Murders are tragic but rare. But what drives some people to kill? Michael Mosley has been looking into research exploring the minds of murderers.

In the 1870s Dr Cesare Lombroso, sometimes called the father of scientific criminology, was studying criminals imprisoned in Turin.

He became convinced that criminals are a step back down the evolutionary ladder, a reversion to a primitive or subhuman type of man. He decided, after years of study, that you could tell a criminal by the shape of their face and the excessive length of their ape-like arms.

"A criminal's ears," he wrote, "are often of a large size. The nose is frequently upturned or of a flattened character in thieves. In murderers it is often aquiline like the beak of a bird of prey."

Find out more The Mystery of Murder: A Horizon Guide, presented by Michael Mosley, is broadcast on BBC Four at 21:00 GMT on 9 March 2015 Or catch up on BBC iPlayer

Sadly, spotting potential murderers turned out to be nothing like as simple as Dr Lombroso claimed and his "scientific" findings were soon discredited. But this was the beginning of a search that has continued for more than a century - to find out if criminals, and in particular murderers, have different brains to the rest of us.

The invention of functional brain scanning in the 1980s revolutionised the understanding of what goes on inside our heads. The first scanning study of murderers was carried out in California by British neuroscientist Prof Adrian Raine. He was attracted to the Golden State not by the beaches but by, as he put it, "the large numbers of very violent and homicidal individuals".

Image copyright ALAMY Image caption Skull studied by Dr Cesare Lombroso

Over the course of many years Raine and his team scanned the brains of numerous murderers and nearly all showed similar brain changes. There was reduced activity in the pre-frontal cortex, the area of the brain which controls emotional impulses, and over activation of the amygdala, the area which generates our emotions.

So it seems that murderers have brains that make them more prone to rage and anger, while at the same time making them less able to control themselves.

But why does this happen?

Image copyright Science Photo Library Image caption The amygdala is the area of the brain that generates our emotions

Raine's studies suggest that part of the reason may be childhood abuse, which can create killers by causing physical damage to the brain. The pre-frontal cortex is especially vulnerable

One of the prisoners that Raine scanned was Donta Page, a man who brutally murdered a 24-year-old woman when she caught him breaking into her home. As a baby Page was frequently shaken by his mother, and as he got older the abuse got worse. His mother would use electrical extension cords, shoes, whatever was handy. These were not once a year beatings, they were beatings that occurred almost daily

"Early physical abuse, amongst other things could have led to the brain damage, which could have led to him committing this violent act," Raine says.

But only a small proportion of those who have a terrible childhood grow up to become murderers. Could there be factors that predispose us to murder?

Image copyright Other Image caption Donta Page was physically abused as a child

A breakthrough came in 1993 with a family in the Netherlands where all the men had a history of violence. Fifteen years of painstaking research revealed that they all lacked the same gene.

This gene produces an enzyme called MAOA, which regulates the levels of neurotransmitters involved in impulse control. It turns out that if you lack the MAOA gene or have the low-activity variant you are predisposed to violence. This variant became known as the warrior gene.

About 30% of men have this so-called warrior gene, but whether the gene is triggered or not depends crucially on what happens to you in childhood.

Jim Fallon, professor of psychiatry at the University of California, has a particularly personal interest in this research. After discovering a surprisingly large number of murderers in his family tree he had himself genetically tested and discovered he had an awful lot of genes that have been linked to violent psychopathic behaviour.

As he puts it: "People with far less dangerous genetics become killers and are psychopaths than what I have. I have almost all of them"

But Jim isn't a murderer - he's a respected professor.

His explanation is that he was protected from a potentially violent legacy by a happy childhood. "If you've the high-risk form of the gene and you were abused early on in life, your chances of a life of crime are much higher. If you have the high-risk gene but you weren't abused, then there really wasn't much risk. So just a gene by itself, the variant doesn't really dramatically affect behaviour, but under certain environmental conditions there is a big difference".

Image caption Prof Jim Fallon has genes linked with violence but is not violent

So it seems that a genetic tendency towards violence, together with an abusive childhood, are literally a killer combination - murderers are both born and made.

We now have a far more sophisticated understanding of the complex interactions between the social and the biological factors that predispose people to violence. But what can we do with that knowledge?

Research is focusing on ways to reduce violent behaviour and there is good evidence that teaching families who are at risk positive parenting skills is effective at improving impulse control.

The hope is that now we know so much more about the causes of murderous behaviour we can spot the early warning signs and intervene before it's too late.

Cesare Lombroso: Father of scientific criminology

19th Century, Italian physician Cesare Lombroso studied the skulls of criminals and said he could identify them by their looks

Biological factors were more important than environmental explanations for criminality

Believed he had found evidence that a criminals brain was different to that of a non criminal

Physical structure and psychological characteristics of his "criminal types" had "atavistic" or "primitive" traits

Criminal types shared facial features, such as large canine teeth, long, sloping foreheads, large ears, chin abnormalities

Work on criminality influenced developments throughout Europe and the US, but has largely been discredited

The Mystery of Murder: A Horizon Guide is on BBC Four at 21:00 GMT, 9 March 2015 - or catch up on BBC iPlayer

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