Whenever a violent crime occurs, one thing that happens with depressing inevitability is the accusation that the perpetrator was mentally ill.

In fairness, you can see where this comes from: a savage machete attack, say, or a seemingly random burst of deadly gunfire on a city street are clearly not the behaviour of someone “thinking straight”. And often, once the reports start pouring in, inevitably the past or present mental health of the perpetrator becomes a talking point. After all, if there’s any record they’ve sought help in the past, it must have been something to do with that, yes?

No. As countless people have pointed out over the years, the link between mental illness and violent crimes, no matter what they are, has never been clear, no matter how many people assume it to be “obvious”. It’s a frighteningly complex issue that would be impossible to do justice to in a single blogpost, but here are just a few things to consider.

Mental Illness is a slippery subject

What is a mental illness? We’re more aware of the issue these days than we ever have been, but it’s still tricky to pin down. Unlike a broken bone or common virus, mental health issues don’t usually present with such clearly visible causes or aberrations that doctors can look at and recognise. Even the term mental illness is something many experts object to on the grounds that it is misleading, drawing parallels with physical ailments that are not accurate or helpful.

What is considered mental illness is constantly in flux. It can be largely subjective. A person talking to an invisible superbeing who created the whole universe 6000 years ago? That’s considered normal by millions. But if they said that superbeing was a giant beetroot named Rasputin, most would consider them unwell. Sometimes it just comes down to how many people agree with you.

This means there is a vast range of mental health issues that can affect a person, in many different ways. One in four people are said to experience mental health problems at some point, but 25% of the population haven’t killed someone or committed a violent crime. That would show up on a census, if nothing else. Therefore, saying someone who committed a violent crime had a mental illness is so vague as to be essentially meaningless. Maybe a violent offender does have mental health issues in their past. But it’s like saying they have brown hair. Or owned a Mazda. Or ate pork. It may be true, but it doesn’t mean it’s a factor, or a direct cause of their behaviour.

The cliche of the “mental patient” who needs to be looked up in a padded room is far from an accurate portrayal of the vast majority of people with mental health concerns, and even then it’s often their own safety which is the biggest issue, not that of others. Photograph: Alamy

No obvious link between mental health problems and violence

Obviously, that previous statement is unfair. Your hair colour or what car you drive won’t directly influence your behaviour (although … BMW owners?), whereas it’s far more likely, inevitable even, that your mental health status will.

It would be extremely naïve/optimistic to argue that nobody with mental health problems is capable of/predisposed to violence. The process of people being sectioned exists to deal with mental health patients who, due to the extreme nature of their symptoms, pose a danger to themselves or others. Whether you agree with the practice is another matter, but that’s ostensibly what it’s for.

But the idea that certain mental illnesses inevitably cause violence is far more prevalent than the evidence warrants. Numerous studies have shown that those with mental health problems are more often the victims of violence, not perpetrators. And many of these are from the USA, where violent gun crime is a very big problem.

Mental illness doesn’t occur in a vacuum. It’s tangled up with everything else in a person’s life. Upbringing, background, poverty, general health, experiences, genetics, and so on. Essentially, if you exclude the contribution of all other possible factors, the link between mental illness and violence becomes increasingly small. It’s far more common for people to be violent if they also have a substance abuse problem. You could argue this implies the mental illness is a consequence, alongside the violent behaviour, of other issues affecting the person. So blaming mental health for crimes is often like blaming the getaway car for a bank robbery; it’s just one part of the whole situation, and not even the most important.

It’s based on stigma, not evidence

The assumption that people with mental health issues are violent is deeply entrenched (cleaver-wielding “lunatic” costumes, anyone?) It often leads to circular reasoning. How often have you heard people claim that committing a violent crime is proof of mental illness? “Only a mentally ill person would kill someone, so anyone who kills someone is automatically mentally ill”. Leaving aside the vast majority of homicides which aren’t committed by people with mental problems, this isn’t evidence based.

So where does it come from? It might be due to our brain’s dislike of uncertainty (people with mental health issues don’t behave “predictably”) or general fear of the “other”, but a lot of it seems to stem from the media. Why?

A way of avoiding responsibility and blame?

Most mainstream media is owned by and aimed at white people, often but not always men, often middle class. You’re reading this in the Guardian, probably the best example of the latter.

Say a serious violent crime happens, with an identified culprit. Political or ideological reasons aside, people have an instinctive tendency to distance themselves from anyone they see as doing something bad, or being involved in bad things. It’s called the attribution bias. When we witness something bad happening to someone, our first reaction is to work out why it wouldn’t happen to us/we wouldn’t do it. It’s a psychological self-defence mechanism.

So say you’re a white middle-class English person who reads papers which contain a lot of anti-immigrant stories, making you hate immigrants, and you’re not shy about sharing your views publicly. Then, a horrible violent crime happens. The perpetrator is Muslim? Well, they’re a radical terrorist. YOU’RE not. The perpetrator is black or some other race? Well, YOU’RE not, that’s just the sort of thing they do isn’t it? The perpetrator is a white English person who hates immigrants … well, they must be mentally ill! Of course. YOU’RE not mentally ill, you wouldn’t do that. The fact that they look the same and are from the same place and share the same views is just a coincidence, it’s the mental illness which is to blame. And thus, you get to distance yourself from the crime.

For American readers, swap “middle class” for “gun enthusiast” in the previous paragraph, it still works.

A lot of the time, invoking mental health problems to explain behaviour that could otherwise be tied to more mainstream causes is just a cop-out. A constant barrage of media scaremongering about immigrants or gay people or anyone different could easily be blamed for someone committing violence against these groups. But if they’re “mentally ill” then all responsibility is absolved (apparently).

You’d think if the people/media who blamed mental illness for violent crime genuinely believed that, they’d be more alarmed about the state of mental health treatment and facilities. But no, there’s rarely a mention as mental health services are put under massive debilitating strain by cuts and ignorance. We live in a world where a pensioner can’t take a bottle of water onto a plane because they might somehow turn it into a bomb and countless rights are sacrificed in the name of security, but those who think people with mental illness are all killers-in-waiting are fine with them not getting adequate help or care.

The fact that people can come to this conclusion and find nothing amiss with it just shows how complicated mental health really is.

Dean Burnett is known for arguing against mental health stigma and sees no reason to stop at present.

The Idiot Brain by Dean Burnett (Guardian Faber, £12.99). To order a copy for £7.99, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min. p&p of £1.99.