The MAO-A gene, also known as the "Warrior gene", is thought to cause increased antisocial and violent behavior patterns in people who have experienced an abusive childhood. [ref] Caspi et. al. found similar results.
I also remember an episode of BBC Horizon (48x05) that studied psychopathy, which touched on the MAO-A gene and its links to anti-social behaviour, psychopathy and murder.
In popular media (e.g. the character Dr Raymond Langston in CSI: Crime Scene Investigation[ref]) people with a mutation in this gene have been depicted as short-tempered and innately violent.
The results seem pretty solid around violent behaviour - a combination of a poor childhood, especially an abusive one, and an abnormal MAO-A gene can lead to violent behaviour and psychopathy.
I recently saw someone present a quote on the subject:
An abnormal MAO-A gene has been shown to be prevalent in serial-killers, mass murderers, paedophiles, sexual sadists and pathological sex offenders.
Sadly, I can't find the source of the quote, nor has the person who posted it got back to me with a source. I can't find any studies or information that study the correlation between an abnormal MAO-A gene and any of those groups.
Can anyone provide links to studies that prove, disprove or even just study such a correlation?