A single gene has been linked with being a psychopath - and it's very controversial

As of yet, no single factor can explain what causes people to behave in ways labeled psychopathic. But research suggests our genes may play a role.

One gene in particular is linked with an increased risk of violent or aggressive behavior, studies have found.

Known as MAOA (monoamine oxidase A), this "warrior gene" controls the production of a protein that breaks down brain-signaling chemicals like dopamine, noradrenalin, and serotonin, which all influence mood.

But the idea of a "psychopath" gene remains controversial.

A gene for psychopathy?

People with a variant of the gene, called MAOA-L, produce less of the protein that breaks down these signaling chemicals, which in turn causes them to build up. An excess of these chemicals, scientists believe, leads to impulsive behavior (such as as hypersexuality), sleep disorders, mood swings, and violent tendencies.

James Fallon, a neuroscientist at the UC Irvine School of Medicine, was studying the brains of murderers and other violent criminals who are thought to be psychopaths, when he accidentally found out he might be one.

Not only did his brain scans look suspiciously like those of the murderers, he later found out that he also possesses the MAOA-L gene that's been linked to aggression and antisocial behavior.

Psychopaths are often described as having a lack of empathy. They may understand when others are in need or in pain, but they don't feel it viscerally the way most people do. Fallon can identify with that. "I don't get the interpersonal warm and fuzzies," he told Business Insider in an interview earlier this month.

But simply having the MAOA-L gene isn't enough to make someone a psychopath. The environment someone is exposed to is likely very important too.

Fallon thinks that the reason he turned out to be a realitvely normal, nonviolent person, despite having this gene, is that he had a good upbringing. But growing up in a harsher environment could have tipped him toward a darker path. "If [the brain] sees a hostile world, the only way to survive is to be hostile," Fallon said, whereas "if it sees a normal world, it will be normal."

And some research supports this idea. One 2002 study followed a large group of male children from birth to adulthood who were abused or maltreated. The study found that boys who had a particular version of the MAOA gene grew up to be more antisocial, compared with those who had a different version of the gene.

And this kind of genetic evidence is starting to make its way into the justice system. In 2009, an Italian appeals court shortened the sentence of a convicted murderer by a year because he had the version of the MAOA gene associated with violence.

Not that simple

But experts have warned against drawing conclusions about a person's character based on this gene alone.

"We don't know how the whole genome functions and the [possible] protective effects of other genes," Giuseppe Novelli, a forensic scientist and geneticist at the University Tor Vergata in Rome, told Nature News.

What's more, the same gene could have different effects in people of different ethnicities. A 2006 study in the United States found that abused children who had high levels of the MAOA that breaks down brain signals were less likely to commit violent crimes, but only if they were white.

So while it's tempting to reduce a complex psychiatric disorder like psychopathy to a single gene, it's almost certainly not that simple.