’s acquisition of information increases as more information is obtained, was given its most forceful articulation by Judge Douglas Ginsburg of the D.C. Circuit in the landmark case that ultimately became United States v Jones. Writing for the Court of Appeals, Judge Ginsburg used a mosaic theory to explain why long-term geolocation surveillance of a vehicle was categorically different from short-term surveillance: Prolonged surveillance reveals types of information not revealed by short-term surveillance, such as what a person does repeatedly, what he does not do, and what he does ensemble.

These types of information can each reveal more about a person than does any individual trip viewed in isolation. Repeated visits to a church, a gym, a bar, or a bookie tell a story not told by any single visit, as does one’s not visiting any of these places over the course of a month. The sequence of a person’s movements can reveal still more; a single trip to a gynecologist’s office tells little about a woman, but that trip followed a few weeks later by a visit to a baby supply store tells a different story.