This lack of contact with others, or isolation from others, is an important factor here, because lack of contact or isolation results in ignorance about the other groups. This ignorance can then lead to this ‘stereotyping’ you mention. This can also lead to more suspicion of the other side. And this suspicion can easily become a kind of fear. And when people act out of fear, aggression is one potential response. In fact, it is one of the most common responses. So you see, out of fear, out of mistrust, then their behavior sometimes becomes more aggressive, of a more violent type.
