Quoted from “Illinois Traffic Stop Of Star Trek Fans Raises Concerns About Drug Searches, Police Dogs, Bad Cops“:
“In the 2005 case Illinois v. Caballes, the U.S. Supreme Court ruled that having a drug dog sniff the exterior of a vehicle during a routine traffic stop does not violate the Fourth Amendment. But in a dissent to that opinion, Justice David Souter pointed to mounting evidence that drug dogs aren’t as infallible as police departments often claim. Souter noted a study that the state of Illinois itself used in its briefs showing that in lab tests, drug dogs fail 12.5 to 60 percent of the time.
“Since then, more evidence has emerged to support Souter’s concerns.
“The problem isn’t that the dogs aren’t capable of picking up the scent, it’s that dogs have been bred to please and interact with humans. A dog can easily be manipulated to alert whenever needed. But even with conscientious cops, a dog without the proper training may pick up on its handler’s body language and alert whenever it detects its handler is suspicious.
“In one study published last year in the journal Animal Cognition, researchers rigged some tests designed to fool dogs into falsely alerting and others designed to trick handlers into thinking a package contained narcotics (it didn’t). Of the 144 total searches performed, the dogs falsely alerted 123 times. More interesting, the dogs were twice as likely to falsely alert to packages designed to trick their handlers than those designed to trick the dogs.
“In 2011, the Chicago Tribune published a review of drug dog searches conducted over three years by police departments in the Chicago suburbs. The paper found that just 44 percent of dog “alerts” led to the discovery of actual contraband. Interestingly, for Hispanic drivers the success rate dipped to 27 percent, again supporting the theory that drug dogs tend to confirm the suspicions (and, consequently, the biases) of their handlers.
“A 2006 statistical analysis (PDF) of police dog tests by University of North Carolina law professor Richard Myers concluded that the dogs aren’t reliable enough to provide probable cause for a search.
“HuffPost obtained the records for one Illinois state police K-9 unit for an 11-month period in 2007 and 2008. Of the 136 times this particular dog alerted to the presence of drugs during a traffic stop over that period, 35 of the subsequent hand searches found measurable quantities of illegal drugs.”