Imagine that a police officer, after searching 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 skepticism. 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. 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. […]