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Supreme Court Considers the Warrantless Sniff


Florida v. Jardines and Florida v. Harrisare scheduled to be heard by the United States Supreme Court on Wednesday, October 31. The cases pivot on whether the use of police dogs to detect illegal drugs violates the Fourth Amendment privacy rights of the person being sniffed.

General background

K-9 units are commonplace across America. (K-9 is shorthand for “canine.”) Police dogs are used to pursue suspects, to locate victims, and to detect banned goods like bombs or cocaine. A detection dog “alerts” his handler either by sitting down or by barking. The dogs are trained intensively to identify specific odors; the police handlers receive special instruction as well.

In the field, the handler uses the dog’s alert as probable cause to conduct a search. (Probable cause is a reasonable belief based on sufficient facts.) But does the sniffing constitute a search in the first place? If it does, then probable cause must exist before the sniffing itself is allowed under the Constitution.

The Fourth Amendment provides,

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and 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.

Florida v. Jardines involves a 2006 arrest. The police received an unverified and anonymous tip that Joelis Jardines was growing marijuana in his home. After the police surveilled the house and witnessed no activity, a handler took his detection dog up to the front door, where the dog alerted to a drug odor. Thereafter, a detective went to the front door and claimed to smell marijuana. On that basis, a warrant was obtained, and Jardines was arrested. But Jardines has sought to have the evidence against him suppressed on the grounds it had been obtained through an illegal search.

Florida v. Harris involves a 2006 traffic stop for expired tags. The officer noticed the driver, Clayton Harris, was nervous and had an open beer in his truck. When Harris refused to consent to a search of his vehicle, a detection dog did a “free air sniff” and alerted to drugs. In the ensuing search, the officer discovered over 200 pseudoephedrine pills and other precursors of methamphetamine. Interestingly, the dog had not been trained to detect pseudoephedrine. Charged with the intent to manufacture meth, Harris sought to suppress the evidence, but he was denied.

Two aspects of these cases are worth examining closely. First, does the sniff of a dog constitute either a search or a probable cause for one? Second, how reliable are the alerts given by detection dogs?

Were the sniffs illegal searches?

The Supreme Court has repeatedly found that dog sniffs in public places are not searches, in part because the dogs merely use the air to detect drugs. The question then becomes whether they constitute the probable cause necessary to obtain a warrant or to conduct a warrantless search.

Florida v. Jardines and Florida v. Harris differ in at least one crucial circumstance: In Jardines, the police conducted the search on a private home (indeed, they even had to cross the private property of the yard to get the dog to the house to sniff it). In Harris, the police made their search on an automobile on the public roads. The strength of Fourth Amendment protections is different for each.

For decades, the 1967 case Katz v. United States was a legal touchstone for applying the Fourth Amendment to determine if a search was legal. Katz involved a man who used a public telephone booth to transmit illegal gambling bets. The police recorded him by placing an eavesdropping device on the outside of the booth. The Supreme Court held that Katz had a reasonable expectation of privacy even though he was in a public place, because he had closed the booth’s door behind him. Moreover, as Justice Potter Stewart wrote, Katz had paid “the toll that permits him to place a call” and he “is surely entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world.”

Thus the “reasonable expectation of privacy” became a legal test. A conversation conducted in a person’s home is protected by the Fourth Amendment, while a chat on a public street is not. By knowingly exposing information or objects to the public, a person loses the expectation of privacy.

Automobiles are more of a gray area. If evidence of a crime is not in public view within a car, then the operator has some legitimate expectation of privacy. That protection is far less than for a home, however. Why? In 1925, through Carroll v. United States, the Supreme Court established what is known as the “motor vehicle exception.”

This legal rule allows law enforcement to search automobiles without the search warrant that is required to examine a home. The exception is based in part on the public nature of driving, and in part on a car’s ability to leave the scene before a warrant can be obtained. Thus, only probable cause is required for a search of a car.

This returns to the key question: Does a detection dog’s alert constitute probable cause?

In the 2005 case Illinois v. Caballes, the Supreme Court ruled that

a dog sniff conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess does not violate the Fourth Amendment.

In other words, there can be no expectation of privacy regarding the possession of illegal substances, and without such an expectation the examination of a car is not a “search” under the Fourth Amendment. The dog’s alert provided the necessary probable cause.

Thus, detection dogs have been called “probable cause on four legs.” But this description is accurate only if a dog’s alerts are reliable. In his dissenting opinion on Caballes, Justice David Souter highlighted the issue of reliability. He wrote,

The infallible dog … is a creature of legal fiction.… Their supposed infallibility is belied by judicial opinions describing well-trained animals sniffing and alerting with less than perfect accuracy, whether owing to errors by their handlers, the limitations of the dogs themselves, or even the pervasive contamination of currency by cocaine.… [D]ogs in artificial testing situations return false positives anywhere from 12.5 to 60% of the time, depending on the length of the search.

The Clever Hans effect

Clever Hans was a horse owned by a German mathematics teacher named Wilhelm von Osten. Around the turn of the 20th century, Hans became an international sensation for his alleged ability to solve math problems, to spell words, and to perform other mental feats by stomping his hoof on the ground. In 1904, the New York Times ran an article entitled “Berlin’s Wonderful Horse; He Can Do Almost Everything But Talk.” (PDF)

In 1907, a scientific investigation concluded that Hans was taking cues from the body language of von Osten and from audience reactions. There was no accusation of malfeasance on von Osten’s part. The man was simply giving off involuntary signals, which the horse picked up. Since then the phenomenon of an animal performing based on involuntary cues from a human being has been labeled the “Clever Hans effect.”

In 2011, Lisa Lit, Julie B. Schweitzer, and Anita M. Oberbauer published a study entitled “Handler Beliefs Affect Scent Detection Dog Outcomes” in the scholarly journal Animal Cognition. The University of California, Davis researchers tested the Clever Hans effect on police dogs. Eighteen law enforcement handler-dog teams were involved in the experiment. All were trained and certified for detecting drugs, explosives, or both. The teams each performed two sets of searches in four rooms with five minutes allocated per search. In advance, the researchers told the handlers that each room could have as many as three target odors, and that some targets would be marked with red paper. They were not told that in fact no target would actually smell of drugs or explosives. In short, any alert from the dogs would be false.

The 144 searches conducted produced only 21 accurate detections; in other words, the dog gave no alert 21 times. The total number of false detections was 225. The highest rate of false alerts by far came in response to targets marked with red paper. The dogs were picking up the unconscious body language of their handlers.

In a Reason article (Feb. 21, 2011) entitled “The Mind of a Police Dog,” Radley Balko concluded,

In the process of domesticating dogs, we have bred into them a trait that tends to trump most others: a desire to please us — and toward that end, an ability to read us and a tendency to rely on us to help them solve their problems.


The use of detection dogs is increasing. One reason can be found in civil-forfeiture laws. Police departments are now able to confiscate vehicles, money, and other possessions that are used in the conduct of illegal activity. Attorney Darpana Sheth of the Institute for Justice, which filed an amicus brief in Florida v. Harris, commented, “Police dog handlers work in departments that are often funded through forfeiture funds, giving them a direct financial incentive to ‘police for profit’ rather than pursue the neutral administration of justice.”

There is also considerable incentive to use dogs for searches that could not otherwise be conducted under the law. Such searches are an end run around the need for warrants, which normally restricts police authority. As detection dogs become more common, however, the courts will be pushed to clarify the constitutional status of a sniff. Indeed, the need for clarity may be why the Supreme Court has agreed to hear these two detection-dog cases.

The implications for the Fourth Amendment are immense. For example, if the Jardines appeal is unsuccessful, there will be legal precedent for police to use detection dogs on the direct exterior of homes. The amicus brief filed by the Cato Institute on Jardines states, “this Court confronts whether government agents could walk a drug-sniffing dog to the front door of every home in America or similarly patrol lines at movie theaters, shopping mall entrances, and such without implicating the Fourth Amendment.” (PDF)

But the Supreme Court’s findings are more likely to rest on circumstance than on constitutional principle. Jardines has the advantage of a person’s reasonable expectation of privacy on his own property, within his own home. Harris is less likely to succeed.

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    Wendy McElroy is an author for The Future of Freedom Foundation, a fellow of the Independent Institute, and the author of The Reasonable Woman: A Guide to Intellectual Survival (Prometheus Books, 1998).