The Irish Examiner called Rottweilers “time bombs on legs.” The Supreme Court of Kansas called pit bulls “a public-health hazard.” Dostoyevski called the bloodhound “a terrible beast.” But new data suggests that stereotypes of breeds and aggression might warrant a closer look.  

My website, Dognition, gathers data on dog behavior by leading paid subscribers through games designed to test their pets’ cognition. Recently, a random sample of people who played the games was asked how aggressive their dogs were in various situations—toward people who were new or familiar, children who were new or familiar, and dogs who were either new, familiar, bigger, or smaller. More than 4,000 dog owners responded.

In almost every measure, out of the 35 most common breeds, Chihuahuas were reported as the most aggressive, especially toward bigger dogs they have not seen before. But before Chihuahua owners start picketing, none of the breeds were particularly aggressive. Chihuahuas peaked out at being moderately aggressive on some measures but were usually on the “sometimes aggressive” end of the spectrum. They only stood out because most other breeds—including pugs, collies and King Charles Cavalier Spaniels—were “seldom aggressive” or “never aggressive.”

The American Pit Bull Terrier—a breed often portrayed as highly aggressive—consistently ranked as one of the least aggressive dogs, with the exception being toward new dogs, where it was still below Miniature Schnauzers. This could be because pit bull owners are conscious of the bias against the breed when self-reporting, but it does agree with the American Temperament Test Society, which also has found that American Pit Bull Terriers were among the most tolerant breeds.

Dogs are carnivores, and sometimes they do bite. More than 300,000 Americans visit the ER for dog bites every year, and an average of 25 of these people die from the injuries. In response, since the 1980s, more than 900 cities have enacted breed specific legislation (BSL). BSL can range from wearing a muzzle to euthanasia.  Although other breeds, like Rottweilers, Chow Chows, and even Chihuahuas are occasionally included, almost every BSL is targeted at pit bulls.

This is problematic, since not only does the evidence suggest that pit bulls are not more aggressive toward people than other breeds, but few people even know what pit bulls are. American Staffordshire Terriers, Staffordshire Bull Terriers, American Pit Bull Terriers, and mixes of any of these breeds all can be called a pit bull, and even people who are familiar with pit bull breeds can have trouble identifying them. In several recent studies,  workers at shelters misidentified dogs’ breeds 50 to 87 percent of the time. When DNA tests identified a dog’s dominant breed as Dalmatian, shelter workers called it a terrier. When the dog was mostly Alaskan malamute, they called it an Australian shepherd dog.

When a hospital records that a dog who bit someone is a pit bull, they rely on the report of the victim, parents, or a witness. No one does a DNA test to make sure. Media coverage of attacks tends to encourage this misidentification: In 2008, a pit bull attack that hospitalized a woman generated 230 articles and televised reports in national and international news. A few days before, a mix-breed dog killed a 16-month-old child. The local paper reported it twice.

Almost every organization, from the Humane Society to the Obama administration, opposes breed-specific legislation, because of the evidence that it does not work. For instance, after banning pit bulls in 1984 and euthanizing thousands of animals, Denver has more people hospitalized for dog bites than anywhere else in Colorado.

In place of these policies, researchers who study animal behavior are trying to better understand the context in which dogs bite, and the interactions that facilitate attacks. And more accurate, predictive genetic tests like Embark mean that with a quick cheek swab, at least pit bulls will no longer be misidentified. Perhaps in the future, the genetics of aggression will be understood well enough to warn people before a dog has bitten someone.

But for now, you can’t tell if a dog is aggressive by the way it looks, any more than you can with a human.