Skip to main content
Find a Lawyer

The Supreme Court Upholds Suspicionless Dog Sniffs


Tuesday, Feb. 01, 2005

Last week, in Illinois v. Caballes, the U.S. Supreme Court held that when a trained police dog sniffs the trunk of a motorist's car, no constitutional issue is even raised.

The Court ruled that the motorist does not undergo a "search" within the meaning of the Constitution's Fourth Amendment. And the Fourth Amendment's requirement of reasonableness only applies to government activities that do amount to searches (or seizures). Accordingly, the Justices ruled that Illinois state troopers did not even implicate, much less violate, the Fourth Amendment, when they brought a drug-sniffing dog to smell the trunk of the car driven by Roy Caballes.

The decision in Caballes was not unexpected. It relied on an earlier precedent, the 1983 ruling in United States v. Place, which had upheld the use of drug-sniffing dogs to "inspect" the luggage of passengers awaiting departure at the airport.

The Court in Caballes reasoned that a sniff by a dog trained to seek out illegal drugs is not a search because the sniff can only reveal evidence of illegal activity. Thus, unlike conventional searches of a dwelling or personal property, the dog sniff does not reveal any information that people have a right to keep private.

That reasoning seems to imply, too, that the police do not need any individualized suspicion to subject people themselves--and not just their cars--to the humiliation and fear that may accompany a dog sniff. And that consequence is troubling.

But precisely this troubling aspect of the decision could also be reassuring. If dog sniffs of people do not raise Fourth Amendment issues, then, for example, dogs trained to detect explosives can be used to foil the plans of a terrorist who conceals a bomb on his person in a crowded public place.

The Facts and Legal Background in Caballes

The case arose when Caballes was stopped for driving 71 miles per hour on an interstate highway on which the posted speed limit was 65. After the state trooper radioed the stop to the dispatcher, another trooper came to the scene with his drug-sniffing dog.

According to the facts as assumed by the Supreme Court, the troopers had no reason to suspect that Caballes was guilty of anything worse than ordinary speeding. Nonetheless, while the first officer wrote out a warning ticket for Caballes, the second officer walked his dog around the car. The dog "alerted" at the trunk--signaling that drugs were present. Then, armed with the knowledge that the trunk likely contained illegal drugs, the troopers searched it, and discovered a substantial quantity of marijuana.

At his trial, Caballes cried foul. He argued that the dog sniff was a "search" within the meaning of the Fourth Amendment, and an "unreasonable" one at that, given that the police had no basis for suspecting that he possessed illegal drugs. Accordingly, he moved to have the drugs excluded.

Caballes lost that argument in the trial court, and was sentenced to twelve years in prison and a quarter million dollar fine. Caballes later enjoyed a reversal of fortune, when the Illinois Supreme Court threw out the conviction.

But his reprieve was short-lived. The U.S. Supreme Court, in turn, reversed the Illinois high court, reinstating the conviction and sentence. By a 6-2 margin (Chief Justice Rehnquist did not participate, due to his illness), the Court ruled that the dog sniff was not a search.

What is a "Search" and Why Doesn't a Dog Sniff Qualify?

Why not? To begin, under longstanding Supreme Court precedent, not all police investigative measures constitute "searches" implicating the Fourth Amendment.

To give an obvious example, if a cop on the beat observes an assault, clearly the mere act of viewing what is plain for everyone to see is not a "search." Or, to give an example closer to the facts of Caballes, if a police officer patrolling a public park smells marijuana coming from a group of teenagers smoking on a bench, that ordinary use of his olfactory senses is not a "search" either.

Under the relevant precedents, police activity only constitutes a Fourth Amendment "search" if it violates a "reasonable expectation of privacy." In our two hypothetical examples, it is not reasonable to expect privacy in activity conducted in public, where it can be seen or smelled by any passersby, including the police.

Of course, the state trooper who shepherded the police dog around Caballes's car was not a mere passerby. He specifically targeted Caballes for scrutiny. And the drugs in Caballes's trunk were not evident to the human eye or nose--or even to the nose of an untrained dog. So why didn't the dog sniff violate a reasonable expectation of privacy?

Because, according to the majority opinion of Justice Stevens, there is no reasonable expectation of privacy in illegal activity.

The reason why the Fourth Amendment limits police investigation at all is to protect people against intrusions into their private space. Police ransacking a dwelling searching for contraband will uncover all sorts of private, potentially embarrassing, information about the target of the search: material relating to his sexual practices; what books he reads; his taste in music and art; even how tidy he is.

The state has no legitimate interest in obtaining such private information about lawful activity. Accordingly, the Constitution only allows a search that reveals such information when it is an unavoidable accompaniment or component of a search that is otherwise supported by suspicion--typically "probable cause" to believe--that the search will reveal evidence of criminal conduct.

A dog sniff, however, is different. A trained police dog either alerts in the presence of illegal drugs, or does not alert in their absence. The sniff reveals no collateral private information (except perhaps to the dog, who has the good manners not to convey it to his human partners).

Thus, Justice Stevens said, the only privacy interest implicated by a dog sniff is the interest in keeping illegal activity private--and that interest is not protected by the Fourth Amendment.

Justice Souter's Odd Dissent

Justice Souter, however, dissented. He said the majority opinion overlooked the fact that police dogs can and do err: As people are only human, so dogs are only canine.

Citing a variety of error rates for different dogs and trainers, Justice Souter observed: "In practical terms, the evidence is clear that the dog that alerts hundreds of times will be wrong dozens of times." For this reason, Justice Souter thought that the majority could not simply equate the dog's alert with the presence of illegal drugs. Accordingly, he would have treated the dog sniff as a search.

With all due respect, Justice Souter's argument is logically irrelevant to the crucial legal issue. The fallibility of police dogs and their handlers does not in any way bear on the nature of the information the alerting dog conveys--and that information, not the dog's accuracy in providing it, underlay the Caballes majority's conclusion that no Fourth Amendment issue was raised.

Suppose, for example, that police dogs have an average false positive rate of one in four. That simply means that when a dog alerts, the human officer knows there is a seventy-five percent chance that illegal drugs are present. The dog sniff still reveals no information whatsoever about other, legitimately private, activities of the target of the sniff.

To be sure, the police may now use the statistical information about the likely contents of the car as the basis for conducting a full-blown search, for now they have probable cause to suspect that they will find evidence of crime. And that full-blown search might reveal information about legitimately private activities.

But the dog sniff by itself still invades no reasonable expectation of privacy. And the narrow question whether the dog sniff, by itself, is a "search," was the focus of the case.

Justice Ginsburg's Dissent

Justice Ginsburg also dissented, though on different grounds from Justice Souter. (In addition to writing his own dissent, Souter also joined Ginsburg's).

According to Justice Ginsburg, the introduction of the drug-sniffing dog changed the nature of the initial traffic stop, itself a "seizure" within the meaning of the Fourth Amendment. The car and the motorist are "seized" by being stopped.

While it may be reasonable for the police temporarily to stop motorists who exceed the speed limit, it is unreasonable, Justice Ginsburg thought, then to subject their vehicles to a dog sniff. Even if the dog's presence does not change the duration of the traffic stop, it changes its scope.

Justice Ginsburg noted, as well, that under the majority's reasoning, the police may subject parked cars to dog sniffs without any reasonable suspicion. They can even have their dogs quickly encircle and sniff cars stopped at red lights if they choose.

What About Dogs Sniffing People?

The majority opinion in Caballes does not directly address a closely related question: Is the Fourth Amendment implicated when a police dog sniffs a person, not just his or her property?

The logic of Caballes suggests that the answer to this question is also no. As with the sniff of a car trunk, so too with the sniff of a person, the dog will only reveal whether there are drugs on the person. The dog may detect odors associated with poor hygiene or other embarrassing facts, but, except when he makes an error, the dog will only alert in the presence of illegal drugs, not these other facts. Those other, irrelevant bits of private information remain safely within the canine mind.

Nonetheless, there may be reason to think that a dog's sniffing a person should, from a constitutional perspective, be analyzed differently from a dog's sniffing a car. By its terms, the Fourth Amendment protects "the right of the people to be secure in their persons . . . against unreasonable searches and seizures." Surely security of the person is jeopardized by the presence of the nose of an uninvited large dog just millimeters away from one's body.

Most people--perhaps even most dog lovers--would be sufficiently intimidated by such a procedure to feel obliged to remain motionless. Thus, it seems appropriate to deem a dog sniff of the human body a "seizure" of the person that requires some individualized suspicion.

The Caballes majority does not discuss this issue, however, and so we will have to await some future case to learn whether the decision's logic applies to dogs' sniffing human bodies as well as car trunks.

What About Bomb-Sniffing Dogs?

The Caballes opinion also does not address the use of dogs to sniff for explosives and weapons. Clearly, the case would allow such police activity wherever dogs are permitted to sniff for drugs, for the state interest in detecting and foiling terrorism is at least as great as--and likely much greater than--the interest in apprehending drug dealers.

But the Court might be tempted to go further. Suppose that, following my analysis above, the Court were to say that the use of dogs to sniff people, rather than cars, does constitute a search or seizure, and therefore cannot occur in the absence of some reason to suspect that the particular person sniffed is in possession of drugs. Could the Court then distinguish the case of dogs sniffing for explosives or weapons rather than drugs? From the perspective of the "sniffee," after all, the procedures are identical.

Although the majority does not consider this issue, Justice Ginsburg's dissent does. She notes that the object of police investigation--here, what the dogs are trained to sniff--can indeed make a difference with respect to the question of whether that police investigation can only be undertaken on the basis of individualized suspicion. And she observes that while the Supreme Court has upheld sobriety checkpoints that aim to detect and prevent drunk driving, it has not permitted such roadblocks that stop all drivers where the police object is to find illegal drugs.

Likewise, Justice Ginsburg suggests, although in her view it is unreasonable for the police to subject stopped cars to suspicionless dog sniffs for drugs, it might well be reasonable to have dogs sniff for explosives or weapons, even absent individualized suspicion. And if that is true for cars, it may well be true for human bodies as well.

Nonetheless, the terse majority opinion in Caballes provides no clear indication of what the remaining Justices would think about suspicionless dog sniffs of human bodies that aimed to detect explosives or weapons. It only addresses the case of the traffic stop.

Given the frequency of such stops, however, that makes Caballes an important case in its own right. For drug traffickers, the lesson is clear: Scrupulously obey all traffic laws.

For the rest of us, the lesson may be this: Keep a supply of dog biscuits in your glove compartment. That way, if the nice police doggy is disappointed when he doesn't find drugs, at least he'll get a tasty treat.

Michael C. Dorf is the Michael I. Sovern Professor of Law at Columbia University in New York City. His book, Constitutional Law Stories, is published by Foundation Press, and tells the stories behind fifteen leading constitutional cases. He lives with two sweet, poorly trained dogs (and three sweet humans of uneven training.)

Was this helpful?

Copied to clipboard