Why Are Some Dogs More Aggressive Than Others?

Aggression comes in all shapes and sizes. Piotr Wawrzyniuk/Shutterstock

A dog is a lean, mean gene machine. In fact, there's a good chance much of your best friend's behavior is etched in her DNA.

But what if a dog is a little too mean? As in, no one can come near the dog without a surly, snapping response?

According to new research from four U.S. universities, that's in the genes too. For the study, published this month in the Proceedings of the Royal Society B, scientists looked at the genetic and behavioral records of 14,000 dogs spanning 101 breeds. They found that between 60% and 70% of behavioral traits — including aggressiveness — are inherited from their parents.

Among the traits most commonly passed on? The need for attention, trainability ... and aggressiveness.

Of course the first two traits — how much attention they demand and their trainability — may be desirable. Hence, breeders might favor these "types" when selecting the ideal parents.

But aggressiveness? Not many people want a dog that owns them, much less, a dog that bites them. The American Society for the Prevention of Cruelty to Animals (ASPCA) describes aggression as "the most common and most serious behavior problem in dogs."

For countless dogs, it's a death sentence. Aggression is a key reason why families give them up to shelters.

The problem may lie in a dog's relatively shallow gene pool. Despite being domesticated for some 17,000 years, dogs don't have the longest breeding history. All those miniature pinschers and dachshunds and Dalmatians only appeared in the last few centuries, as humans figured out how to tinker with their genes. As a result, there isn't a lot of genetic diversity to spread around.

Personality traits come in different sizes

Photo: David Sokoler/Shutterstock

Not so long ago, dogs were bred for very specific purposes.

"Some were highly prized for their guarding and protective tendencies, others for their hunting prowess, others for their fighting skills, and others for their 'gameness' and tenacity," the ASPCA notes.

In other words, there's a good chance that a miniature poodle has someone up the family tree who was a guard dog — and passed along those mean genes to that adorable poodle who causes adults to flee in terror.

In all, the new study identified 131 genetic variations associated with a dog's behavior. And while there's no single gene for any trait, including aggressiveness, they interact with other genes to make a "character" cocktail that may literally have some bite.

"Dogs exhibit striking parallels to traits in humans," researchers note in the study. "For example, common genetic mechanisms contribute to individual differences in social behavior in dogs and humans."

Close-up of a snarling chihuahua
Dogs may vary wildly in appearance, but when it comes to personality, they don't have a lot of genes to choose from. Jiri Vaclavek/Shutterstock

And, as with humans, personality traits come in different serving sizes. Aggressiveness could be limited to territory — as in, no one shall pass this threshold that isn't family. And absolutely no postal service workers. Or it might manifest as dog-on-dog violence, which is particularly problematic in urban areas.

Then there's predatory aggression, defined by the Royal Society for the Prevention of Cruelty to Animals (RSPCA) as the silent stalking of small animals and birds. But for some dogs, toddlers may also fit the bill.

But when it leads to dog-on-human violence, aggression always brings a bad result for all parties.

Fortunately, being born under a genetic sign isn't an automatic death sentence for dogs. There are plenty of ways to curb a dog's surliness, especially once his motives are established. A professional trainer, rather than a shelter, should be the first step.

"Taking into account the behavior modification techniques that affect aggression, our current understanding is that the incidence and frequency of some types of aggression can be reduced and sometimes eliminated," the ASPCA notes. "However, there's no guarantee that an aggressive dog can be completely cured."