Marvin Minsky believes that our brain works in several levels:
Reflective Brain
Deliberative Brain
Reactive Brain
External World

As the external world changes, the reactive brain might create an instant instinctive reaction. Negative reactions out of fear, for example, almost always "light up" the amygdala portion of the brain. But the amydgala also is known to shrink in human males after they are castrated. So this part of the brain is both involved in reactive and agressive forms of action.