Scientists have discovered a gene that sparks off an enzyme considered responsible for a person's ability to control his or her emotions: the monoamine oxydase A (MAOA) enzyme. Scientists also found that those genes which had a lesser content of MAOA were more consistently found in people who were unable to control their emotions when exposed to facial expressions depicting emotions or while recalling emotional experiences. The individuals who otherwise displayed very little brain activity in areas related to impulse-control, displayed intense brain activity in emotion-related brain structures. Those individuals reacted less emotionally and with more control over their impulses when exposed to the same stimuli who had a strong MAOA gene. This led to the conclusion that a weak MAOA gene is what contributes to uncontrollable violent behaviour.

Source : Spider