The Fourth Amendment requires “reasonable suspicion” to seize a suspect. As a general matter, the suspicion derives from information a police officer observes or knows. It is individualized to a particular person at a particular place. Most reasonable suspicion cases involve police confronting unknown suspects engaged in observable suspicious activities. Essentially, the reasonable suspicion doctrine is based on “small data” – discrete facts involving limited information and little knowledge about the suspect.
But what if this small data is replaced by “big data”? What if police can “know” about the suspect through new networked information sources? Or, what if predictive analytics can forecast who will be the likely troublemakers in a community? The rise of big data technology offers a challenge to the traditional paradigm of Fourth Amendment law. Now, with little effort, most unknown suspects can be “known,” as a web of information can identify and provide extensive personal data about a suspect independent of the officer’s observations. New data sources including law enforcement databases, third party information sources (phone records, rental records, GPS data, video surveillance data, etc.), and predictive analytics, combined with biometric or facial recognition software, means that information about that suspect can be known in a few data searches. At some point, the data (independent of the observation) may become sufficiently individualized and predictive to justify the seizure of a suspect. The question this article poses is can a Fourth Amendment stop be predicated on the aggregation of specific, individualized, but otherwise non-criminal factors?
This article traces the consequences in the shift from a “small data” reasonable suspicion doctrine, focused on specific, observable actions of unknown suspects, to the “big data” reality of an interconnected information rich world of known suspects. With more targeted information, police officers on the streets will have a stronger predictive sense about the likelihood that they are observing criminal activity. This evolution, however, only hints at the promise of big data policing. The next phase will be using existing predictive analytics to target suspects without any actual observation of criminal activity, merely relying on the accumulation of various data points. Unknown suspects will become known, not because of who they are but because of the data they left behind. Using pattern matching techniques through networked databases, individuals will be targeted out of the vast flow of informational data. This new reality subverts reasonable suspicion from being a source of protection against unreasonable stops, to a means of justifying those same stops.
The impact of big data is discussed in five places in the Treatise, and all three sections of Chapter 5 (see e.g. § 5.01) on information privacy and the third party doctrine.
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"If it was easy, everybody would be doing it. It isn't, and they don't." —Me
"Life is not a matter of holding good cards, but of playing a poor hand well." –Josh Billings (pseudonym of Henry Wheeler Shaw), Josh Billings on Ice, and Other Things (1868) (erroneously attributed to Robert Louis Stevenson, among others)
“I am still learning.” —Domenico Giuntalodi (but misattributed to Michelangelo Buonarroti (common phrase throughout 1500's)).
"Love work; hate mastery over others; and avoid intimacy with the government."
—Shemaya, in the Thalmud
"It is a pleasant world we live in, sir, a very pleasant world. There are bad people in it, Mr. Richard, but if there were no bad people, there would be no good lawyers."
—Charles Dickens, “The Old Curiosity Shop ... With a Frontispiece. From a Painting by Geo. Cattermole, Etc.” 255 (1848)
"A system of law that not only makes certain conduct criminal, but also lays down rules for the conduct of the authorities, often becomes complex in its application to individual cases, and will from time to time produce imperfect results, especially if one's attention is confined to the particular case at bar. Some criminals do go free because of the necessity of keeping government and its servants in their place. That is one of the costs of having and enforcing a Bill of Rights. This country is built on the assumption that the cost is worth paying, and that in the long run we are all both freer and safer if the Constitution is strictly enforced." —Williams v. Nix, 700 F. 2d 1164, 1173 (8th Cir. 1983) (Richard Sheppard Arnold, J.), rev'd Nix v. Williams, 467 US. 431 (1984).
"The criminal goes free, if he must, but it is the law that sets him free. Nothing can destroy a government more quickly than its failure to observe its own laws, or worse, its disregard of the charter of its own existence." —Mapp v. Ohio, 367 U.S. 643, 659 (1961).
"Any costs the exclusionary rule are costs imposed directly by the Fourth Amendment."
—Yale Kamisar, 86 Mich.L.Rev. 1, 36 n. 151 (1987).
"There have been powerful hydraulic pressures throughout our history that bear heavily on the Court to water down constitutional guarantees and give the police the upper hand. That hydraulic pressure has probably never been greater than it is today." — Terry v. Ohio, 392 U.S. 1, 39 (1968) (Douglas, J., dissenting).
"The great end, for which men entered into society, was to secure their property." —Entick v. Carrington, 19 How.St.Tr. 1029, 1066, 95 Eng. Rep. 807 (C.P. 1765)
"It is a fair summary of history to say that the safeguards of liberty have frequently been forged in controversies involving not very nice people. And so, while we are concerned here with a shabby defrauder, we must deal with his case in the context of what are really the great themes expressed by the Fourth Amendment." —United States v. Rabinowitz, 339 U.S. 56, 69 (1950) (Frankfurter, J., dissenting)
"The course of true law pertaining to searches and seizures, as enunciated here, has not–to put it mildly–run smooth." —Chapman v. United States, 365 U.S. 610, 618 (1961) (Frankfurter, J., concurring).
"A search is a search, even if it happens to disclose nothing but the bottom of a turntable." —Arizona v. Hicks, 480 U.S. 321, 325 (1987)
"For the Fourth Amendment protects people, not places. What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. ... But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected." —Katz v. United States, 389 U.S. 347, 351 (1967)
“Experience should teach us to be most on guard to protect liberty when the Government’s purposes are beneficent. Men born to freedom are naturally alert to repel invasion of their liberty by evil-minded rulers. The greatest dangers to liberty lurk in insidious encroachment by men of zeal, well-meaning but without understanding.” —United States v. Olmstead, 277 U.S. 438, 479 (1925) (Brandeis, J., dissenting)
“Liberty—the freedom from unwarranted intrusion by government—is as easily lost through insistent nibbles by government officials who seek to do their jobs too well as by those whose purpose it is to oppress; the piranha can be as deadly as the shark.” —United States v. $124,570, 873 F.2d 1240, 1246 (9th Cir. 1989)
"You can't always get what you want / But if you try sometimes / You just might find / You get what you need." —Mick Jagger & Keith Richards, Let it Bleed (album, 1969)
"In Germany, they first came for the communists, and I didn't speak up because I wasn't a communist. Then they came for the Jews, and I didn't speak up because I wasn't a Jew. Then they came for the trade unionists, and I didn't speak up because I wasn't a trade unionist. Then they came for the Catholics and I didn't speak up because I wasn't a Catholic. Then they came for me–and by that time there was nobody left to speak up."
—Martin Niemöller (1945) [he served seven years in a concentration camp]
“Children grow up thinking the adult world is ordered, rational, fit for purpose. It’s crap. Becoming a man is realising that it’s all rotten. Realising how to celebrate that rottenness, that’s freedom.” – John le Carré, The Night Manager (1993), line by Richard Roper
"The point of the Fourth Amendment, which often is not grasped by zealous officers, is not that it denies law enforcement the support of the usual inferences which reasonable men draw from evidence. Its protection consists in requiring that those inferences be drawn by a neutral and detached magistrate instead of being judged by the officer engaged in the often competitive enterprise of ferreting out crime." —Johnson v. United States, 333 U.S. 10, 13-14 (1948)
The book was dedicated in the first (1982) and sixth (2025) editions to Justin William Hall (1975-2025). He was three when this project started in 1978.
