The entry for monoamine oxidase a in the OMIM database seems to indicate a strong link between variation in this X-linked gene and violent behavior, and the recent news that a New Zealand study links a deficient form of the MOA-A gene to an increased risk of adult antisocial behavior in maltreated boys seems in accord with this information. Of course, one must be careful that this isn't a case of researchers finding precisely what they had set out expecting to find.