The Fourth Amendment’s Hidden Intrusion Doctrine

*Assistant Professor of Law, SMU Dedman School of Law. Many thanks to Aliza Hochman Bloom, Laura Dolbow, Donald Dripps, Sarah Gottlieb, Paul Heaton, Shannon Heery, Anjelica Hendricks, Rachel Kincaid, Kate Levine, Evelyn Malavé, Sandra Mayson, Ross Miller, Jason Nance, Shaun Ossei-Owusu, David Rudovsky, Jenia Turner, and the participants in the Decarceration Law Professors 2024 Workshop for helpful conversations and feedback on earlier drafts. Special thanks to the faculty and staff of the Quattrone Center for the Fair Administration of Justice at the University of Pennsylvania Carey Law School for friendship, camaraderie, and fellowship support for this research. Also, thanks to Hanna Anssari and Pratyusha Mulpur for research assistance, and to the terrific editors of the Virginia Law Review for their meticulous work on this piece. Any errors are my own.Show More

The Fourth Amendment’s concept of probable cause is the linchpin of legal standards governing law enforcement actions such as arrests, searches, and seizures. This Article challenges the assumption that the same quantum of evidence can meet the probable cause standard regardless of whether law enforcement seeks to conduct a search, to seize evidence, or to make an arrest, and regardless of the intrusiveness of such search or seizure. This Article demonstrates that the Supreme Court implicitly considers the degree of intrusion into privacy or liberty, not just the quantum or quality of evidence, when determining whether probable cause exists. In doing so, I bring to light the Supreme Court’s “hidden intrusion doctrine.”

By failing to explicitly state that the degree of intrusion is a factor in the probable cause analysis, the Supreme Court injects ambiguity that has many consequences. Some lower courts and law enforcement agencies already balance the quantum or quality of evidence with the severity of intrusion, even without explicit Supreme Court guidance, but others do not. The ambiguity in the doctrine therefore fosters inconsistency and expands police discretion. Moreover, as technological advancements from facial recognition to digital searches reshape investigative techniques, the need for a clear articulation of the probable cause standard is increasingly urgent.

This Article suggests both doctrinal and policy-based proposals that would bring the Supreme Court’s intrusion doctrine out of the shadows and require deliberate consideration of the degree of intrusion in probable cause determinations. Such an approach would preserve law enforcement flexibility while safeguarding individual rights amidst evolving technological landscapes.

Introduction

In 2022, Alonzo Sawyer was arrested after facial recognition software generated a list of potential matches to surveillance images of a man who had recently assaulted a bus driver.¹ 1.Eyal Press, Does A.I. Lead Police to Ignore Contradictory Evidence?, New Yorker (Nov. 13, 2023), https://www.newyorker.com/magazine/2023/11/20/does-a-i-lead-police-to-ignore-contradictory-evidence.Show More Using the facial recognition software as the basis for probable cause, police arrested Mr. Sawyer and detained him for approximately a week with hardly any other investigation.² 2.Id.Show More After all, the image and software created a nexus between Mr. Sawyer and the crime. There was no question that the person depicted in the surveillance footage was the assailant.

The problem is that Mr. Sawyer was not the person in the image. After police arrested Mr. Sawyer, officers conducted several less invasive investigatory steps: police visited the house where he had been staying to search the premises for the clothing the assailant had been wearing in the surveillance photo and found nothing.³ 3.Id.Show More They interviewed his relatives, who confirmed Mr. Sawyer’s alibi that he had been at home many miles away at the time of the assault.⁴ 4.Id. Police also showed the surveillance photograph from the assault to Mr. Sawyer’s probation officer, who at first said he believed the photo was of Mr. Sawyer. Id. The probation officer, however, was told before he made the identification that Mr. Sawyer had been identified by facial recognition. Id. In addition, he had only met Mr. Sawyer wearing a COVID pandemic-era mask. Id. The probation officer later questioned his own identification and reached out to police to say that he had changed his mind—he no longer thought the surveillance image depicted Mr. Sawyer. Id.Show More These actions were not only far less intrusive than arresting Mr. Sawyer; they also revealed that the facial recognition software had been wrong.

Mr. Sawyer’s arrest turned on the definition of probable cause. Police used a match from new, relatively untested software to justify his arrest and detention for several days. Was that evidence enough to meet the probable cause standard to deprive him of his liberty, arguably the most severe intrusion implicated by the Fourth Amendment? What would have happened if police had been required to investigate further using less intrusive methods before arresting Mr. Sawyer? Could his week in jail have been avoided? Does the law require such an intermediate investigation when the only evidence is from new, untested technology?

This Article examines these questions through the lens of the Fourth Amendment’s probable cause standard, challenging the predominant understanding that the same quality or quantum of evidence—the evidentiary inputs—can satisfy the standard regardless of the degree of intrusion police want to exercise. The term probable cause comes directly from the text of the Fourth Amendment, which requires that “no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”⁵ 5.U.S. Const. amend. IV (emphasis added).Show More Drawing on this text, probable cause has become the standard⁶ 6.In this Article, I focus only on the Fourth Amendment intrusions that rely on probable cause to demonstrate how that analysis also includes the degree of intrusion, even though the Supreme Court does not explicitly acknowledge it as it does in the reasonable suspicion context. There are other types of search and seizure that do not require probable cause, such as searches incident to a lawful arrest, inventory searches, and sobriety checkpoints, which are not justified by any degree of suspicion that a specific person has committed a crime, see infra notes 16, 69, 104 and accompanying text, and so-called Terry stops, which are supported by reasonable suspicion, a lower standard than probable cause. See infra note 37.Show More by which law enforcement can, while investigating a crime, conduct three separate and distinct actions that implicate individuals’ privacy or liberty interests⁷ 7.Law enforcement may also collect evidence through many different types of actions that require little, if any, individualized suspicion that evidence will be found in a particular place. See, e.g., United States v. Tuggle, 4 F.4th 505, 510–11 (7th Cir. 2021); Smith v. Maryland, 442 U.S. 735, 745–46 (1979); 18 U.S.C. § 2703(d) (allowing disclosure of certain electronic communications based on the government’s representation that records are “relevant and material to an ongoing criminal investigation”). Nevertheless, as discussed elsewhere in this Article, the probable cause standard is required for many of the actions law enforcement conduct that most deeply intrude into individuals’ privacy. See infra Section I.A.Show More: arrest,⁸ 8.See, e.g., Maryland v. Pringle, 540 U.S. 366, 368 (2003). Notably, police do not need a warrant to arrest someone in public, as long as the arrest is based on probable cause. United States v. Watson, 423 U.S. 411, 423–24 (1976).Show More search of a constitutionally protected area,⁹ 9.See, e.g., Illinois v. Gates, 462 U.S. 213, 246 (1983).Show More and seizure of evidence.¹⁰ 10.See, e.g., Chambers v. Maroney, 399 U.S. 42, 51–52 (1970). Probable cause is also the standard for many other iterations of these basic intrusions. See infra notes 20–25 and accompanying text.Show More

Most scholars would say that the same standard of probable cause applies¹¹ 11.See 2 Wayne R. LaFave, Jerold H. Israel, Nancy J. King & Orin S. Kerr, Criminal Procedure § 3.3(a), at 118 n.8 (4th ed. 2015) (“Thus, in Spinelli v. United States, 393 U.S. 410 . . . (1969), where the issue was whether the challenged search warrant had been issued on probable cause, the Court found its earlier decision in Draper v. United States, 358 U.S. 307 . . . (1959), involving grounds to arrest, to be a ‘suitable benchmark’ for resolving that issue. Similarly, Illinois v. Gates, 462 U.S. 213 . . . (1983), which significantly altered the probable cause formula in a search warrant context, has readily been deemed applicable to arrest warrants as well. State v. Schroeder, 450 N.W.2d 423 (N.D. 1990).”). The treatise authors note that the question of “probable cause of what” will differ in various contexts, but the general quantum of evidence and the analysis is the same. Id. § 3.3(a). See also Craig S. Lerner, The Reasonableness of Probable Cause, 81 Tex. L. Rev. 951, 953–54 (2003) (“[P]robable cause is widely viewed in the legal community as a fixed standard.”).Show More regardless of whether police wanted to get a warrant for Mr. Sawyer’s phone records or arrest him. I argue that the law requires more, or stronger, evidence to justify a greater intrusion into an individual’s privacy or liberty than the evidence necessary to justify a lesser intrusion. Under such an approach, for example, the facial recognition match might, alone, provide probable cause to obtain phone records, but not to arrest Mr. Sawyer and deprive him of his liberty.

In this Article I show that, contrary to widespread assumptions, the Supreme Court has implicitly acknowledged that whether probable cause exists to justify any given search or seizure depends in part on the degree of intrusion the particular search or seizure entails. For example, the Court has required more or stronger evidence to justify the search of a home or a custodial arrest than it has for the search of a car, even though “probable cause” is the standard for all three. But the assumption that the degree of intrusion does not factor into the totality of the circumstances¹² 12.See Gates, 462 U.S. at 246.Show More test used to evaluate probable cause is understandable because the Supreme Court has failed to say so explicitly. This Article identifies and defines these considerations in what I term the Fourth Amendment’s “hidden intrusion” doctrine.

The Supreme Court’s vagueness on its intrusion doctrine and the ensuing failure of lower courts and law enforcement to consistently consider the degree of intrusion in determining whether probable cause exists have injected ambiguity that has many consequences,¹³ 13.See Section I.C.Show More including arrests like Mr. Sawyer’s. It allows police to proceed less cautiously, moving straight to an arrest without any intermediate investigation. It gives cover to police who arrest when they could first search to confirm their suspicions, and it allows them to conduct a more intrusive search when a less intrusive search would suffice. It insulates police decision-making from judicial review and civil liability. It sows confusion for trial courts applying the probable cause standard, for judges reviewing warrant applications, and for officers on the street trying to conduct police work that comports with the Fourth Amendment. It undermines transparency and consistency in the application of the probable cause standard. It generally expands police power.

Though some scholars have argued that balancing the quantum or quality of evidence collected by police with the degree of intrusion involved in a search or seizure should be part of the Fourth Amendment analysis,¹⁴ 14.See Christopher Slobogin, Let’s Not Bury Terry: A Call for Rejuvenation of the Proportionality Principle, 72 St. John’s L. Rev. 1053, 1053–54 (1998) [hereinafter Slobogin, Let’s Not Bury Terry] (“That framework, which I call the proportionality principle, is very simple: A search or seizure is reasonable if the strength of its justification is roughly proportionate to the level of intrusion associated with the police action.”); Andrew Manuel Crespo, Probable Cause Pluralism, 129 Yale L.J. 1276, 1343 (2020). Over thirty years ago, Christopher Slobogin recognized the value of incorporating the degree of intrusion into search and seizure analysis in his article imagining how the government would consider a search and seizure doctrine if the Fourth Amendment did not exist. See Christopher Slobogin, The World Without a Fourth Amendment, 39 UCLA L. Rev. 1, 49–50 (1991) [hereinafter Slobogin, The World Without a Fourth Amendment]. Slobogin argued that the “proportionality principle” was not part of existing Fourth Amendment doctrine but represented the proper balance of interests between the State and the populace. Id. (“Put another way, because the cost to individual interests of a mistake by the police is greatest when the intrusion is greatest, and diminishes as the intrusion lessens, the tolerance for such mistakes should vary inversely with the level of intrusion. Note further that the cost to the state of a police mistake is also greatest when the intrusion is greatest, because the public is more likely to perceive unnecessary police intrusions as illegitimate when they are significant.” (emphasis omitted)). This Article argues that Slobogin’s “proportionality principle” is already being applied by the courts, even if it has never been explicitly incorporated into Fourth Amendment doctrine.Show More this Article is the first to argue that such balancing is already part of the jurisprudence. The common, unitary understanding of probable cause—that one should entirely ignore the degree of intrusion in deciding whether probable cause exists to justify a particular search or seizure—misapplies important nuances in Supreme Court doctrine.

In addition, I show that there is no need for the Supreme Court’s intrusion doctrine to remain hidden and that bringing it to light would increase transparency and consistency in decisions made by lower courts and law enforcement. There is in fact broad understanding among system actors that the degree of intrusion is relevant in evaluating whether a search or seizure is justified. Some courts and police are already doing such balancing on an ad hoc basis,¹⁵ 15.See infra Section II.A.Show More requiring more evidence to justify the search of a home than a car, or to justify a search of someone’s crotch than less private parts of their body. But without guidance from the Supreme Court sanctioning such consideration, they are merely acting on the instinct that the degree of intrusion should be part of the search and seizure calculus. The result is a hodgepodge legal doctrine of probable cause that rarely discusses degree of intrusion, even if it is implicitly part of the analysis. Police and judges are left with no coherent limiting principle—some follow their instinct that more or more reliable evidence should be necessary to justify a greater intrusion into individuals’ privacy, but others do not.

An examination of how the police apply the probable cause standard in two areas of emerging technology provides useful case studies for the necessity of a clear intrusion doctrine. As untested technologies, like the facial recognition technology used in Mr. Sawyer’s case, are used to develop leads that then justify searches, seizures, and arrests, courts and police need guidance to help understand how new technologies should be used in the probable cause analysis. A clear intrusion doctrine would guide courts and policy-makers as they respond to an ever-changing technological law enforcement landscape, providing guardrails that protect citizens from undiscovered flaws in new technology while still allowing law enforcement agencies to use new technology. An explicit understanding of how the degree of intrusion applies in the probable cause analysis would encourage police to tread carefully with untested technology. It would encourage them to search before they arrest and to minimize harm from unanticipated errors.

Likewise, bringing the Supreme Court’s hidden intrusion doctrine into the light—by making clear that whether probable cause exists depends in part on the degree of intrusion of a particular search or seizure—could help solve the vexing problems around seizure and search of cell phones. As with any physical evidence they seize in an investigation, police must generally have probable cause to seize a cell phone that they believe contains evidence of a crime.¹⁶ 16.See, e.g., Chambers v. Maroney, 399 U.S. 42, 51–52 (1970). There are some limited circumstances in which the seizure of physical evidence need not be based on probable cause, such as in the course of an inventory search. See, e.g., Colorado v. Bertine, 479 U.S. 367, 371 (1987).Show More As the Supreme Court recognized in Riley v. California, “[m]odern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans ‘the privacies of life.’”¹⁷ 17.Riley v. California, 573 U.S. 373, 403 (2014).Show More Police may have reason to believe that some particular type of evidence may be stored in a cell phone, but does that give them license to search every part of the phone? And if it does, should it? Courts authorizing search warrants have grappled with this question and have come to differing opinions on whether the same evidence to seize a phone or search part of it would justify a search of the entire phone.¹⁸ 18.See infra notes 239–41.Show More Recognition of the Fourth Amendment’s hidden intrusion doctrine would help resolve this tension, requiring more evidence to justify a search of a phone than a seizure of a phone.

In a world in which emerging technologies like facial recognition technology are implemented quickly, often before any meaningful analysis of their reliability or risks, or where such technologies allow police to glean an entirely different degree of information about individuals’ private lives as done through phone searches, courts and policy-makers should be willing to move more slowly. This Article argues that in light of the Supreme Court’s hidden intrusion doctrine, they must move more slowly, and it proposes an approach that I call “graduated probable cause.” Under this approach, to comply with the (previously hidden, now explicit) intrusion doctrine, police would have to collect more or stronger evidence to justify greater intrusions like an arrest or a search of a home. If they had less evidence, or less reliable evidence, they could conduct a lesser intrusion, such as the search of a car or seizure of a cell phone, as an intermediate step. Such a process would maintain law enforcement flexibility, but also protect individuals’ privacy and liberty interests in the face of a constantly changing technological landscape.

This Article proceeds in four Parts. In Part I, I provide an overview of probable cause, examining both how the Supreme Court has defined the standard and how previous scholars have attempted to untangle that messy doctrine, to illustrate that Supreme Court probable cause precedent already implicitly considers the degree of intrusion a particular search or seizure makes into an individual’s privacy or liberty. In Part II, I examine case law from lower federal courts and state courts to demonstrate how, despite broad statements about a unitary standard, courts have already drawn distinctions in practice about the quantum of evidence necessary to provide probable cause based on the degree of intrusion. I also demonstrate how police trainings and policies can either reinforce the idea of a unitary standard to preserve police discretion or provide a model demonstrating the workability of a probable cause analysis that does include consideration of the degree of intrusion. In Part III, I examine in depth the case studies of facial recognition software and phone searches to demonstrate the confusion created by a unitary probable cause standard and the ways in which explicit consideration of the degree of intrusion could improve probable cause analysis in a rapidly changing digital environment. In Part IV, I articulate possible doctrinal and policy solutions for rearticulating the probable cause standard based on the degree of intrusion. Acknowledging that the degree of intrusion is already implicitly part of the analysis, I argue for explicitly incorporating it into the totality of the circumstances analysis that courts already apply. In other words, the courts do not need a new framework; they just need to honestly articulate the interest balancing that already drives decisions. I also set forth policy proposals for police, based on a graduated approach to investigation already endorsed by some law enforcement agencies, that would direct police to first conduct less intrusive searches and seizures in circumstances where they are relying on less, or less reliable, evidence.

1.  Eyal Press, Does A.I. Lead Police to Ignore Contradictory Evidence?, New Yorker (Nov. 13, 2023), https://www.newyorker.com/magazine/2023/11/20/does-a-i-lead-police-to-ignore-contradictory-evidence. ↑
2.  Id. ↑
3.  Id. ↑
4.  Id. Police also showed the surveillance photograph from the assault to Mr. Sawyer’s probation officer, who at first said he believed the photo was of Mr. Sawyer. Id. The probation officer, however, was told before he made the identification that Mr. Sawyer had been identified by facial recognition. Id. In addition, he had only met Mr. Sawyer wearing a COVID pandemic-era mask. Id. The probation officer later questioned his own identification and reached out to police to say that he had changed his mind—he no longer thought the surveillance image depicted Mr. Sawyer. Id. ↑
5.  U.S. Const. amend. IV (emphasis added). ↑
6.  In this Article, I focus only on the Fourth Amendment intrusions that rely on probable cause to demonstrate how that analysis also includes the degree of intrusion, even though the Supreme Court does not explicitly acknowledge it as it does in the reasonable suspicion context. There are other types of search and seizure that do not require probable cause, such as searches incident to a lawful arrest, inventory searches, and sobriety checkpoints, which are not justified by any degree of suspicion that a specific person has committed a crime, see infra notes 16, 69, 104 and accompanying text, and so-called Terry stops, which are supported by reasonable suspicion, a lower standard than probable cause. See infra note 37. ↑
7.  Law enforcement may also collect evidence through many different types of actions that require little, if any, individualized suspicion that evidence will be found in a particular place. See, e.g., United States v. Tuggle, 4 F.4th 505, 510–11 (7th Cir. 2021); Smith v. Maryland, 442 U.S. 735, 745–46 (1979); 18 U.S.C. § 2703(d) (allowing disclosure of certain electronic communications based on the government’s representation that records are “relevant and material to an ongoing criminal investigation”). Nevertheless, as discussed elsewhere in this Article, the probable cause standard is required for many of the actions law enforcement conduct that most deeply intrude into individuals’ privacy. See infra Section I.A. ↑
8.  See, e.g., Maryland v. Pringle, 540 U.S. 366, 368 (2003). Notably, police do not need a warrant to arrest someone in public, as long as the arrest is based on probable cause. United States v. Watson, 423 U.S. 411, 423–24 (1976). ↑
9.  See, e.g., Illinois v. Gates, 462 U.S. 213, 246 (1983). ↑
10.  See, e.g., Chambers v. Maroney, 399 U.S. 42, 51–52 (1970). Probable cause is also the standard for many other iterations of these basic intrusions. See infra notes 20–25 and accompanying text. ↑
11.  See 2 Wayne R. LaFave, Jerold H. Israel, Nancy J. King & Orin S. Kerr, Criminal Procedure § 3.3(a), at 118 n.8 (4th ed. 2015) (“Thus, in Spinelli v. United States, 393 U.S. 410 . . . (1969), where the issue was whether the challenged search warrant had been issued on probable cause, the Court found its earlier decision in Draper v. United States, 358 U.S. 307 . . . (1959), involving grounds to arrest, to be a ‘suitable benchmark’ for resolving that issue. Similarly, Illinois v. Gates, 462 U.S. 213 . . . (1983), which significantly altered the probable cause formula in a search warrant context, has readily been deemed applicable to arrest warrants as well. State v. Schroeder, 450 N.W.2d 423 (N.D. 1990).”). The treatise authors note that the question of “probable cause of what” will differ in various contexts, but the general quantum of evidence and the analysis is the same. Id. § 3.3(a). See also Craig S. Lerner, The Reasonableness of Probable Cause, 81 Tex. L. Rev. 951, 953–54 (2003) (“[P]robable cause is widely viewed in the legal community as a fixed standard.”). ↑
12.  See Gates, 462 U.S. at 246. ↑
13.  See Section I.C. ↑
14.  See Christopher Slobogin, Let’s Not Bury Terry: A Call for Rejuvenation of the Proportionality Principle, 72 St. John’s L. Rev. 1053, 1053–54 (1998) [hereinafter Slobogin, Let’s Not Bury Terry] (“That framework, which I call the proportionality principle, is very simple: A search or seizure is reasonable if the strength of its justification is roughly proportionate to the level of intrusion associated with the police action.”); Andrew Manuel Crespo, Probable Cause Pluralism, 129 Yale L.J. 1276, 1343 (2020). Over thirty years ago, Christopher Slobogin recognized the value of incorporating the degree of intrusion into search and seizure analysis in his article imagining how the government would consider a search and seizure doctrine if the Fourth Amendment did not exist. See Christopher Slobogin, The World Without a Fourth Amendment, 39 UCLA L. Rev. 1, 49–50 (1991) [hereinafter Slobogin, The World Without a Fourth Amendment]. Slobogin argued that the “proportionality principle” was not part of existing Fourth Amendment doctrine but represented the proper balance of interests between the State and the populace. Id. (“Put another way, because the cost to individual interests of a mistake by the police is greatest when the intrusion is greatest, and diminishes as the intrusion lessens, the tolerance for such mistakes should vary inversely with the level of intrusion. Note further that the cost to the state of a police mistake is also greatest when the intrusion is greatest, because the public is more likely to perceive unnecessary police intrusions as illegitimate when they are significant.” (emphasis omitted)). This Article argues that Slobogin’s “proportionality principle” is already being applied by the courts, even if it has never been explicitly incorporated into Fourth Amendment doctrine. ↑
15.  See infra Section II.A. ↑
16.  See, e.g., Chambers v. Maroney, 399 U.S. 42, 51–52 (1970). There are some limited circumstances in which the seizure of physical evidence need not be based on probable cause, such as in the course of an inventory search. See, e.g., Colorado v. Bertine, 479 U.S. 367, 371 (1987). ↑
17.  Riley v. California, 573 U.S. 373, 403 (2014). ↑
18.  See infra notes 239–41. ↑