The U.S. Supreme Court building in Washington, D.C. (Carlos Barria/Reuters)

My friend and colleague Will Baude blogged yesterday about the positive law model of the Fourth Amendment and its possible role in the Carpenter Supreme Court case. I thought I would chime in on why I disagree. In my view, Will’s positive law approach is seriously flawed as a method of interpreting the Fourth Amendment.

Let me start with some context. In 2007, I wrote an article, “Four Models of Fourth Amendment Protection,” on how courts interpret the “reasonable expectation of privacy” test. I found that, as a descriptive matter, the Justices mixed and matched among four different and often contradictory models for what makes an expectation of privacy “reasonable.” In some cases, they looked to a probabilistic model and considered the likelihood of a privacy invasion. In some cases, they looked to a private facts model and considered whether the outcome of the conduct was acquisition of particularly private information. In some cases, they looked to a positive law model and considered whether the government violated a law other than the Fourth Amendment. And in some (most) cases, they looked to a policy model and considered whether it was desirable to regulate the government practice. Most Supreme Court opinions mixed and matched among the models, sometimes invoking multiple models and sometimes rejecting others.

I argued in my “Four Models” article that this pluralism was desirable. In applying the reasonable expectation of privacy test, the court was trying to distinguish less invasive practices from more invasive practices. But there was no universal method to do that. The court had naturally developed proxy tests for distinguishing more invasive practices that accurately tracked invasiveness in some cases but not all cases. Because no one test accurately distinguished more and less invasive practices, the court couldn’t adopt any one model. Instead, it had developed localized models to guide lower courts: The Supreme Court tended to use whatever model(s) accurately divided less from more invasive practices in that kind of case, and then lower courts would reason by analogy and apply those same models to similar cases. The result was a relatively clever way to distinguish less from more invasive police practices in a decentralized system in which there are thousands of lower court decisions and few Supreme Court rulings.

In their article, “The Positive Law Model of the Fourth Amendment,” Baude and our mutual friend James Stern take one of the four models and say it should be the exclusive test for what is a search. From their abstract:

The touchstone of the search-and-seizure analysis should be whether government officials have done something forbidden to private parties. It is those actions that should be subjected to Fourth Amendment reasonableness review and the presumptive requirement to obtain a warrant. In short, Fourth Amendment protection should depend on property law, privacy torts, consumer laws, eavesdropping and wiretapping legislation, anti-stalking statutes, and other provisions of law generally applicable to private actors, rather than a freestanding doctrine of privacy fashioned by courts on the fly.

At the outset, it’s worth noting that this is a pretty radical theory. The court has looked to positive law in some cases, primarily in the context of property law. But I believe the new theory would expand the positive law focus from property law to any law so long as it is “generally applicable to private actors.” Off the top of my head, I’m not sure which existing Supreme Court precedents on Fourth Amendment search doctrine could survive that revolutionary standard.

The broader conceptual problem with a universal positive law model is that it has no connection to what the Fourth Amendment search test is trying to do. Although positive law accurately tracks notions of invasiveness in some contexts, often positive law has nothing whatsoever to do with anything relevant to Fourth Amendment concerns. Laws are enacted for an extraordinary range of reasons, and a universal positive law model would have to give Fourth Amendment relevance to all of them.

Here’s what I wrote about this in “Four Models” in 2007:

The positive law model has . . . flaws as a universal guide. Like the probabilistic model, the positive law model provides an accurate proxy for the reasonableness of police practices in many cases. Positive law that restricts access to information and places often reflects widely shared notions of which accesses cause significant harms and which do not. Courts, legislatures, and agencies will tend to promulgate rules restricting access to a person’s private material when unfettered access will be harmful, which is a major factor in determining whether a particular government practice is reasonable per se or reasonable only in context. This means that government practices that violate positive law will often be reasonable only in context, and those practices that do not will often be reasonable per se.

Physical intrusions into the home provide an obvious example. Breaking into to your house interferes with your property rights; it also reflects a deeply invasive affront to your privacy and security. On the other hand, watching your house from a public street does not violate positive law; while it may be a bit creepy, it does not amount to a severe invasion of your privacy. In this context, positive law nicely matches our intuitions as to which kinds of police practices should be permitted without a warrant. The positive law model provides a useful proxy to determine when a government invasion violates a reasonable expectation of privacy.

But much like the probabilistic model, the positive law model does not work in every case. In many circumstances, positive law will not accurately capture whether police practices are per se reasonable. This is true for two primary reasons. First, positive laws are enacted for a wide range of reasons that may have nothing to do with whether access by criminal investigators would be reasonable per se. Consider the FAA regulations analyzed in Florida v. Riley, the helicopter flyover case. The FAA presumably drafted those regulations to minimize noise and deter accidents, not to limit the police. Whether the police happened to fly over or under FAA airspace limits has no significant connection to whether particular police flyovers are reasonable only if justified by a warrant. Similarly, imagine a person lives on a 100-acre farm with their small house in the center. Trespassing a few feet onto that person’s property will violate his property rights but not infringe on his privacy. In that setting, positive law does not accurately track the reasonableness of the government’s investigation.

The positive law model may also be insufficient when technology or social practice changes rapidly. The story is a familiar one, as it tracks the standard explanation for the move from Olmstead to Katz. Technology tends to shift the privacy implications of different law enforcement steps. New technologies can divorce privacy and social norms from property law and other statutory and regulatory protections. New technological surveillance tools make it possible to invade privacy without interfering with property or other laws, and they can also make it possible to establish privacy without harnessing positive law. As a result, technological change can make the function of positive law largely arbitrary; it no longer serves the same function it might if the technology remained stable. A test that focuses entirely on positive law such as property will be underprotective with technological surveillance techniques and may prove overprotective with other techniques.

Like the probabilistic model, positive law accurately distinguishes per se and contextually reasonable practices in some cases but not others. As a result, it cannot provide an exclusive guide to what makes an expectation of privacy reasonable.

Baude’s and Stern’s particularly severe version of the positive law model would create some startling results. For example, imagine a police officer sees a car driving at 70 miles per hour in a 40-miles-per-hour zone. The officer, wanting to catch up to identify the car and driver, and write a ticket or make an arrest, speeds at 80 miles per hour to catch the speeder. Under the positive law model, I gather, the officer’s speeding would be a “search” that would presumptively require a warrant.

But this seems entirely arbitrary. The purpose of speeding laws is to ensure safe driving. And the purpose of allowing the officer to speed to catch the speeder is also to ensure safe driving, as an officer can’t stop speeding if the officer can’t speed himself to catch speeders. It seems odd to say that the enactment of speeding laws should make the officer’s chasing after the speeder a “search.” It doesn’t seem to have anything to do with any traditional concerns of the Fourth Amendment.

A second problem with the universal positive law model is that it would become the tail wagging the dog. I wrote about this at length in my recent article, “The Effect of Legislation on Fourth Amendment Protection, 15 Mich. L. Rev. 1117 (2017).” Here’s the problem. If you say that Fourth Amendment law hinges on positive law, then debates over positive law will become proxy battles for the Fourth Amendment. Consider the law, 47 U.S.C. § 222(a), that Baude suggests may create Fourth Amendment rights in cell-site records. If anyone knew, at the time 47 U.S.C. § 222(a) was enacted, that it would have the Fourth Amendment consequence that Baude thinks it should have, then the Justice Department would have fought the passage of that statute just as hard as it fought amendments to the Stored Communications Act that would have imposed a warrant standard for cell-site records.

Put another way, the only reason 47 U.S.C. § 222(a) even exists is that no one thought it had any Fourth Amendment relevance when it was being considered. At time, it didn’t occur to anyone that passing 47 U.S.C. § 222(a) could secretly invalidate 18 U.S.C. § 2703(c), the law that Congress thought was the governing rule on law enforcement access to historical cell-site records. (I had never heard the argument that the statute was relevant to the Fourth Amendment until Richard Re’s blog post in July 2017, and I have been studying and writing about the legal framework for government access to cell-site records since the late 1990s.) If the theory existed at the time, and had been adopted by courts, you wouldn’t have the positive law that is driving the claim. And ex ante, the legislature could no longer provide the benefits of those other laws, because their enactment would be overshadowed by the Fourth Amendment consequences.

Here’s how I summarized the problem of hinging Fourth Amendment interpretation on positive law in “The Effect of Legislation on Fourth Amendment Protection:

Displacement and independence risk losing the benefits of a dual system of search and seizure in two ways that the independence approach preserves. First, the prospect that legislation will influence Fourth Amendment rulings under influence or displacement can limit the legislature’s ability to enact helpful statutory protections beyond the Fourth Amendment. Influence and displacement take away control from the legislature, either forcing it to enact constitutional-style rules or else making the effect of its statutory design uncertain. Independence frees the legislature to impose whatever rules appear best.

Second, the prospect that legislation will influence the Fourth Amendment creates an incentive to manipulate investigative legislation. Because the executive branch both has veto power over investigative legislation and litigates Fourth Amendment cases, it would likely coordinate the two roles. Legislative debates could become a proxy for Fourth Amendment litigation, with legislative attention turned away from enacting the best statutory rule and towards what state of statutory law might set up the most advantageous constitutional case. Independence cuts the feedback loop and allows the legislative process to proceed without constitutional interference.

A final problem with applying the positive law model in Carpenter is that, like other third-party doctrine cases, it’s properly understood as a case about exhibiting subjective expectations. That is, it’s a case about whether you can maintain Fourth Amendment rights based on how you expose information, not whether you can have Fourth Amendment rights in the first place. Whatever one thinks of the merits of applying a positive law model to what is a search generally, it’s particularly hard to see why it should apply to a case that is about the consent-based concerns of the exposure doctrine and not the search-based concerns driving the four models. For more on this, see Part IV of my amicus brief.

For those reasons, I don’t think the positive law approach is helpful in Carpenter.