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In a recent study, researchers examined drug-sniffing dogs and their handlers. They conducted an experiment in which handlers were sent with their dogs into different environments to seek out drugs.

In certain instances, the handlers were led to believe that the likelihood of finding drugs was high. In such cases, the dogs overwhelmingly signaled that drugs were present–even though there were none. Meanwhile, in a separate study where handlers had no indication of whether there were drugs–or where they were located–the dogs performed accurately.

Dogs are excellent at reading humans. In fact, they’re some of the best readers of body language on the planet. They notice where you’re looking, how you’re gesturing, even whether you’re calm or agitated. This highly sensitive connection a dog establishes with a human–particularly with their owner or handler–can lead to some unintended prejudices for police dogs.

If a police officer is suspicious of a certain person or environment, he is likely to unwittingly communicate this bias to his dog. For instance, if an officer believes drugs to be located in a certain area of a house, he could unknowingly signal to the dog that they are on the right path as they approach the target area.

Police argue that if a dog signals when no drugs are present, it is not necessarily indicative of police bias. Dogs can also smell the residue of drugs, which could be present in a suspect’s clothing or in upholstery.

Nonetheless, the above study is putting pressure on police departments to demonstrate that they’re conducting themselves in an unprejudiced manner. Some people are pushing for a regulation requiring handlers to keep track of how often their dogs alert when no drugs are present. News of the study is also making police bias an increasingly common defense used in drug cases.