Study Finds It’s a Hormone, Not the Breed, That Makes Dogs Aggressive

In Denver, Miami, Montreal and other areas (most recently, Springfield, Mo.), pit bulls are banned because of the misconception that it’s a dangerous breed. German shepherds, Rottweilers and Dobermans are also often perceived as being aggressive, which is why, as their owners can tell you, it’s difficult to get home insurance if you happen to have one of these dogs.

Dog experts and almost all major animal welfare organizations oppose these unfair bans and other breed-specific legislation (BSL) because they unfairly single out certain types of dogs. BSL has also proven to be ineffective, since public safety has not improved where it’s been enacted.

Hopefully the results of a recent study at the University of Arizona will help raise awareness that certain dog breeds aren’t naturally aggressive.

“Dog aggression is a huge problem,” said Evan MacLean, the leader of the study that was published in the journal Frontiers in Psychology. “Thousands of people are hospitalized every year for dog bites, especially kids, and aggression is one of the main reasons that dogs get relinquished to shelters. If there are ways to intervene and affect biological processes that produce aggression, that could have a huge benefit both for people and dogs.”

The subjects of this study were pet dogs of various breeds who tended to be aggressive on walks (aka leash aggressive), according to their owners. For comparison purposes, pet dogs that weren’t aggressive were also included in the study.

Each dog was held on a leash by its owner as, across the room, an audio of a dog barking was played behind a curtain. The curtain was then pulled back to reveal a lifelike dog model with a human handler. The dogs were also presented with everyday noises and objects like a trash bag and inflated yoga ball.

The responses and hormone levels of the dogs were measured before and after the interactions. Although none of the dogs were aggressive toward the objects, many of the leash-aggressive dogs barked, growled and lunged at the fake dog.

All of those dogshad higher levels of the hormone vasopressin which, just as it does in humans, causes aggression in dogs.

Previous research has found that testosterone and serotonin can cause aggression in dogs and other mammals, but this was the first study to focus on the effects of vasopressin as well as the feel-good hormone oxytocin, which inhibits aggression.

Why do some dogs have higher levels of vasopressin? It’s not necessarily because of their breed.

“There’s a lot of work showing that experiences in your lifetime can change the way hormones function,” MacLean said. “For a lot of dogs that have aggression problems, the owners report that the onset of the aggressive symptoms happened after some sort of traumatic experience. Often it was that the dog was attacked by some other dog and is in a hypervigilant state after that event almost like a post-traumatic reaction.”

If your dog is leash or otherwise aggressive, you can try spending more time in more friendly interaction with your pet which, no surprise, is a win-win for both of you.

“Previous work shows dog-human friendly interactions can create a release in oxytocin in dogs, and when dogs interact with people, we see that their vasopressin levels go down over time,” MacLean said. “These are bidirectional effects. It’s not just that when we’re petting a dog, the dog is having this hormonal response we’re having it, too.”

Photo credit: Teerasuwat


Melania P
Melania P12 days ago

See? A reason science can explain why dogs are aggressive. They are not born "evil"! Thanks for sharing

Marie W
Marie W21 days ago

Thank you for sharing!

Renee M
Renee M6 months ago

Most rational humans know it has never been the breed that is the issue...
At least hormones and life experiences make a lot more sense when it comes to aggressive behaviors. And of course by life experiences that includes the human(s) who train or don't train their dogs.

Mike R
Mike R6 months ago


Philippa P
Philippa Powers6 months ago

My Chihuahua, Zoe, is so aggressive towards other dogs that I have to hold her by her harness on the platform of my scooter as we whizz by.

Shirley S
Shirley S6 months ago

My son had a lovely cross border collie cattle dog that had a nice nature. But on two occasions was forced to defend itself against attacks from two dogs together that instigated the attack. The first attack was from 2 Labradors & the second was 2 Pit-bulls whose owner had let them loose.

Carl R
Carl R6 months ago


Bill E
Bill Eagle6 months ago

Hormones also make people dangerous as well. Just ask my wife.

Elisabeth H
Elisabeth H6 months ago

interesting, thanks

Richard A
Richard A6 months ago

Thank you for this interesting article.