This Article offers a defense of the Fourth Amendment’s third-party doctrine, the controversial rule that information loses Fourth Amendment protection when it is knowingly revealed to a third party. Fourth Amendment scholars have repeatedly attacked the rule on the ground that it is unpersuasive on its face and gives the government too much power. This Article responds that critics have overlooked the benefits of the rule and have overstated its weaknesses.
The third-party doctrine serves two critical functions. First, the doctrine ensures the technological neutrality of the Fourth Amendment. It corrects for the substitution effect of third parties that would otherwise allow savvy criminals to substitute a hidden third-party exchange for a previously public act. Second, the doctrine helps ensure the clarity of Fourth Amendment rules. It matches the Fourth Amendment rules for information to the rules for location, creating clarity without the need for a complex framework of sui generis rules.
Finally, the two primary criticisms of the third-party doctrine are significantly weaker than critics have claimed. The third-party doctrine is awkward for reasons of form rather than function; it is a consent rule disguised as an application of Katz’s “reasonable expectation of privacy” test. Claims that the doctrine gives the government too much power overlook the substitutes for Fourth Amendment protection in the use of the third parties. Those substitutes include entrapment law, common law privileges, the Massiah doctrine, the First Amendment, internal agency regulations, and the rights of the third parties themselves.