Understanding Aggression in Dogs: It's Complex, but Fear is a Common Cause

The risk factors for dogs being aggressive towards people include being fearful, older, small, and someone’s first dog, new research shows. 

A Lagotto Romagnolo on a dog bed. Fear is commonly behind aggression in dogs, study shows
Photo: Bianca Ackermann/Unsplash

By Zazie Todd, PhD

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When you can persuade a lot of dog guardians to complete a questionnaire, you can answer some really interesting questions about dog behaviour. Like, what factors are associated with dogs being aggressive towards people?

New research in Scientific Reports looks at just this question. The results come from a large subsample (more than 9,000) of questionnaire results from a total sample of 13,700 dogs. The overall research has already looked at the factors associated with social fear (e.g. of strangers and other dogs) and non-social fear in dogs, both of which I previously reported on. Now, the scientists have turned their attention to aggression.  

For this study, aggression was defined as “frequent growling, snapping and biting or trying to snap or bite.” (This is a narrower definition than used in some other research).

The results show that aggression is most commonly associated with:

  • Being fearful. Both moderately and very fearful dogs were more likely to be aggressive than dogs that were not fearful.
  • Being older. Older dogs were more likely to be aggressive than young dogs.
  • Being a male dog, rather than female.
  • Being a small dog, rather than a medium or large dog.
  • Being someone’s first dog, rather than living with someone who has had a dog before.
  • Being an only dog, instead of having other dogs in the same household. 

The researchers also looked at the effect of breed. As you might expect, Golden Retrievers and Labrador Retrievers were least likely to be aggressive. 

The breeds that were more likely to be aggressive included the Rough Collie, Miniature Poodle, Miniature Schnauzer, German Shepherd, and Lagotto Romagnolo. (Incidentally, if you want to see radar charts for behavioural characteristics of the Miniature Schnauzer and Lagotto Romagnolo, they are included in my previous report on dogs' fears and anxieties).  

PhD student Salla Mikkola (University of Helsinki), first author of the paper, says, 

"Dogs' fearfulness had a strong link to aggressive behavior, with fearful dogs many times more likely to behave aggressively. Moreover, older dogs were more likely to behave aggressively than younger ones. One of the potential reasons behind this can be pain caused by a disease. Impairment of the senses can contribute to making it more difficult to notice people approaching, and dogs' responses to sudden situations can be aggressive."

It’s important to note that this data is correlational so it does not tell us about the interplay between genetic and environmental factors for different breeds. For example, the researchers note that some breeds may be more likely to be in pain, which can cause aggression. They also suggest that breeders take aggression into account when choosing which dogs to mate. It's best to pick friendly and non-fearful dogs rather than aggressive or anxious ones. 

The finding about small dogs is in line with some other studies, and not with others, so more research is needed. Earlier research has shown that people spend less time training little dogs, so it’s possible that this is a factor in them being more likely to be aggressive. 

The finding that experienced owners are less likely to have an aggressive dog ties in with other research. One possibility is that more experienced owners are likely to do a better job of socializing their puppy and have a better understanding of dog behaviour. The scientists say that better education of dog owners may help to prevent aggression.

As for the finding about single dogs, it could be that if someone has an aggressive dog they are less likely to get another dog, but that seems unlikely to be the whole story. Another intriguing idea suggested in the report is that in a home with multiple dogs, the best-behaved dogs are likely to get more attention, and so they may all try to behave better. Of course it could also be something to do with the kind of people who get more than one dog. Again, more research is needed. 

This research did not look at training methods, but other research has found a link between aversive training methods and fear and aggression. As well, it’s important to know that any dog can bite, and breed-specific legislation does not protect people (see e.g. Breed Specific Legislation had no effect on dog bites in Odense, Denmark).

The study did not distinguish between aggression towards familiar and unfamiliar people. 1,791 dogs in the study had shown aggression towards people, while the others had not. The size of the sample in this study is impressive.

Overall these results show that aggression in dogs is a complex issue with multiple causes. The implication for breeders in that temperament should be taken into account when breeding. And for dog guardians, if you’re getting a puppy, ask about (and preferably see) the friendliness of both parents (see: how to choose a puppy). And be sure to socialize your puppy to help them grow up to be sociable and confident.  

The link between aggression and fear is not surprising. If your dog is fearful, see eight tips to help a fearful dog feel safe. And it’s always better to seek help sooner rather than later for fear and aggression cases. See my post on how to choose a dog trainer.  As well, here are some tips on how to help a reactive dog

The paper is open access if you would like to read it in full (link below).

If you liked this post, you'll love my new book, Bark! The Science of Helping Your Anxious, Fearful, or Reactive Dog which is available for pre-order now. Karen Fine DVM, author of the NYT-bestseller The Other Family Doctor, says “Bark! should be required reading for every veterinarian and anyone who loves an anxious, fearful, or reactive dog.”

The cover of Bark! The Science of Helping Your Anxious, Fearful, or Reactive Dog by Zazie Todd
Pre-order Bark! The Science of Helping Your Anxious, Fearful, or Reactive Dog


Mikkola, S., Salonen, M., Puurunen, J. et al. Aggressive behaviour is affected by demographic, environmental and behavioural factors in purebred dogs. Sci Rep 11, 9433 (2021).  

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