Jacob Sullum

Imagine that a police officer, after taking it upon himself to search someone's car, is asked to explain why he thought he would find contraband there. "A little birdie told me," he replies.

Most judges would react with appropriate skepticism to such a claim. But substitute "a big dog" for "a little birdie," and you've got probable cause.

Or so says the U.S. Supreme Court, which last week unanimously ruled that "a court can presume" a search is valid if police say it was based on an alert by a dog trained to detect drugs. The court thereby encouraged judges to accept self-interested proclamations about a canine's capabilities, reinforcing the alarmingly common use of dogs to justify invasions of privacy.

Drug-detecting dogs are much less reliable than widely believed, with false-positive error rates as high as 96 percent in the field. A 2006 Australian study found that the rate of unverified alerts by 17 police dogs used to sniff out drugs on people ranged from 44 percent to 93 percent.

Police and prosecutors commonly argue that when a dog alerts and no drugs are found, "the dog may not have made a mistake at all," as Justice Elena Kagan put it, writing for the Court. Instead, it "may have detected substances that were too well hidden or present in quantities too small for the officer to locate."

This excuse is very convenient -- and completely unfalsifiable. Furthermore, probable cause is supposed to hinge on whether there is a "fair probability" that a search will discover evidence of a crime. The possibility that dogs will react to traces of drugs that are no longer present makes them less reliable for that purpose.

So does the possibility that a dog will react to smell-alike odors from legal substances, distractions such as food or cues from their handlers. Given all the potential sources of error, it is hard to assess a dog's reliability without looking at its real-world track record. That is why the Florida Supreme Court, in the 2011 decision that the U.S. Supreme Court overturned, said police should provide information about a dog's hits and misses.

"The fact that the dog has been trained and certified," it said, "is simply not enough to establish probable cause," especially when, as in most states, there are no uniform standards for training or certification.

Jacob Sullum

Jacob Sullum is a senior editor at Reason magazine and a contributing columnist on Townhall.com.
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