Narcissism: Genetics or Abuse, Nature or Nurture?

Is pathological narcissism the outcome of inherited traits - or the sad result of abusive and traumatizing upbringing? Or, maybe it is the confluence of both? It is a common occurrence, after all, that, in the same family, with the same set of parents and an identical emotional environment - some siblings grow to be malignant narcissists, while others are perfectly "normal". Surely, this indicates a predisposition of some people to developing narcissism, a part of one's genetic heritage. (From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)