Well it is our $64,000 question and here is as good an explanation as I have heard. Laurence Miller, Ph.D., author of ‘Criminal Psychology – Nature, Nurture, Culture’ explaining the biological, sociological, and psychological reasons for violent behaviour.
This book just went on my to read list for certain. The phrase “Active Gene Environment Correlation” is a very good way of describing how like finds like.
Really interesting as he includes so many variable and influences rather than just concentrating on eg brain chemistry.