The Surveillance Regulation Toolkit: Thinking Beyond Probable Cause

Privacy and Security and Innovation and Economic Growth

Article Snapshot


Paul Ohm


in The Cambridge Handbook of Surveillance Law, David Gray and Stephen E. Henderson, eds., Cambridge University Press, 2017, pp. 491-508


Limits on government surveillance are often based on judicial review of “probable cause” alone. New technologies and surveillance techniques mean that the “probable cause” determination no longer protects privacy and liberty adequately.

Policy Relevance

Judges should recognize that surveillance can be unconstitutional for many reasons. Legislators should improve public reporting of surveillance.

Main Points

  • The main limit on government surveillance today is judicial review, coupled with the “probable cause” standard (or, sometimes, a lower standard like “reasonable suspicion”); if a police officer cannot show a judge enough evidence to support a finding of probable cause, some forms of surveillance are off limits.
  • Technological change means that the probable cause standard fails to effectively limit intrusive surveillance.
    • Search warrants grant police access to devices with massive data storage, such as laptops, leading to broad searches like the unconstitutional “general warrant.”
    • The probable cause inquiry does not limit surveillance by machine learning systems, which use data in the public domain to predict crime, without focusing on a particular crime or suspect.
  • In 1967, the Supreme Court ruled that surveillance could be unconstitutional even if the probable cause standard was met, when the police failed to identify a particular crime or target a particular message, failed to notify the target, and continued the surveillance too long; judges should follow this precedent in going beyond probable cause to limit surveillance.
  • Policymakers can use on five types of regulatory tools to limit surveillance, including the following:
    • Evidentiary justification standards, such as probable cause.
    • Meaningful review by judges or other authorities.
    • Limits on the amount or scope of surveillance, such as time limits.
    • Accountability measures, such as sanctions on police officers who lie.
    • Transparency measures, such as notice to the target of the surveillance.
  • Meaningful review by an external authority such as a judge or higher-ranked official should not just be a rubber-stamp; the authority should be empowered to scrutinize facts, and should be detached and neutral.
  • Limits on surveillance can include time limits, the expiration of surveillance of authority when certain conditions are met, and limits on the types of targets (such as the requirement that the target by an agents of a foreign power).
  • Improved transparency measures would include ensuring that notice or delayed notice of the surveillance be given to targets of surveillance; also, “gag orders” barring third parties who assist with surveillance from notifying the targets should be restricted.


Get The Article

Find the full article online

Search for Full Article