The Mind of a Police Dog

For the first few years I had her, I was impressed by my late
dog Harper’s uncanny ability to assess people’s character. She
hated every crappy landlord and bad roommate. Barked at them.
Snarled at them. Wouldn’t go near them. But if I brought home a
date I liked,
, a Shar Pei/Labrador mix, would curl up right next to
the woman and turn on the charm. It took me several years to figure
out that my dog wasn’t a good judge of character; she was just good
at reading me. She liked the people I liked and disliked the people
who rubbed me the wrong way. For dogs descended from lines bred for
protection and companionship, this talent makes sense. A dog adept
at distinguishing friend from foe was likely to be kept around and
bred, and one very good way to tell friend from foe is to read your
master’s body language.

My confusion about what was going on in Harper’s head reflects a
common misconception that is also apparent in the ways dogs are
used in criminal investigations. When we think dogs are using their
well-honed noses to sniff out drugs or criminal suspects, they may
actually be displaying a more recently evolved trait: an urgent
desire to please their masters, coupled with the ability to read
their cues.

Several studies and tests have shown that drug-sniffing dogs,
scent hounds, and even explosive-detecting dogs are not nearly as
accurate as they have been portrayed in court. A recent Chicago

of traffic stops by suburban police departments from
2007 to 2009, for example, found that searches turned up contraband
in just 44 percent of the cases where police dogs alerted to the
presence of narcotics. (An alert is a signal, such as barking or
sitting, that dogs are trained to display when they detect the
target scent.) In stops involving Hispanic drivers, the dogs’
success rate was just 27 percent. The two largest departments the
Tribune surveyed—the Chicago Police Department and the
Illinois State Police—said they don’t even keep track of such

But don’t blame the dogs; their noses work fine. In fact, the
Pentagon’s Defense Advanced Research Projects Agency recently

, after 12 years and millions of dollars of research,
that the canine snout, fine-tuned by millions of years of
evolution, is still far more sensitive and reliable than any
technology man has been able to muster when it comes to detecting
explosives in places such as Iraq and Afghanistan.

The problem is our confusion about when dogs are picking up a
scent and when they are responding to cues from their handlers.
The Economist‘s “Babbage” blog summarizes
a recent study led by Lisa Lit, a neurologist (and former dog
handler) at the University of California-Davis, that demonstrates
the startling consequences of that confusion:

[Researchers] asked 18 professional dog handlers and their mutts
to complete two sets of four brief searches. Thirteen of those who
participated worked in drug detection, three in explosives
detection, and two worked in both. The dogs had been trained to use
one of two signals to indicate to their handlers that they had
detected something. Some would bark, others would sit.

The experimental searches took places in the rooms of a church,
and each team of dog and human had five minutes allocated to each
of the eight searches. Before the searches, the handlers were
informed that some of the search areas might contain up to three
target scents, and also that in two cases those scents would be
marked by pieces of red paper.

What the handlers were not told was that two of the targets
contained decoy scents, in the form of unwrapped, hidden sausages,
to encourage the dogs’ interest in a false location. Moreover, none
of the search areas contained the scents of either drugs or
explosives. Any “detections” made by the teams thus had to be
false. Recorders, who were blind to the study, noted where handlers
indicated that their dogs had raised alerts.

The results? Dog/handler teams correctly completed a search with
no alerts in just 21 of the 144 walk-throughs. The other 123
searches produced an astounding 225 alerts, every one of them
false. Even more interesting, the search points designed to trick
the handlers (marked by the red slips of paper) were about twice as
likely to trigger false alerts as the search points designed to
trick the dogs (by luring them with sausages). This phenomenon is
known as the “Clever Hans effect,” after a horse that won fame in
the early 1900s by stomping out the answers to simply arithmetic
questions with his hoof. Hans was indeed clever, but he couldn’t do
math. Instead he was reading subtle, unintentional cues from the
audience and his trainer, who would tense up as Hans began to click
his hoof, then relax once Hans hit the answer. 

In her wonderfully written (but strangely titled) book
of a Dog
, Columbia University psychology professor
Alexandra Horowitz further illustrates how humans can unconsciously
influence canine behavior. She describes various tests in which
researchers measure the problem-solving skills of domestic dogs in
comparison to wolves. The tests include activities in which dogs
and wolves are tasked with finding a ball or treat hidden in a
room, tucked behind a screen, or sealed in a container. When the
task involves minimal human contact, domestic dogs perform about as
well as their more primitive cousins. But the more the experiments
incorporate interaction with humans, the more poorly domesticated
dogs fare. They tend to give up, then simply wait for the human
researcher to get the prize for them. Horowitz explains:

By standard intelligence tests, the dogs have failed…I
believe, by contrast, that they have succeeded magnificently. They
have applied a novel tool to the task. We are that tool….We solve
the puzzles of closed doors and empty water dishes….We humans are
brilliant enough to extract hopelessly tangled leashes from around
trees.…Dogs are terrific at using humans to solve problems, but not
as good at solving problems when we’re not around.

It is hard to overstate the implications of these findings for
the use of dogs in police work. The dogs who failed Lit’s scent
tests did not lose their sense of smell. But in the process of
domesticating dogs, we have bred into them a trait that tends to
trump most others: a desire to please us—and toward that end, an
ability to read us and a tendency to rely on us to help them solve
their problems. Any training program that does not take this
tendency into account will produce dogs who frequently issue false

The consequences of those mistakes are profound. As my colleague
Jacob Sullum has explained,
the U.S. Supreme Court says a dog sniff is not invasive enough to
qualify as a “search” under the Fourth Amendment, so police do not
need a warrant or probable cause to have a dog smell your luggage
or your car. At the same time, however, the courts treat an alert
by a drug-sniffing dog as probable cause for an actual,
no-question-about-it search, the kind that involves going through
your pockets, opening your luggage, looking in your trunk, and
perusing your personal belongings. The problem is that a dog
barking or sitting may be responding not to a smell but to his
handler’s hunch about a suspect’s guilt. The reason we have a
Fourth Amendment is precisely to prevent searches based on

The consequences of misusing police dogs go well beyond
unconstitutional searches. A drug dog’s alert can help establish a
connection between a suspect’s property and drug activity, allowing
police to seize the property for possible forfeiture.
Even if the owner is never charged with a crime, the burden is on
him to go to court to win back what was his, a process that often
costs more than the property is worth. In a case
I reported last year, for example, college student Anthony Smelley
had $17,500 in cash that he’d won in an accident settlement seized
when police in Indiana pulled him over and a drug dog alerted to
Smelley’s car. It took Smelley more than a year to win the cash
back in court, even though a subsequent hand search turned up no
illegal substances.

Canine testimony can also play a key role in murder
. Last September the Texas Court of Criminal Appeals

the 2004 murder conviction of Richard Winfrey Sr.
because the case against him was based on an unreliable,
pseudoscientific “scent lineup” in which Fort Bend County Deputy
Sheriff Keith Pikett (now retired) claimed his team of bloodhounds
alerted to the murder victim’s scent on Winfrey’s clothing. Pikett
and his dogs have assisted in thousands of criminal investigations
by police departments all over Texas. As late as last year,
prosecutors were trying
to use the results from one of Pikett’s scent lineups to retry
Anthony Graves after a federal appeals court threw out his murder
conviction. Graves, who served 18 years on death row, has since
been exonerated and freed.

Pikett is now the target of a
class-action lawsuit
brought by people who say they were
wrongly detained or convicted based on his dogs’ alerts. Similar
questions have been raised about the methods used by the late John
Preston, a former Pennsylvania state trooper who found a second
career as a freelance dog handler in police investigations, mainly
in Florida. Preston’s dog helped convict dozens of Floridians in
the 1980s. At least three murder convictions secured through
Preston’s testimony have since been overturned.

In 2006 University of North Carolina law professor Richard Myers
a statistical analysis
(PDF) of police dog accuracy tests and
concluded that the animals were not reliable enough to produce
probable cause for a search, let alone serve as the cornerstone of
a conviction. At least five states have
banned or restricted
the use of scent lineups in criminal
cases, but they are still frequently used in courtrooms across the

Dogs can be valuable investigative tools. They are great, for
example, at following a scent in searches for suspects or sniffing
out survivors after a disaster. The bomb-detecting dogs in Iraq and
Afghanistan are successful because their handlers have no
preconceptions about where bombs may lie. Indeed, they are putting
their lives in the dogs’ paws. With no cues from their masters to
cloud their judgment, the dogs are free to go about their task
unbiased. But while Canis domesticus retains many of its
wilder relative’s sensory abilities, it is in many ways a man-made
animal. When we don’t take that reality into account, a dog can be
worse than useless. But that’s not the dog’s fault. It’s ours.

Radley Balko is a
senior editor at
Reason magazine.