http://somethingincredible.blogspot.com/2007/01/aggressive-personalities-or-magic-of.html
In response to these studies, a new physiological model was made, to distinguish between controlled and impulsive aggression. This model drew upon a body of work showing that low serotonin levels led to general impulsive behavior. Researchers surmised that people who were impulsively aggressive had low serotonin levels. People who exhibited dominance behavior, on the other hand, were driven by testosterone. High serotonin levels allowed them to control their impulses and to use their aggression to establish dominance. A study in chimpanzees found that unprovoked assaults and biting was associated not with high testosterone but low serotonin, and that dominance behaviors such as mounting of other males were associated with high testosterone and high serotonin levels.