Actual Expectations of Privacy, Fourth Amendment Doctrine, And The Mosaic Theory

70 Pages Posted: 11 Jul 2015 Last revised: 19 Oct 2017

Date Written: October 29, 2015


In the landmark case of United States v. Jones, as many as five Supreme Court justices indicated that tracking the geolocation of a car for a month would be a Fourth Amendment search even though tracking the same car for a day would not be. This duration distinction is based on an influential theory of the Fourth Amendment, dubbed the mosaic theory, which posits that the aggregation of several non-searches of the same person might amount to a search. Jurists have justified the mosaic theory’s duration-sensitivity by grounding it in their sense of “popular attitudes” regarding privacy expectations. Through an empirical examination of survey responses from three large nationally representative samples totaling over 2800 US citizens, we show that Americans’ actual privacy expectations run directly counter to the mosaic theory. Where the mosaic theory says that tracking duration affects citizens’ expectations of privacy, ordinary Americans overwhelmingly say it does not. Our data also reveal that younger Americans and those Americans holding the most firmly anti-authoritarian views have significantly greater expectations of privacy in geolocation information than their fellow citizens. Americans do say that longer duration surveillance is more intrusive than shorter duration surveillance, but the magnitude of this effect remains small.

We explore the implications of these findings for the mosaic theory by considering the role of public opinion data in Fourth Amendment doctrine more generally. We ultimately propose a clarified approach to the classic Katz v. United States “reasonable expectations of privacy” framework that formalizes the role of public opinion by reframing the first prong of Katz to ask whether people in general expect privacy in a given context, and the question of what “society is prepared to recognize as reasonable” in Katz as one for which the perceived intrusiveness of a search is germane. To show how survey data could shed light on current Fourth Amendment controversies, we also provide contemporary data about American citizens’ privacy expectations when faced with various scenarios. The paper presents new data on popular expectations of privacy with regard to police use of stingray devices, cell tower geolocation, email content analysis, hotel guest registry searches, and various sorts of surveillance cameras.

Keywords: Fourth Amendment, privacy, reasonable expectations of privacy, mosaic theory, law & psychology, constitutional law

Suggested Citation

Kugler, Matthew B. and Strahilevitz, Lior, Actual Expectations of Privacy, Fourth Amendment Doctrine, And The Mosaic Theory (October 29, 2015). 2015 Supreme Court Review 4 (2016), University of Chicago Coase-Sandor Institute for Law & Economics Research Paper No. 727, U of Chicago, Public Law Working Paper No. 534, Available at SSRN: or

Matthew B. Kugler

Northwestern University - Pritzker School of Law ( email )

375 E. Chicago Ave
Chicago, IL 60611
United States

Lior Strahilevitz (Contact Author)

University of Chicago Law School ( email )

1111 E. 60th St.
Chicago, IL 60637
United States
773-834-8665 (Phone)
773-702-0730 (Fax)

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