The crime gene: Should bad genetics mean a shorter sentence for criminals?

If a predisposition toward aggressive behaviour is written in your genes, should you be held fully accountable for your actions if you commit a violent crime? Neuroscientists have argued that this kind of evidence doesn't stand up in court and shouldn't be used yet.

Certain genes have strong links to criminal behaviour. Studies have estimated that 5-10% of all violent crime in countries such as Finland could be attributed to people who have just two genes linked to this behaviour.

For example, a low-activity MAOA gene – known as the 'warrior gene' – is linked to a much greater likelihood of committing violent crime. MAOA is involved in regulating the brain's dopamine and serotonin levels, but exactly how it is related to violence is not clear.

Some lawyers have argued that this genetic influence even undermines the legal notion of free will. The influence of such genetic traits might be seen to lessen the responsibility of the defendant for their actions. If it was someone's genes that 'made them do it' rather than a conscious choice, are they fully accountable for it?

In many countries, defence lawyers have successfully used these arguments to shorten defendants' sentences. In Italy, one convicted murderer, who killed her sister and attempted to kill her parents, had their sentence reduced from life without parole to 20 years in prison because she had a low-activity MAOA gene.

But the state of the science to back up these claims isn't advanced enough to support their use in court, psychiatrist Paul Appelbaum of New York State Psychiatric Institute argues in a comment piece in the journal Nature Human Behaviour, with his co-author Nicholas Scurich of the University of California, Irvine.

"Associations between gene variants and criminal behaviour – which is the current state of the science – tell us nothing about mechanisms of influence or the degree of control that might nonetheless be possible," Applebaum told IBTimes UK.

"Thus, many of the inferences that jurors and judges are being asked to draw based on such information, for example, that the defendant should be punished less severely because he has a genetic variant associated with criminal behaviour, are flawed."

As such, Applebaum and Scurich predict that the use of behavioural genetics should decline in the courtroom, until the science is advanced sufficiently to give stronger evidence.

"If we ever get to the point of understanding how these genetic influences operate and how they affect behavioural control, there may be greater value to their introduction into criminal proceedings," Applebaum said. "We're not there yet."

However, the use of such evidence is not likely to decline any time soon, said Paul Catley, head of the Open University Law School. This is part of a trend in the broader use of genetics in court cases, including in determining whether a death was deliberate or accidental in shaken baby cases.

"Actually the only large scale study on this in the US, suggests it's on the increase," Catley said. "We've also found a similar trend in England and Wales in a recent study."

While evidence from behavioural genetics might still have a way to go, it can still be valuable in criminal cases, he said.

"I think that if evidence can help a court to understand the situation, it should be used. I think that before evidence is used in a court, it should satisfy some requirements for its reliability," he said. "Excluding evidence that could be reliable could well lead to injustice."