It is true in most common psychiatric disorders and addictions. Dementia, bipolar, PND and Schizophrenia seem to have unrefutable research that there are genes that help to influence your liabilty to suffer with them.
I think environment goes some way but just in my family for example, genes have to some sort of influence. I have two schizophrenic cousins and yet their other brother is fine.
My nan is an absolute nutjob but that's definitely through the environmental factor of her paedo father but then she raised 6 children, really, really badly and half are alcoholics, and 2 of those have othe psychiatric disorders. My aunt definitely suffers with psychosis and my uncle is bipolar and yet my dad and two of his brothers are just fine. All joking aside, I have terrible problems with addiction issues and I am almost certain I have some sort of borderline personailty disorder and all of my family on both sides go senile. I have already asked my brother to smother me when I start accusing everyone of stealing my things because hey all seem to get a bit nasty with it.