Aggressive dogs bite people more than 4.5 million times each year, according to the American Veterinary Medical Assocation. About 20 percent of those dog bites need medical attention.

If you're thinking about adopting a dog, it's natural to worry about whether they will become aggressive and bite you or someone you love. Find out what makes dogs aggressive, what the most aggressive breeds are and how you should factor breed aggression into your decision to get a dog.

What Makes a Dog Aggressive?

A 2021 University of Helsinki study collected data from 13,715 dogs, including 1,791 dogs with frequent episodes of aggressive behavior. The researchers identified several factors contributing to aggressiveness:

• Fearfulness. Fear was the most significant contributor to aggressive behavior. Much more than breed or other factors.
• Age. Older dogs were more likely to be aggressive. Possibly because pain from chronic illness makes them less patient with humans.
• Gender. Male dogs were more aggressive than females. Sterilization did not affect aggressiveness.
• Size. Small dogs were more aggressive than larger breeds. Possibly because they are more fearful towards humans and animals much larger than them.
• Owner experience. Pets of first-time dog owners were more likely to be aggressive.
• Socialization. Dogs who didn't have canine companions were more likely to be aggressive towards people.
• Breed. Dog breeding is another factor that influences aggression, but its influence is not overwhelming. Training can help dogs from more aggressive breeds get along with humans and other animals.

The University of Helsinki published data on aggressiveness in 22 dog breeds. These 10 dogs are more aggressive than average:

1. Rough Collie

Average Weight: 40 to 65 lbs

Average Height: 20 to 24 inches

Life Expectancy: 14 to 16 years

The loyal and loveable Lassie popularized this breed. But like the TV star's antics, its reputation as a friendly dog might be fiction. Rough collies (or long-haired collies) were the most aggressive breed in the University of Helsinki's study.

These dogs are known to be very energetic and alert. That might contribute to their aggressiveness towards strangers. They are also very loyal and may snap when their family is threatened.

2. Miniature Poodle

Average Weight: 10 to 15 lbs

Average Height: 10 to 15 inches

Life Expectancy: 13 to 15 years

Like other small dogs in the University of Helsinki's research, miniature poodles behave aggressively to make up for their size. They are very energetic, and their hyperactivity can sometimes escalate to more aggressive behavior. Miniature poodles often come across as "yappy," rather than dangerous, but they are still one of the most aggressive dog breeds according to the study.

3. Miniature Schnauzer

Average Weight: 12 to 20 lbs

Average Height: 12 to 14 inches

Life Expectancy: 12 to 15 years

A smaller version of the standard schnauzer. Miniature schnauzers were bred in Germany in the 19th century as guard dogs. Their stature may make them more aggressive than their larger cousins.

Miniature schnauzers are also fearless, despite their size. They won't back down in the face of a fight, which can cause normal situations to escalate.

4. German Shepherd

Average Weight: 49 to 88 lbs

Average Height: 22 to 26 inches

Life Expectancy: 9 to 13 years

German shepherds frequently serve as police or military dogs, but not because of their aggressiveness. Instead, it's because they respond well to training. German shepherd owners can use this to their advantage and train their dogs to behave well.

These jobs help them burn off all their energy. Without enough stimulation, they can turn their liveliness towards aggression.

5. Spanish Water Dog

Average Weight: 31 to 49 lbs

Average Height: 15 to 20 inches

Life Expectancy: 10 to 14 years

These medium-sized dogs, known for their woolly curls, were originally bred to retrieve downed waterfowl. That may contribute to their above-average aggressiveness. Today, they're prized as athletic family dogs that can act as vigilant watchdogs.

6. Lagotto Romagnolo

Average Weight: 24 to 53 lbs

Average Height: 17 to 18 inches

Life Expectancy: 15 to 17 years

The curly-coated Lagotto Romagnolo breed began as a truffle-hunting dog in northeastern Italy. Like other working dogs, they can be very energetic and need plenty of stimulation, so they don't turn aggressive. Fortunately, they also respond well to training, so owners can help reign in any aggressive tendencies.

7. Chinese Crested Dog

Average Weight: 7 to 12 lbs

Average Height: 9 to 13 inches

Life Expectancy: 13 to 15 years

Chinese crested dogs are one of the smallest dog breeds in the world. The breed is most known for its hairless variety, which has only tufts of hair around its paws and head.

Their size contributes to their aggression, along with their energetic temperament. These dogs are also highly intelligent. Owners can take advantage of that trait and teach them tricks. This burns off energy and manages their aggressive dispositions.

8. German Spitz Mittel

Average Weight: 15 to 24 lbs

Average Height: 12 to 15 inches

Life Expectancy: 13 to 15 years

These small dogs have poofy coats and a reputation for yapping. They're devoted to their owners, but very distrustful of strangers. They can make great watchdogs, and they'll always sound the alarm when they see an intruder.

9. Coton de Tuléar

Average Weight: 8 to 13 lbs

Average Height: 9 to 12 inches

Life Expectancy: 14 to 16 years

Cotons are famous for their fluffy white coats and vocal personalities. They form very close bonds with their owners, and they make excellent companions. But as small dogs with lively temperaments, they're prone to aggressive behavior.

10. Wheaten Terrier

Average Weight: 30 to 40 lbs

Average Height: 17 to 19 inches

Life Expectancy: 12 to 15 years

These bearded dogs come from Ireland, where they were bred to hunt and kill vermin. Their soft coats might give you the impression that these dogs have a soft temperament, but they were originally bred as working dogs. They carry over the energy and aggression from those early days, but with training, they can be loving family dogs.

What Are the Least Aggressive Dogs?

Golden retrievers and labrador retrievers were vindicated by the University of Helsinki study. They were the least aggressive dogs among the breeds studied.

Shetland sheepdogs and Lapponian herders were all near the bottom of the list, making them more peaceful than many other dogs.

The Staffordshire bull terrier rounded out the list as the fifth least aggressive dog. This dog commonly appears on "most dangerous breed" lists as a member of the pit bull category, but data from the University of Helsinki doesn't support that idea.

The University of Helsinki's research should cause us to reexamine many of our beliefs about the most aggressive dog breeds. Small dogs and miniature breeds appear frequently in the top 10, while the usual suspects like pit bulls or rottweilers are proven to be less aggressive based on the probability of aggressive behavior.

Of course, a pit bull's bite would prove deadlier than a chihuahua's—so it's crucial to train larger dogs, even if their breed is technically less aggressive.

Should You Avoid Aggressive Dog Breeds?

No. If you find a dog you like, don't stop yourself from adopting it just because of its breed.

Breed is just one of many factors that can contribute to aggressiveness. Its contributions are minor compared to training, environment and how you behave around the dog.

Breeds can also be hard to determine. Animal shelter workers frequently misidentify dogs as pit bulls, a breed thought to be more aggressive than others.

The bottom line: dogs don't bite because of their breed. And concerns about breed aggressiveness should not stop you from adopting a particular dog.

FAQs About Aggressive Dog Breeds

If we don't have an answer to your question about aggressive dogs, ask your vet or a professional dog trainer.

Can You Train a Dog to Be Less Aggressive?

Yes. Training is a major factor in managing aggressiveness. And it could be one of the reasons why first-time dog owners tend to raise more aggressive pets.

To manage your dog's aggression, try working on:

• Socialization. Take your dog to the park where they can practice socializing with dogs and humans.
• Recall. If your dog responds quickly when you recall them, you can pull them out of dicey situations before things turn aggressive.
• The cause. Dogs usually aren't aggressive for no reason. They respond to situations or stimuli. If you can find out what sets your dog off and address it, you can reduce their aggression.

However, training an overly aggressive dog takes skill and patience. If you haven't trained dogs before, call a professional trainer to help manage your pet's aggressive behavior.

Is Dog Reactivity Different from Aggressive Behavior?

Yes. Dog reactivity and aggressive behavior are not the same.

Reactive dogs overreact to common stimuli. They lunge, bark and growl in situations where other dogs would behave normally, like seeing other dogs at the park or meeting strangers. This might be because they're fearful of an uncertain situation or hyperstimulated.

Reactive dogs are not aggressive dogs. However, they can be pushed into aggression. If your dog is reactive, make sure to give them space and speak with a trainer about managing their reactive behavior.

Dr. Katherine Houpt, the James Law Professor Emeritus of Behavior Medicine at Cornell University, recommends advocating for your dog's personal space. In the short term, keep them away from stimuli that trigger reactivity. This will show them they can feel safe and calm outdoors.

In the long term, train them to behave well around the troubling stimuli. Houpt recommends keeping treats on hand and rewarding your dog as soon as their trigger appears. This will help them associate the trigger with something positive.

Do Apartments Allow Aggressive Dog Breeds?

It depends on the apartment. Apartments commonly ban these dog breeds, commonly thought to be aggressive:

• Chow chows
• Huskies
• Rottweilers
• German shepherds
• Pit bulls

However, some of the most aggressive dog breeds, like rough collies, are rarely on apartment breed restriction lists. Apartment breed restrictions may have more to do with size and perceived danger than actual aggressiveness.

If you have a well-behaved dog that's part of a banned breed, talk to the apartment manager. You may be able to get an exception.

Emotional support animals are also exempt from breed restrictions, thanks to the Fair Housing Act. But you will need an emotional support animal (ESA) letter from a licensed mental health professional.

Does Insurance Cover Aggressive Dog Breeds?

Home insurance companies often have dog breed ban lists. Journalists working for Forbes analyzed ban lists at major home insurance providers and found that these dog breeds frequently appeared on them:

• Doberman Pinschers
• Pit Bulls
• Rottweilers
• Chow Chows
• Wolf dogs and wolf hybrids

Insurance providers won't offer you a policy if you have a dog on their ban list. However, policies and ban lists vary between providers. Make sure to check with your insurance provider.

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Breed is one of many factors influencing aggressiveness in dogs. However, that shouldn't stop you from adopting a dog that's more aggressive than average. Training can make any dog a good citizen, regardless of breed.