I think temperament is an inborn thing. Some are born inherently nasty, whilst others have a naturally gentle disposition, for example. Just like if you exposed 10 different children to the same upbringing - good or bad - they would all react to the environment differently and find coping mechanisms based on their own temperamental tendencies. Hence, how you get siblings that are all so profoundly different from one another! No doubt external factors can alter someones behavior, and make them act differently to how they instinctively want to, based on previous experiences. But ultimately I believe people are what they are.