Today the U.S. Supreme Court unanimously ruled that "a court can presume" an alert by a drug-sniffing dog provides probable cause for a search "if a bona fide organization has certified a dog after testing his reliability in a controlled setting" or "if the dog has recently and successfully completed a training program that evaluated his proficiency in locating drugs." The justices overturned a 2011 decision in which the Florida Supreme Court said police must do more than assert that a dog has been properly trained. They deemed that court's evidentiary requirements too "rigid" for the "totality of the circumstances" test used to determine when a search is constitutional. In particular, the Court said it was not appropriate to demand evidence of a dog's performance in the field, as opposed to its performance on tests by police. While the Court's decision in Florida v. Harris leaves open the possibility that defense attorneys can contest the adequacy of a dog's training or testing and present evidence that the animal is prone to false alerts, this ruling will encourage judges to accept self-interested proclamations about a canine's capabilities, reinforcing the use of dogs to transform hunches into probable cause.

Writing for the Court, Justice Elena Kagan accepts several myths that allow drug dogs to function as "search warrants on leashes" even though their error rates are far higher than commonly believed:

Myth #1: Field performance is a misleading indicator of a dog's reliability. When a dog alerts and no drugs are found (as happened twice in this case), "the dog may not have made a mistake at all," Kagan says. Instead it "may have detected substances that were too well hidden or present in quantities too small for the officer to locate," she suggests. "Or the dog may have smelled the residual odor of drugs previously in the vehicle or on the driver's person." This is a very convenient, completely unfalsifiable excuse for police and prosecutors. But probable cause is supposed to hinge on whether there is a "fair probability" that a search will discover evidence of a crime, and the possibility that dogs will react to traces of drugs that are no longer present makes them less reliable for that purpose.

Myth #2: Police department testing is the gold standard by which a dog's reliability should be judged. Kagan says the uncertainties of the real world "do not taint records of a dog's performance in standard training and certification settings," because "the designers of an assessment know where drugs are hidden and where they are not." That is precisely the problem when the designers are the dog trainers, as is usually the case, because they may deliberately or subconsciously indicate the locations of the drugs. Lawrence Myers, a veterinarian and neurophysiologist at Auburn University who is an expert on dogs' olfactory capabilities, observes: "Typically if a cop says, 'I train the dog every week,' he's hiding things and then going around and finding the things he's hidden. Putting something out, you as the handler, then taking the dog through, you are going to seriously skew the training; you're going to cue. You can't help it; you know exactly where the damned thing is."

Myth #3: Police have a strong interest in making sure their dogs are reliable, so we can trust their assurances on that score. "After all," writes Kagan, "law enforcement units have their own strong incentive to use effective training and certification programs, because only accurate drug-detection dogs enable officers to locate contraband without incurring unnecessary risks or wasting limited time and resources." Here she echoes Justice Antonin Scalia, who during oral argument in this case last October seemed flabbergasted by the idea that a drug dog might be unreliable or that police might exaggerate its reliablity. Yet police may value a dog that alerts promiscuously if their main goal is to justify searches they want to conduct based on suspicions that otherwise would not amount to probable cause (or if they are looking for cash to seize). Once a search has been conducted, the officer seeking to justify it has a clear interest in exaggerating his dog's abilities.

Facing the courts' pro-dog bias, defense attorneys still can call expert witnesses to cast doubt on a police department's training methods, evaluation standards, or handling procedures. But that is expensive, which may help explain why the defense attorney in this case failed to do so (as the Supreme Court emphasizes). They can seek data on a dog's hits and misses, but many departments do not keep track of false positives, so there are no data to examine (as was true in this case). They can even question whether a dog did in fact alert, but that is difficult to do if the police do not videotape the encounter (which was also true in this case). The upshot is that courts generally will allow searches based on an officer's unsubstantiated claim about how an animal of undetermined reliability reacted to a person, a suitcase, a car, or a house. As Florida defense attorney Jeff Weiner puts it, the justices "have given law enforcement a green light to do away with the Fourth Amendment merely by uttering the magic words, 'My dog alerted.'"

The Supreme Court has not yet issued a ruling in Florida v. Jardines, the other drug dog case it heard last October. Jardines raises the question of whether police need a warrant to use a drug-sniffing dog at the doorstep of a home, and there is reason to believe the Court's answer will be less favorable to the government than the blanket endorsement of current practices it issued today.

I discuss the implications of both cases in the March issue of Reason.