He’s associated damage to the temporal lobe with psychopathic killers. The epigenetic effects, brain damage, and environments appears to be an MAOA variant on the X chromosome with experiencing violence around 3 years old.
Males only get the X from their mother. Men are much more likely. Girls get one X from mother and one from father which dilutes. Bathing the brain in serotonin too early makes the brain insensitive to the calming serotonin later.
TED Jim Fallon: Exploring the mind of a killer