Neurologist Dr Jim Fallon has spent three and a half decades studying behaviour, and more specifically psychopathic killers.
A professor at University of California Irvine, Fallon gave a TED talk on the mind of killers.
Psychopathic killers are distinct from other killers in that they can act without motive.
Avoiding producing psychopaths is part of Fallon's area of study, by determining what factors can produce one.
Ideas such as earlobe length or skull size determining psychopathy have passed out of most medical circles, yet Fallon does focus on physical factors.
These include a person's genetics, epigenetics (things happening externally to genes), damage to their brain, and also environment factors.
Fallon studied 70 brains and found these factors were common to the psychopathic killers among them:
1. Brain damage
All of the psychopaths that Fallon studied has damage to their orbitofrontal cortex (above the eyes) and to the interior part of the temporal cortex. In addition, all of them had some other of brain damage in a variety of combinations. What kind of psychopathic killer is created from this melting pot of factors is determined by exactly when damage to the brain occurs.
2. Usually male
Possession of the high risk violence gene, the MAOA is also common to psychopathic killers. The gene MOAO is sex linked, and exists in the X chromosome. As such it comes from mothers, and Fallon believes this is why most psychopaths are boys because the X chromosome comes solely from their mother, whereas daughters have one from their father and one from their mother.
3. Too much serotonin
According to Fallon people who have the MAOA gene in utero, are overexposed to serotonin. The efficacy of this neurotransmitter which usually calms the brain, is neutered by the overexposure. The fetus develops an immunity to it. As such, the control of anger and stress relief is impotent in later life.
4. Witnessing violence
'The deciding factor is exposure to extreme violence at a young age. The three other factors can exist without leading to murder, but the trigger is witnessing or becoming involved in a traumitising incident.
Fallon explains that in order for this gene to be expressed, the potential psychopath must witness or be involved in an an extremely violent event prior to puberty.
Fallon believes that war zones produce generations of children exposed to violence, and that the violent genes are concentrated as more aggressive or violent people who are more likely to survive, reproduce with one another.
See the full video from Dr Fallon's TED talk below, and also learn about own genetic history of psychopathic killers.