Review of The Digital Fourth Amendment

Kerr, Orin. The Digital Fourth Amendment: Privacy and Policing in Our Online World. Oxford University Press. 2025.
 
The Fourth Amendment to the United States Constitution limits how the government may conduct searches and seizures, with the effect that evidence can be excluded from trial if it was obtained in violation of that amendment. Technological advances such as the automobile, phone booth, and Internet have required courts to reinterpret Fourth Amendment law to accommodate these novelties.
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Lower courts often differ in their interpretations and, after the technologies and law have settled a bit, the Supreme Court gets involved. Author Orin Kerr, the William G. Simon Professor at the University of California, Berkeley Law School, presents a framework for adjudicating digital searches and seizures in The Digital Fourth Amendment: Privacy and Policing in Our Online World, published by Oxford University Press. His approach, equilibrium-adjustment, “takes the legal rules that existed before the technological shift as a starting point, and it asks how to preserve the values served by those rules amid the gale-force winds of technological change.”

Background
Professor Kerr’s book reinforces his status as a leading law professor and Fourth Amendment theorist—it provides a thorough background in privacy law, starting back in Entick v. Carrington, a 1765 decision from the English courts proscribing overly broad warrants, and continues through foundational cases in U.S. jurisprudence such as Katz v. United States (1967), which addressed placing a microphone on the outside of a telephone booth, as well as United States v. Knotts (1983) and United States v. Karo (1984) that dealt with beepers hidden in five-gallon drums of chemicals used in drug production. The Supreme Court found that tracking the beepers in public did not violate the defendant’s privacy expectation but did as soon as the devices moved inside a home.

I appreciate the thoroughness and efficiency of Kerr’s background coverage. Specifically, he resists the temptation to mention prominent cases such as Griswold v. Connecticut (1965) that aren’t on-point for this argument and saves space for more relevant citations.

Local Devices
Part II addresses local devices such as computers and mobile phones, presenting Kerr’s arguments for how courts should adjudicate searches and warrant requirements based on his equilibrium-adjustment paradigm. After presenting valuable background on how these searches are conducted (usually by creating an image of the device, if possible), he argues for his preferred interpretation of searches in light of how the enormous storage capacity of a digital device compares to the traditional paper filing system on which Fourth Amendment jurisprudence evolved.

Part III moves on to networks and the third-party doctrine, which states that information freely exposed to a third party, such as the address on the outside of an envelope or numbers dialed on a phone network, can be accessed routinely by law enforcement. Other topics include the use of a GPS tracker placed on a defendant’s car, as was examined in United States v. Jones (2012); automated license plate readers; and pole cameras installed near a place of interest to provide sustained surveillance. Per that last point, through his Bluesky account Professor Kerr shared an 11th Circuit Court ruling that 10 months of such observation didn’t constitute a search because the areas surveilled were in public view.

A Vague Standard at Best
Kerr devotes all of Chapter 9 to Carpenter v. United States (2018) because of its seismic impact on the digital Fourth Amendment. The opinion found that requesting cell-site location information (CSLI) from a mobile services provider required a search warrant for…reasons. The lack of a concrete rule for lower courts to follow has caused “deep uncertainty” and led to courts proceeding cautiously. To remedy that uncertainty, Kerr proposes a three-element test to determine if a warrant is required:

• The records must be of a new type made available by the digital world.
• Generating the records must be unavoidable.
• The information must be from the kinds of records that reveal the privacies of life.

These tests are simple to state but require significant interpretation and analysis. I plan to revisit his discussion in the near future and believe this chapter offers substantial insight for me as a non-lawyer but especially for attorneys and judges addressing these issues.

Moving Forward
The remainder of The Digital Fourth Amendment discusses matters at the forefront of the law, including government buying data from private sellers, geofencing warrants gathered from Google Maps and similar services, and the “unanswerable questions” of what Kerr calls mosaic theory as argued in United States v. Maynard (2010), a companion case to United States v. Jones. The DC Circuit’s ruling in Maynard found that tracking a car briefly using a GPS device would not constitute a search, but 28 days of such tracking without a warrant allowed the authorities to create an “intimate picture” of the defendant’s life that was not reasonable to allow.  While the Massachusetts Supreme Judicial Court has wholeheartedly embraced mosaic theory, Kerr isn’t convinced of its utility because it seems impossible to draw a bright line demarking where a collection of data points taken in aggregate crosses into areas protected by the Fourth Amendment.

Conclusion
I believe The Digital Fourth Amendment provides an outstanding summary of the state of play in Fourth Amendment jurisprudence. In addition, Professor Kerr’s paradigm of equilibrium-adjustment offers a path forward as technology changes. While new laws and Supreme Court decisions (should they ever decide another Fourth Amendment case) will shift the playing field, one could do much worse than using equilibrium-adjustment to maintain the existing value structure.

One might ask whether equilibrium-adjustment is required…in many ways it seems obvious and merely a description of the status quo. I believe the paradigm might seem obvious in hindsight but laying it out explicitly is useful. In the second edition of Legal Argument: The Structure and Language of Effective Advocacy, James A. Gardner writes that “the judge wants nothing more than to be led out of this jungle by a confident, reliable guide . . . .” I believe Professor Kerr has provided such a guide though we can expect the specifics to be debated for years to come.