I will attempt to demonstrate why many scientists believe that there is a genetic variable in individuals such as these.
Many scientists believe that a mutation exists on a certain gene that causes aggressive behavior.
At the University Hospital in Nijmegen, the aforementioned Dutch family was examined. This seemed significant because the enzyme encoded by this particular gene helped to break down several neurotransmitter substances that, if built up, could cause a person to act excessively, if not violently. Another study conducted in Finland analyzed men who acted on impulse and were unable to control their behavior. Mutant mice and wild type mice were separately placed into a situation with an intruder and the mutant mice attacked the intruder much quicker and at a higher incidence than the wild type mice. In conclusion, many scientists postulate that there may be a genetic foundation as to how violent people behave.