I just don't buy into it. I believe very much in cause and effect, and I think there is always a reason why people are the way they are, and, like Attila, I think the biggest reason is the kind of parenting they had, and the family dynamics in general.
This isn't always obvious on the surface - parents might appear to be loving and caring to the outside world, they might think of themselves as being so, but there is always (in these cases) some darkness somewhere, some warp in the programme that governs the family dynamics. It is virtually always unconscious, the result of something from one or both parents' own upbringing which they have never challenged, often because it is well hidden and also because to do so is unspeakably painful.
To me, the evidence is in the outcome. If there is an (adult) child who is dysfunctional in some serious way, that is the proof that something was amiss with the parenting. That to me is the prompt for a parent to investigate their own family dynamics in depth and see where this dysfunction comes from. But that is the point at which most parents, IME, choose to blame the (adult) child instead, because it's an easier option.
There is a very deep vein of child-blaming that runs through the heart of our culture, IMO, and we are all susceptible to it; those of us whose lives have been significantly impacted by this tendency are inevitably the ones who have most motivation to challenge it and perhaps have to dig around in the dark, unpleasant truths that some others can conveniently hide from.
(As an aside: Recent scientific studies that I have heard of seem to show not only that environment plays a much bigger role than genetics in the development of personality, but that the kind of mothering you receive actually affects the physical development if the brain.)
Sorry OP I know this is turning into a bit of a thread derail. But I think one thing there is almost a consensus on is that parents who really loved their child would not have let things go for 10 whole fucking years and voluntarily missed out on so much of their DC's and DGC's lives.
And I think that's the main point for me. Why would any child who felt really, deeply, truly and unequivocally loved by their parent want to destroy that relationship, deprive themselves of that irreplaceable, deep nurturing? And even if somehow they did, wouldn't the parent who really, deeply, truly and unequivocally loved their child do anything and everything to restore it to them? Wouldn't that be the single most important thing in your life, bar none?
Because at the end if the day it's not really about good parenting as some kind of task that must be performed to a certain standard; it's just about love and devotion. If you are genuinely devoted to your child, I think they know, however much you may fuck up (and of course we all fuck up as parents, each and every one of us). I do find it really hard to believe that when the love is that strong, it will not find a way, or at least keep on trying and trying. And if it isn't that strong... It's not the child's fault.
I know mine isn't a widely held or very popular viewpoint. But there we go.