In intriguing research conducted at the University of California and other locations, it appears that psychopathic killers often have lower intelligence, which can be the result of brain damage; often from severe chronic maltreatment as a child. Three factors appear to be present among violent offenders:
1. Several "violence" genes.
2. Damage to certain areas of the brain
3. Exposure to extreme trauma and poor parental bonding in childhood.
Among genetic markers related to aggression and mood is MAOA. The high risk variant of this gene gets inherited more by males than females. MAOA regulates serotonin, which affects mood (which is why medications like Prozac affect mood. These medications are called SSRIs: Selective Serotonin Reuptake Inhibitors. They function to keep more serotonin in the synapses between brain cells.). In the womb the high-risk version of MAOA can lead to a buildup of serotonin in the brain making the brain less sensitive to the normally calming effects of serotonin. Other research indicates that people who inherit the high-risk gene and who are raised in abusive homes may be more prone to violent behavior.
In another study pbulished in Comprehensive Psychiatry early in 2009, it was found that boys who inherit a mutated variant of MAOA are more likely to be in a gang than those without the mutation and are more likely than those without the mutation to be the most violent gang members.