One question that has been floating around this topic is whether the motivation to be aggressive is internal or external.    If internal, why would we have such a motivation which ultimately often leads to the harm and possibly to the destruction of members of our species?   Aggression may be separated into two categories: hostile and instrumental.  A hostile aggression example may be murder, while one for instrumental aggression may be a robbery.
     Hostile aggression is often the result of something painful, uncomfortable, or frustrating.  The results generally do not benefit the aggressor, but he/she may take out their anger on something completely unrelated at the slightest provocation.  Instrumental aggression occurs when the aggressor acts aggressively in order to benefit somehow.
     We know that aggression can be caused by the environment, such as when a child watches violent t.v. shows.  In addition, it is possible that testosterone affects aggression.   Strong correlation has been shown between high levels of testosterone and aggressive behavior; however, these studies are inconclusive insofar as they do not explain whether the environment causes an increase in testosterone and aggression or if an increase in testosterone causes an increase in aggression.