Does the Dog Smell Drugs, or is it Just Doing What it Thinks its Handler Wants?

UC Davis study finds detection dogs may exhibit the "Clever Hans" effect.

(SACRAMENTO, Calif.) — The performance of drug- and explosives-sniffing dog/handler teams is affected by human handlers’ beliefs, possibly in response to subtle, unintentional handler cues, a study by researchers at UC Davis has found. The study, published in the January issue of the journal Animal Cognition, found that detection-dog/handler teams erroneously “alerted,” or identified a scent, when there was no scent present more than 200 times — particularly when the handler believed that there was scent present.

“It isn’t just about how sensitive a dog’s nose is or how well-trained a dog is. There are cognitive factors affecting the interaction between a dog and a handler that can impact the dog’s performance,” said Lisa Lit, a postdoctoral fellow in the Department of Neurology and the study’s lead author. "These might be as important — or even more important — than the sensitivity of a dog’s nose.”

“Dogs are exceptionally keen at interpreting subtle cues, so handlers need to be cognizant of that to optimize the overall team performance,” said Anita M. Oberbauer, chair of the Department of Animal Science and the study's senior author...

Clever Hans was a horse exhibited as being able to do arithmetic, and solve other problems.  It would stamp out the numbers.  Upon close examination, it was eventually determined that Hans was reading unconscious  clues given by his owner that it was time to stop stamping.
 
Yep, I can easily see that.  Dogs will do a lot to try to make their human companions happy; it's meant a good life out of the wilderness for many thousands of years.  It is, in fact, probably the defining difference between dogs and wolves; dogs are attentive to human emotions, and wolves are unable to interpret them or simply don't care.

Clearly, people who use drug sniffing dogs are not employing them in a random manner; they profile, and search people who they suspect may be carrying drugs or other contraband.  How easy would it be for a dog to learn to see when the handler expects an alert, and then respond accordingly?  Based on what I think I know about dogs, pretty easy.

What will this mean legally?  Could successful searches based on drug dogs now be tossed out of court as lacking probably cause?  How about past convictions?  Could a drug smuggler demand a retrial, and get the evidence from a dog search tossed out as an illegal search?