For years, researchers have tried to understand what drives us to commit acts of violence.

Is it the environment we were raised in?

The socio-economic situation we find ourselves in?

Or is it driven by something completely different—something we have no control of—our genes?

Or specifically, the so-called 'warrior gene'?

The story of the warrior gene starts in 1978, when a woman walked into a hospital in the Netherlands, and not because she was worried about herself, but because she was worried about her family.

All the men in the family had a history of really terrible violent crimes.

There were murders, there were rapes, going back generations and generations.

Researchers started gathering DNA samples from the members of her family, and they discovered that all the men who had this history of very violent crime,

they had the same genetic change in a gene called MAOA, or monoamine oxidase A, that completely knocked out the function of the gene. It wasn't working at all.

We all have two copies of this 'warrior gene'.

It makes a molecule in your brain that breaks down a chemical called serotonin.

This is a neurotransmitter, it sends signals between the cells in your brain, and affects your behavior.

So if this gene isn't working as well as it could do, it's thought that perhaps it makes you more impulsive, it makes it harder to control urges.

So the Dutch study was the first time that this gene, MAOA, had been linked to violent behavior, but then a study in New Zealand took this further.

The Dunedin study followed 1,000 New Zealanders for decades, looking for connections between childhood experiences and violent behavior later in life.

They found that having a variation that just lowers the level of activity of this gene, not knocks it out completely, but just lowers the activity, was associated with a history of violence.

But curiously, mostly only in people who also had a very difficult childhood as well.

And that caught the eye of researchers in Finland working with prisoners in a very high security prison.

We collected a cohort of around 800 prisoners, and about two-thirds of them had conducted at least one violent crime.

But a little bit more than 10% or 15% had committed at least 10 violent crimes, murders, attempted murders, manslaughters.

I was skeptical, but what we did find was indeed if an individual had two or more violent crimes, then he or she had significantly more of the low activity MAOA.

We know that up to six in ten people are walking around with a low activity version of MAOA, but of course, six out of ten people don't commit horrific violent crimes.

So obviously we can't say that if you've got this gene variation you are going to be a violent criminal. Genetics doesn't really work like that.

But this idea that there's a gene behind violent behavior, could you use that as a defense in court?

You know, "It wasn't me, guv. It was my genes that made me do it."

While it might sound a bit strange, but that is actually what's happening.

It's only really been used in two countries, two in an Italian court and the rest have been in American courts.

It's been used as a way of saying that the person lacked control.

I think it's the sort of evidence that is attempting to be used to dazzle a jury.

In the US, where you have the death penalty for these kinds of incredibly violent crimes, the stakes are obviously, literally, life and death.

So lawyers will try and introduce any kind of evidence they can to argue against the death penalty for their client.

To say that there is a gene, like a time bomb that none of us are aware of, I think is wishful thinking.

So humans carry different versions of the MAOA gene, and those that reduce the activity of this gene have been linked to increased risk of aggressive behavior.

Some studies also suggest a link between an abusive childhood, the MAOA gene not functioning adequately, and an increased risk of developing antisocial personality disorder, which may result in committing violent criminal acts.

But whilst it can be tempting to paint a simple picture of how genetics influence our behavior, in reality, it's an incredibly complex issue.

Everything that happens to us interacts with our genes and influences who we are and how we behave.

The fact that we can find a connection between genes and violent behavior doesn't mean that we're going to be heading towards some terrible genetic dystopia where little Johnny is locked up because he's got 'bad genes' that might make him do bad things.

That simply isn't how genetics work.

We have agency, we have independence, we can make our own free choices about who we are and how we behave.