produces protein breaks down neurotransmitters 

 1) MAOA-L, 2) MAOA-H 

 other gene taking part with the aggressive behaviour maybe not just the MAOA.

 If MZ both showing aggression, we can conclude that their aggression is probably genetic 

  If the child is aggressive and their biological parents were aggressive, maybe it is genes causing this aggression 

 adoptees who had criminal convictions, also had biological parents that had the same convictions 

 women are less affected because they can prevent the abnormal variation 

 found that there may be other genes contributing, not just MAOA e.g. MAOA-L+CDH13

 it is both the nature and nurture causing this aggression - cannot say that it is just genes