What If We Looked Inside the Dogs We Decided Were Too Dangerous to Save?

I recently read another social media post about a dog that had sadly killed someone and was subsequently euthanised.

As often happens with these posts, the comments quickly became divided. People debated what should have happened to the dog, what the police should have done, whether the owners were responsible, and whether euthanasia was justified.

But something else caught my attention.

More than one person commented something along the lines of:

*"I've just read that the dog had a growth in its brain."*

Except, when I looked through the comments, nobody said where they'd read it.

There was no source. No link. No explanation of who had diagnosed it, or whether it had actually been confirmed.

It may have been true.

It may have been completely untrue.

And that got me thinking.

**How often do we actually know what is happening inside the dogs we describe as aggressive?**

I don't mean that every aggressive dog has a brain tumour. There is absolutely no evidence that we should assume that.

I mean something much simpler.

When a dog is euthanised because of severe aggression, how often do we actually investigate whether there was an underlying medical or neurological reason for the behaviour?

I started looking.

And this is where things became surprisingly difficult.

We know that medical conditions can affect behaviour. Pain can change how a dog responds to the world. Neurological disease can affect behaviour. Sensory loss, hormonal disorders and other medical problems can potentially contribute to changes in temperament and aggression.

In fact, a 2023 review specifically examining the relationship between aggression and physical disease in dogs concluded that medical conditions can sometimes trigger aggression—for example, intracranial tumours—or aggravate an existing aggression problem, as can painful conditions. The authors argue that assessing physical health should be part of the diagnostic process when investigating canine aggression.

That doesn't mean that every aggressive dog has a medical condition.

But it does mean that the possibility shouldn't simply be dismissed.

We also know that aggression is complicated.

It can be influenced by genetics, early development, fear, learning, trauma, pain, environment, stress and the individual dog's experiences. Often, it isn't one thing. It is a combination of many things.

But when I tried to find out how often dogs euthanised because of serious aggression are given a post-mortem examination, I couldn't find a straightforward answer.

I couldn't find a large-scale study that told me:

**This is how many dogs were euthanised because they were considered dangerous.**

**This is how many were subsequently examined after death.**

**And this is how many were found to have a physical or neurological abnormality that might have contributed to their behaviour.**

That doesn't mean those investigations never happen.

Researchers have, for example, examined brain tissue from dogs euthanised because of aggressive behaviour.

One study, carried out by researchers in Norway, examined brain tissue from 11 dogs euthanised because of aggressive behaviour and compared it with tissue from nine non-aggressive dogs. They looked at gene expression in several areas of the brain, including the amygdala, frontal cortex, hypothalamus and parietal cortex.

The researchers found differences in gene expression between the aggressive and non-aggressive groups, including increased expression of two genes associated with the aggression phenotype.

That is fascinating.

But it isn't the same thing as finding that a certain percentage of aggressive dogs had brain tumours, neurological disease or other physical abnormalities.

And that distinction matters.

Because there is a temptation, particularly on social media, to fill gaps in our knowledge with certainty.

A dog is aggressive, so someone says it must have been abused.

Or it must have been badly bred.

Or it must have a brain tumour.

Or it must have been the owner's fault.

Or it must simply be a "bad dog".

Sometimes we don't know.

And I think we need to become much more comfortable saying that.

The question I'm left with isn't:

**"Do aggressive dogs have something wrong with their brains?"**

That would be an entirely different—and much bigger—claim.

The question is:

**"How often do we actually look?"**

If a dog has bitten someone seriously, and that dog is subsequently euthanised, our focus is understandably on the people who have been harmed and on preventing another incident. Those things matter enormously.

Sometimes euthanasia may be the safest and most responsible decision.

But once that decision has been made, I wonder whether there is also an opportunity to learn.

What if, wherever it was ethically and practically possible, we systematically studied the dogs we euthanise because of serious aggression?

Not to excuse what they did.

Not to suggest that every dangerous dog could have been saved.

Not to shift blame away from owners or deny the experiences of victims.

But simply to ask a scientific question:

**Is there something we are missing?**

How many had undiagnosed pain?

How many had neurological abnormalities?

How many had endocrine or metabolic problems?

How many had evidence of previous injury or disease?

And how many had no obvious physical abnormality at all?

The last question is just as important as the others.

Because if we investigated 1,000 dogs and found that almost all had no relevant physical abnormalities, that would tell us something too.

At the moment, however, I can't find good evidence that allows us to answer these questions.

And perhaps that's the uncomfortable part.

We have research into why dogs are euthanised for behavioural reasons.

We have research into the medical conditions that can influence behaviour.

We have research into the neurobiology and genetics of aggression.

But there seems to be much less research connecting those things together.

A 2024 study examining behavioural euthanasia in pet dogs found that aggression—particularly aggression towards people—was the most commonly reported reason for owners choosing behavioural euthanasia. But this research focused on the behavioural circumstances surrounding the decision. It did not systematically investigate what physical or neurological abnormalities the dogs might have had.

So there is research looking at **why** dogs are euthanised.

There is research looking at **how medical conditions can influence behaviour**.

There is research looking at **what happens in the brains of some aggressive dogs**.

But what happens when we put all of those questions together?

What happens when we take the dogs that have been labelled dangerous and actually investigate them?

Not just their behaviour.

Not just their history.

But the whole dog.

I don't think the answer is to assume that every aggressive dog has an undiscovered brain abnormality.

But I also don't think the answer is to assume that there is nothing left to learn.

Maybe that dog I mentioned at the beginning really did have a growth in its brain.

Maybe it didn't.

I still don't know.

And that's the point.

The fact that I couldn't easily find out—and that several people were willing to repeat the claim without a source—is exactly why I think we need better evidence.

Perhaps, before we decide that a dog is simply aggressive, dangerous or beyond help, we should remain curious about what we don't yet know.

Because once that dog is gone, we don't get the opportunity to ask the question again.

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### Sources and further reading

**The relationship between aggression and physical disease in dogs (2023)**

A review discussing how medical conditions—including pain, neurological disease and intracranial tumours—can trigger or worsen aggression, and why physical health assessment should be part of investigating canine aggression.

[Read the research on PubMed](https://pubmed.ncbi.nlm.nih.gov/37714772/)

**Differential gene expression in brain tissues of aggressive and non-aggressive dogs (2010)**

A Norwegian study examining brain tissue from 11 dogs euthanised because of aggressive behaviour and nine non-aggressive dogs. The researchers found differences in gene expression between the groups. Importantly, this was not a study showing that aggressive dogs commonly have brain tumours or other visible abnormalities; it was an investigation into gene expression and the biology of aggression.

[Read the full open-access research paper](https://pmc.ncbi.nlm.nih.gov/articles/PMC2898780/)

**Factors associated with behavioural euthanasia in pet dogs (2024)**

A study involving owners who had made the decision to euthanise their dogs because of behavioural problems. Human-directed aggression was the most frequently reported reason. The study is useful for understanding why behavioural euthanasia happens, although it does not answer the question of how often these dogs have underlying physical or neurological abnormalities.

[Read the full open-access research paper](https://pmc.ncbi.nlm.nih.gov/articles/PMC11091869/)

**Genetic and Gene-by-Environment Influences on Aggressiveness in Dogs: A Systematic Review (2025)**

A more recent systematic review examining research into the genetic and environmental influences on canine aggression. It highlights the complexity of aggression and the difficulty of identifying a single cause, while also pointing to the need for better research and more consistent methods.

[Read the full open-access research paper](https://pmc.ncbi.nlm.nih.gov/articles/PMC12345521/)