A case of damage done, then, as far as my teenagers are concerned?
"Pretty much," he replies. And for his, too? James has a daughter and son, Olive and Louis, 14 and 11, with his wife, journalist Clare Garner. "Yes, pretty much."
Brief answers, but usually it is Oliver James who is asking the questions, with clients he sees in his London consulting rooms on Monday, or for the rest of the week by Skype from his office at home in a Cotswolds village. Today, however, it's the UK's favourite shrink who is the one on the couch, stretching out his long, thin frame on a grey sofa in his living room, his trademark spiky black hair resting against an ill-matched pile of cushions.
"Consciously, the only reason I have written the book," he explains, "is that I became very aware of all the evidence about the Human Genome Project (HGP), which startled me."
The role of genes in mental health conditions
That evidence is that when it comes to conditions such as ADHD, bipolar disorder and schizophrenia, all of which exact a terrible toll, genes play little or no part.
"No gene has been shown to have any significant effect on any psychological traits," James insists. "That's just a fact. Now the HGP scientists haven't concluded from that that genes do not have an effect. They have said they just haven't found [the genes] yet. There have been so many studies of genes now - with mental illness there have been 115, all finding squiddly diddle."
For too many, he says, the "genes thing" has become "like a religion". If you sideline genes, when considering why youngsters suffer mental health problems, that leaves parents in the firing line.
Many of us know loving and long-suffering parents, I suggest, who devote themselves to the care of troubled offspring. Is he really saying such parents have caused those troubles?
"Definitely," he replies without hesitation. "There are a lot of things that people think are nature but which are really nurture. It's much, much more comfortable to think nature."
Is nurturing to blame?
James says he doesn't want to blame nurturers. "I have no time for blame. Insight is all. Blaming your parents gets you nowhere. It is there in the second verse of Philip Larkin's poem - 'they were f---ed up in their turn'. That is where redemption lies." But any redemption, he suggests, will be addressing the symptoms - through therapy and drugs - rather than the causes.
James made his name by taking no hostages. As a TV presenter, he famously caused Peter Mandelson to cry when he questioned him about his father in the BBC Two series The Chair. It has made him a controversial figure, even among his professional colleagues.
Even the title of his new book, though, demonstrates his habit of giving most prominence to the eye-catching parts of his latest thesis - in this case, damning the cherished notion that gene therapy could offer a cure to mental illnesses. Yet, when you dig down, James's message is more nuanced. He may rule out genes, but he is not dismissing nature altogether.
"I can see very strong evidence from personal experience that physical transmission of traits is possible." He uses the example of playing football in the garden with his son. As a child, James was good at dribbling a ball and now so is Louis, but not because his father has hot-housed him - James has had multiple sclerosis for 27 years, so giving a practical demonstration of dribbling is now beyond him.
But what exactly does he mean, then, by "physical transmission"? "My son may have inherited his dribbling skills through a physical mechanism, because he couldn't have learnt it from me. There's a black box of physical transmission there. That's all I am saying." That "black box", he says, might be to do with chemical patterns passed on from father to son through a process known as epigenetics.
"I am also not ruling out physical causes for psychological traits. There's a whole business about the long-term effects of what happens in pregnancy that will, perhaps, one day turn out to be hugely important. However, with psychosis - whether it be bipolar or schizophrenia - there is just a mass of evidence that something has gone horribly wrong in the family."
He illustrates his point again with a story from his own life. "I grew up with Teddy St Aubyn [the award-winning novelist, Edward St Aubyn]. It didn't occur to me, or my parents, that Teddy had been sexually abused by his father. The awful truth about families is that there is a lot that goes on behind closed doors that nobody knows about."
James is not breaking any confidences here. St Aubyn has written of the long-term damage done to him by his childhood, but with other cases discussed at length in the book - notably the deaths, 14 years apart, from heroin overdoses of TV presenter, Paula Yates, and her daughter Peaches Geldof - James gets closer to the line. He knew Yates well - they once worked together on a TV series, Sex With Paula - and says that his detailed account of the mother/daughter relationship is based on information "from people who knew [her] well. The main sources are impeccable."
Does he worry that so publicly picking over the details of a young woman's tragic death, less than two years after it happened, might add to the grief of her father, Bob Geldof, or her family? "I've thought about this an awful lot," he replies, suddenly vulnerable. "I'm very aware of what Bob's been through and I hope that, if he does read it, he will feel it makes sense of something that otherwise didn't make sense."
Blundering into parenting
Writing about alleged failings in other people's parenting inevitably shines a spotlight back on his own skills. "Maltreatment can be everyday," he agrees. "I went blundering into [parenting], just like everybody else. But as I have got older, I have become gradually more conscious of how awful it can be for them to be my children."
So what is life with James as a dad really like? He makes light of his grumpy moods and his MS. When I look closely, I notice his walking gait is a little stiff, but nothing more. I wonder if his illness has had any impact on the quality of his nurturing of his children.
"A minimal effect on my daughter - the only thing is the tiredness. Sometimes I'm perhaps a bit more irritable because I'm tired. It's a sadness, but I would say from their point of view, it is very insignificant. What really matters to a child is whether someone is tuned in to you. It's all about love."
At last, an optimistic note. Even if it is all down to parents, James is acknowledging, we do have the potential to get it right.
• Not in Your Genes: The Real Reasons Children Are Like Their Parents by Oliver James (Vermilion.
