Studies show that dogs reveal the hidden prejudices of the officers who deploy them. Photograph by Scott Olson / Getty

The Department of Justice’s report on the Ferguson Police Department is full of eye-catching numbers that reveal a culture plagued by significant racism. Statistically significant. For instance, nearly ninety per cent of the people who prompted a “use of force” by the F.P.D. were black. Even among such skewed percentages, there are some standouts. Among cases in which a suspect was bitten by an attack dog and the suspect’s race was recorded, what percentage were black?

A hundred per cent.

There is little nuance in the incidents described in the report; the police simply sicced their dogs on unarmed black males. According to the F.P.D’s own guidelines, handlers should not release the hounds “if a lower level of force could reasonably be expected to control the suspect or allow for the apprehension.” But the report reveals that the F.P.D. is quick to set loose its trained attack dogs—often on black children.

In one account, a dog was sent after an unarmed sixteen-year-old who was also Tasered. The electric shock of that weapon partially paralyzes a person. If that were to happen while a dog was tearing at your arms and legs, all you could do would be to watch in immobilized horror.

Another case involved four police officers, including a canine handler, trapping an unarmed fourteen-year-old in an abandoned basement. The crime? Trespassing. The D.O.J. report recounts what the boy says happened to him: “When he saw the dog at the top of the steps, he turned to run, but the dog quickly bit him on the ankle and then the thigh, causing him to fall to the floor. The dog was about to bite his face or neck but instead got his left arm, which the boy had raised to protect himself. F.P.D. officers struck him while he was on the ground, one of them putting a boot on the side of his head. He recalled the officers laughing about the incident afterward.”

The starkness of these accounts prompts some questions. Were no white suspects ever subdued with dogs? Were all the white suspects perceived as less dangerous (than a kid playing hookie)? Was this just overt Bull Connor-style racism? Or was there something more subtle going on in these incidents—an unconscious cuing by the dog’s handlers as to whom the animals should attack the most aggressively.

The use of canines in American policing and surveillance has spiked since 9/11. They have been hired to work as bomb-detection dogs, methamphetamine-sniffing dogs, and cadaver dogs, and they have been trained to detect mines. There are arson dogs, of course, but there are also dogs trained to detect bed bugs and dogs that specialize in locating invasive quagga mussels at boat landings. Then there are dogs trained to find orca scat, or forbidden mobile phones in prison, or Townsend’s Big-Eared Bats.

The conventional wisdom is simply that dogs have supersensitive, trustworthy noses. We hear about these dogs that perform amazing feats, such as sniffing out cancer, and it reinforces our common faith that they are nearly flawless in this regard—a trust that is further reinforced when we recall our childhood dog, who loved us unconditionally (Wendy, you big loveball), or maybe our favorite on-screen canine star (depending on your generation: Petey, Lassie, Old Yeller, Comet, Hooch, Brian Griffin). The Supreme Court agrees, resisting evidence that a dog’s power to smell and detect might be anything less than an unquestionable fact. In a 2006 paper about detector dogs, Richard Myers, a law professor at the University of North Carolina, summarized the lazy rhetoric that courts often use to avoid questioning supposedly tried-and-true methodologies: "The use of [insert technology here] is well-settled in the law of this [state/circuit] and we need not revisit it here."

It would appear that science supports giving dogs’ noses the benefit of the doubt. Humans have about five million olfactory cells, while dogs have as many as two hundred and twenty million. And one study of the canine snout found that it is able to detect scents at concentrations as low as five hundred parts per trillion. But there’s a catch, which most of us (including the Supreme Court, a few years back) don’t consider: dogs are also astute at picking up on subconscious cues from their handlers—a talent that, like their sense of smell, is exquisite and astonishingly fine-tuned.

Science fans may recall the story of Clever Hans—the German horse that, at the turn of the twentieth century, wowed audiences by telling time, reading calendars and solving math problems. Two Berlin academics, named Pfungst and Stumpf, figured out that Hans was reading not only his handler but also the faces of the audience to know when to stop pawing the ground as he had arrived at the right mathematical answer. Around the same time, Americans were being entertained by a similarly talented horse, named Beautiful Jim Key. In later years there came Lady Wonder, the telepathic horse, and Jim the Wonder Dog, another psychic. Domesticated animals are pretty good at figuring out what we want them to do, and for good reason.

Training a dog to smell, say, salamander dung (it’s been done) requires that the trainer teach the animal cues at the beginning of the training. The dog is eventually weaned off its reliance on human signals, but the cues remain the foundation of the learned behavior. Moreover, dogs have been co-evolving with humans for roughly fifteen thousand years, learning to read us. It shouldn’t come as a surprise that a trained dog has a powerful inclination to do what we secretly want them to do.

A 2011 study published in the journal Animal Cognition found that even expertly trained dogs and the most professional handlers cannot evade what is called the Clever Hans effect. In tests, dogs trained to detect explosives and drugs were sent, with their handlers, into a series of rooms to find non-existent contraband. In one room, there was a decoy that had been scented with sausage; in another, there was an unscented decoy accompanied by a sign telling the handler, falsely, that it smelled of contraband; a control room had no decoys. The investigators found, overall, that “human more than dog influences affected alert locations": the meat decoy attracted more false alarms than anything in the control room, but the decoy with the sign prompted nearly twice as many false alerts as the one with the tempting scent. In other words, the dogs found their handlers' unconscious cues significantly more compelling than the sausage. Trained animals, it turns out, are arguably better at reading our cues than we are at suppressing them.

A friend of mine recently saw a detection dog at a Metro-North station in Connecticut go into full alert when a fifty-something white businessman in a suit walked by. The trainer pulled the dog back and nodded for the man to pass. My friend noted, “Training a dog is not something that happens once and then it’s over. They are always being trained.” And so that Metro-North dog has begun a kind of continuing education. Repeat that action and the dog will learn: give middle-aged white guys in brogues a pass.

In 2011, the Chicago _Tribune _analyzed three years’ worth of data related to police dogs working in suburban Chicago. The analysis revealed how easy it is for certain cultural stereotypes—in this case, of drug-dealing Hispanics­—to travel straight down the leash. When dogs alerted their handlers to the presence of drugs during traffic stops, the officers found drugs forty-four per cent of the time overall. In cases involving Latino drivers, that number fell to twenty-seven per cent. In other words, the dogs were erroneously implicating people of Hispanic descent far more frequently than other people. When this disparity was made public, the question naturally arose of whether handlers’ unconscious cues might be the cause? Paul Waggoner, a scientist at Auburn University's Canine Detection Research Institute, who was interviewed by the Tribune, provided the answer. A “big, resounding yes,” he said.

There is a paradox in police-dog work, post-Ferguson. Depending on its training, a dog may or may not be good at alerting us to drugs or bombs. But there is one social ill that all detection dogs, even the poorly trained ones, reveal with searing accuracy: the hidden racial prejudices of the police officers who deploy them.