Previous research showed that low levels of an enzyme called monoamine oxidase A is involved in aggression. If MAO sounds familiar, it may be because of MAOI, monoamine oxidase inhibitors. MAOs catalyze the uptake of serotonin, and inhibiting them can be used to fight depression. MAOIs are also used to treat anxiety, migraines and even Parkinson's. MAOs are naturally occurring enyzmes, and mice (and men) who lack them respond violently to stress. This is especially true when the lack of MAO is combined with some type of traumatic event or neglect in early life.