Dogs are still the most popular pet in the U.S. (sorry, cats) and only getting more popular. Just last year, the percentage of homes with at least one dog rose to nearly 45 percent, according to the American Veterinary Medical Association.

For many of these first-time pup parents, smaller breeds may seem the easier or safer option. But looks can be deceiving. 

According to numerous studies over the past two decades, certain pint-sized breeds are stubborn as mules and prone to barking, biting, snapping and snarling — while plenty of larger dogs take “man’s best friend” to the extreme. And researchers have some pretty good hypotheses for why.

Read More: Why Do Small Dogs Live Longer Than Large Dogs?

Most Aggressive Dog Breeds

A 2021 study, conducted by the University of Helsinki and published in Scientific Reports, looked into the factors contributing to aggressive behavior (or a lack of such behavior) in more than 9,200 purebred dogs. The researchers found that small dogs indeed have significantly higher odds of aggression compared to their medium and large counterparts.

The second and third most aggressive breeds on their list? Miniature poodles and miniature schnauzers. Labrador retrievers and golden retrievers, on the other hand, were found to be the least aggressive.

A study published in Applied Animal Behaviour Science in 2008 also endeavored to determine breed-specific aggressiveness. Noting that we tend to believe stereotypes about larger and more powerful dogs because of their disproportionate risk of injury, the researchers surveyed the owners of more than 30 different dog breeds to uncover the truth.

According to their questionnaire results, dachshunds, chihuahuas and Jack Russell terriers are the most likely to attempt to bite humans — and more than one-fifth of the Jack Russell terriers additionally displayed serious aggression toward unfamiliar dogs.

Read More: Why Does My Dog Bark So Much?

Why Are Small Dogs So Aggressive?

But hey, it’s a dog-eat-dog world out there, right? Surely there’s a reason why these pint-sized pets are willing to bark and bite to be seen as top dog. 

Blame the Parents

We’re all a product of our upbringing, and our pets are no different. So, if you’re the less-than-proud owner of a difficult small dog, it might be time to reconsider your training chops. Of course, you aren’t alone. 

Researchers say that pet parents of smaller breeds tend to be more lenient when correcting bad behavior; after all, it’s way cuter when a five-pound chihuahua jumps all over a guest than when a 75-pound German shepherd attempts to do the same.

As a result, smaller breeds, on average, tend to have fewer social skills and be less obedient in general.

In Their Genes

Now imagine that generation after generation of our littlest companions receive the same insufficient training.

Somewhere in the middle of the “nature versus nurture” argument, some researchers argue that modern-day small dogs are so aggressive because for millennia, humans simply didn’t bother to breed these bad behaviors out of them during domestication.

Lending credence to this line of reasoning is a 2016 study published in BMC Genomics, which mapped the genomes of several hundred dog breeds in search of genetic indicators of fear and aggression. Sure enough, among their findings was a link between aggression and a growth factor gene that gives small dogs their miniature stature.

Scared Silly

Perhaps the most obvious reason pint-sized pups are more likely to lash out, however, is that they are more vulnerable than bigger dogs and therefore have more to be fearful of. As a result, they’re quicker to go on the defense.

The same 2021 University of Helsinki study from above also found that “highly fearful dogs had over five times higher odds of aggressive behavior than non-fearful dogs.” And it’s known that small dogs, when compared with large dogs, tend to have more issues with separation anxiety and excitability and hyperactivity upon their owner’s return.

Some of these behaviors can be reduced by keeping your dog away from problematic situations and environments, or by consistently reinforcing good behaviors with rewards. But because of the special dangers that canine aggression causes, VCA Animal Hospitals recommends meeting with a veterinarian or even a board-certified veterinary behaviorist as a first step.

Read More: What To Do If Your Pet Is Struggling With Anxiety