The sky hangs over London like a dark, oppressive blanket. It has been drizzling all morning and is about to break into a downpour. I am in Highbury Fields looking for the flat of the clinical psychologist and writer Dorothy Rowe, renowned author of the worldwide bestseller Depression: The Way Out of Your Prison, and 11 other books including the latest, My Dearest Enemy, My Dangerous Friend: Making and Breaking Sibling Bonds, a definitive exploration of how siblings can both wreck or enrich one another's lives depending on how the cookie crumbles. I am lost and late. 'It's the ugliest, most uncared for building on the block,' Rowe says bluntly when I call for guidance. I find it immediately.

It's in this same matter-of-fact tone that Rowe, in her new book, tells of the unhappy first 40 years of her own life, disclosing for the first time candid details of a tyrannical childhood and marriage in her native Australia. Her mother was a depressive. 'She let me know I was fat, ugly and lazy and went into a crazy rage and would beat me and declare that she was going to kill me and then herself ,' she writes. Her elder sister too, 'would look at me curiously and comment with the fascinated disgust of someone who had inadvertently kicked over a stone and found a particularly strange and loathsome slug hiding there'. Then there was a passive father, and, later, a cheating, lying husband who made Crocodile Dundee look like a fairy.

Rowe opens the door to her garden flat and ushers me in from the rain. She is small with an elfin face, wire-rimmed spectacles and dark, unruly brows under shorn, silver hair. Rowe once observed that 'bitterness is so bad for the complexion' and she seems to have heeded her own advice. In well-cut jeans, loafers and a cornflower-blue shirt, she looks considerably younger than her 76 years. She is clearly not bitter if her books are anything to go by. There is, however, a poster in the porch saying 'A Woman Needs A Man ... Like a Fish Needs a Bicycle', but who can blame her for that?

Her husband cheated on her not long after their son was born, fathered a lovechild and then suggested that Rowe become his mistress instead. Three years later, Rowe and her 10-year-old son packed up and left for Britain, the other side of the world, a drastic measure, but one that was to be her saving.

She leads me into the sitting room, simultaneously grandmotherly (which incidentally she isn't) and modern. Plush fitted carpets and hulking brown furniture vie with stark white walls hung with abstract paintings. In the fireplace there is a collection of framed photographs, none of her mother or sister, but plenty of her son Edward, now 49 and in Australia, including a particularly touching one of them together when he was a toddler in which Rowe is bending down to talk to him, a young woman in her twenties, wearing a turtle neck and pencil skirt. Her hair is thick and white blonde and combed into an elegant chignon. The photograph must have been taken roughly five years before she found out about her husband's betrayal - one of the two periods of her life, which she calls the unhappiest, and the closest she came to experiencing depression herself. 'I couldn't have been properly depressed,' she tells me now, in her matter-of-fact Australian accent, 'because I was studying for a diploma, holding down a job and bringing up my son. Depressed people don't do that. They don't do anything.' She goes on, 'depression, once a state of mind of which to be ashamed, is less stigmatised, but it is discussed endlessly and described as a vast epidemic sweeping the planet - "depressed" and "depression" are applied to any dysphoric feeling such as unhappiness, disappointment, lack of enthusiasm, even irritability.'

Rowe's view that the diagnosis of depression, or rather the doling out of drugs to treat depression, has become far too prevalent, is particularly pertinent right now. (When I mention a depressed friend who grew up with a depressed mother and was recently prescribed Prozac herself, Rowe shakes her head and says, 'Dear oh dear ... Prozac ... awful, terrible.')

The latest figures show that, in the past 10 years, Britain really has become a Prozac nation. The consumption of antidepressants is at a record high - 31 million prescriptions were written out by GPs last year, a six per cent rise, which many ascribe to patient demand for the drugs, rather than diagnosis of psychological need. As if to prove the point, there was a 10 per cent rise in the use of Seroxat and Prozac, the most popular and well-known of the selective serotonin reuptake inhibitors (SSRIs), despite the fact that three years ago the National Institute for Health and Clinical Excellence (Nice) recommended that they should not be used as a first-stop remedy.

Depression is estimated to affect as many as one in five people at some point during their lives. At any given moment, there are 1.5 million sufferers between 16 and 75, hence the prescriptions. But in a recent report issued by Mind, the leading mental-health charity, current advice is much more in favour of counselling, therapy and exercise.

Dorothy Rowe was sceptical about the prolonged, isolated chemical treatment of depression as long as 20 years ago (she concedes drugs can be useful in the very short-term). Depression: The Way Out of Your Prison was first published in 1983, long before the emergence of Prozac and all these scary statistics. Her stance, then as now, is broadly this: depression is a prison which we build for ourselves as a defence mechanism to survive. She calls it 'a period of unhappy withdrawal, an uncomfortable hibernation where the person comes to realise that something has gone wrong with his life and that something needs to be put right'. The 'putting-right' part is the tricky bit because, ultimately, human beings are terrified of change. It's not that the depressed person wants to stay in bed all day with a pillow over his head, it's more that the idea of confronting the alternative, and what it might reveal about their previously held ideas of themselves and others is so supremely terrifying.

In layman's terms, the pattern translates roughly like this: something dreadful happens to you - it doesn't matter how big or small the world measures it to be, the key is that to you it seems vast and unmanageable - and then your body goes into a kind of emotional and physical paralysis in order to survive. The ensuing, deep unhappiness becomes a prison, infinitely more attractive because of its familiarity, than the painful process of change and reassessment of life needed to get yourself out of it. The backbone of the theory is that, as children, we are brought up to believe in a just world, the pursuit of being 'good', so that only we are responsible if things go wrong. So, if your mother hated you, your father abused you, your boss took away your job and then you lost your house and your wife, it must be your fault. 'The biggest cause of depression is to turn natural sadness into depression,' Rowe tells me. 'All you have to do is blame yourself for the disaster that has befallen you. Only good people get depressed - good people are those people who feel that they are never good enough, they are experts in blaming themselves and feeling guilty. Ninety-nine per cent of suffering isn't caused by natural disasters, it's caused by the ideas we hold and if we believe these ideas are absolute truths then we suffer and we force other people to suffer. But if we understand that our ideas are created by us, then we know we are free to change them.'

The rise in depression today, she says, is down to many things. Among them is the fact that more people feel able to talk about it, whereas in the past women were 'depressives' and men were simply 'alcoholics'. Children get it; it's often a misused label for anxiety; and the standards by which we measure our lives now - our so-called 'life project' - are largely to do with wealth, tangible success and even a degree of celebrity: 'In the past, the "project" was to get to heaven,' Rowe explains, 'and at least you were singing hymns for eternity. Now even what we regard to be success is transitory.'

Her friend and former colleague, consultant clinical psychologist Craig Newnes, says, 'There is nothing remarkable about Dorothy's approach. She is kind, attentive and absolutely believes in the power of people to change. Depression is of course a scam - when we were training together in the Seventies virtually no one was diagnosed with depression but then the drug companies realised they needed a new market and claimed distress was caused by biochemistry. For as long as I have known her, Dorothy has stood up for a conversational and caring approach to helping people.'

Rowe doesn't seek to belittle personal tragedy, far from it. Heaven knows, she has experienced a bellyload of it herself. But what she advocates is trying to interpret it in a different way without saying that it doesn't matter. In the current climate, where pharmaceuticals reign supreme, her ideas, almost 25 years old now, continue to feel fresh. Certainly her following has not diminished, hardly surprising since it comes as a relief to believe that, in most cases, depression is not 'chemical'; not a result of us all being lifelong 'victims' in denial or in 'recovery' from something, a view peddled largely by the 'You-can-do-it!' American self-help culture, which often fails to recognise tangible problems caused by society - such as poverty or abuse; that what really matters to recovery is not the event itself but how we interpret it.

The writer Tim Lott discovered Rowe's work while he was working on his first book about his mother's suicide and his own nervous breakdown, cured by antidepressant drugs. He was sceptical at first, even a little angered by what he considered to be Rowe's unhelpful stance, but after reading her books he became one of her millions of followers throughout the world: 'She changed my life, as she has changed many others,' he says. 'The idea that she is simply a self-help writer is deeply inadequate and misguided. She is more a practical philosopher than anything else.'

Linda Grant, another writer who has known Rowe for around 15 years, agrees: 'At one point in the mid-Nineties, you couldn't go to a party without meeting at least two people whose lives had been changed by Dorothy. She is read by deeply intelligent people, mostly because she has uncomfortable truths for all of us. The reason why she isn't even more well-known than she is, is because her books are considered in a genre of "self-help", but in fact she is telling us much harder, philosophical truths about the world in which we function. She is trying to portray a world full of darkness, and is constantly bombarding us with difficult and complicated ideas. This is not the realm of the boosterish quick fix. It is more a philosophy of life.'

So what exactly does she do? One of Rowe's followers, a woman who suffers from depression, emailed me the following: 'What Dorothy writes feels like the truth, due to the accurate case histories, together with her uncanny ability to view human frailties from the reader's perspective. Through her eyes, you come to know yourself, or if you are lucky enough not to have suffered from depression, you are given a very clear projection of all the potential aspects of this complex condition. In Dorothy's view, the best person to know your pain is yourself, and you can therefore be in charge of your way out of the worst too. For the reader, it means that you have to come to terms with being your own instrument of change; not relying on others to pull you out of the mire. Therefore, you cannot remain totally closed off in the prison of depression if you have taken the decision to try and allow yourself to take some part, however small, in the world again.'

In 2004, Saga Magazine published 'The Wise List', the people in the UK whom the magazine stated had 'that laudable quality of wisdom'. Fifty people were on the list, and of those, the readers were asked to pick the six wisest of all. Rowe was one of those six, alongside fertility expert Lord Winston, chief rabbi Dr Jonathan Sacks, and judge Dame Elizabeth Butler-Sloss. OK, I say to her, I consider myself happy, but how can I ensure I remain so? How can I ensure I bring up happy, well-adjusted children, who are also loving siblings? 'A book about bringing up children is the one book I will never write,' she explains, despite the fact that childhood seems key to the whole thing. (Childhood does, in fact, feature heavily in the recent sibling book, more of which later.) 'I will not add to the argument of a "right" way and a "wrong" way. All I can say is that children learn by example. If they are treated with dignity and respect and see their parents being loving and generous to one another, that is the best you can hope for them.' She pauses, 'I cannot tell people how exactly to go about being happy, but I can certainly tell them how to stop themselves from being happy. Happiness is not something you can "achieve". It's a by-product of what you do. What you do if you want to be happy is value and accept yourself. I became an expert in my own happiness. I had to deal with happiness and unhappiness in my personal life and my professional life as a clinical psychologist and the turning point for me finally came when the intellectual knowledge I had as a psychologist - that I actually could eradicate my self-doubt and instead value myself - became knowledge of the heart and not just the head.'

Dorothy Rowe's unhappiness started at birth, in 1930 in Newcastle, Australia, when her depressed mother gave birth to her, three years after aborting a boy, and then never forgave her for being alive. In My Dearest Enemy, My Dangerous Friend, she writes: 'Once, on a rare occasion when there were visitors to dinner, my father said something about how my mother had had breast abscesses after I was born. Mother gave him one of her fierce black looks and Dad shut up like a man shot dead ... I do not doubt that she blamed me for those breast abscesses.'

Her six-year-old sister was sent away for the birth and, coming back to find Dorothy in the crib, she too spent the rest of her life punishing her for existing. This book was clearly born out of Rowe's destructive relationship with her sister.

There are many parts of the book that might easily have been exploited as the latest lucrative misery memoir. As it is, she dissects and explores the complicated dynamic of the sibling relationship, whether it be love, hate, resentment, lifelong competition to be 'the goodest' or, for the lucky ones, enduring or new-found solidarity.

For years she resisted publishers' requests to write the book. But only when her sister was elderly did the time feel right. (She admits even now to feeling jangled after the most innocuous telephone conversation with her sister.) It is hardly surprising since the most dominant feature of the first half of Rowe's life seems to be betrayal. In one painful paragraph, Rowe sums up it without a trace of sentimentality: 'There can be painful events in our childhood from which we can recover and grow stronger, but betrayal by someone who should care for us and whom we trust is not one of them. We may be able to work out a way to live with the memory of the betrayal without denying it or becoming crippled by self-hatred, but we lose forever our innocent joy of being alive.'

Rowe provides many anecdotes relating to her mother and sister's betrayal, but one in particular makes the heart lurch. The first is of the adult Dorothy hearing her sister tell a story, during lunch at Fortnums, of a family member trying to train her pet. Rowe writes of what her sister told her: 'She said, "The kitten piddled on the floor and she picked it up and rubbed its nose in it. I suddenly remembered when you were a baby. You were propped up in your high chair and you wet and it went all over the seat and dripped on the floor. Mother rubbed your nose in it.' Dorothy adds, 'My sister told this story in the "fancy-that" tone of voice much used in my family to drain the significance out of any unusual event ... She was apparently unaware of the effect her story had on me.'

Her mother is long dead and her sister hasn't read the sibling book: 'My sister keeps telling me she doesn't remember things now. I've tried to talk to her but she would just change the subject. She can not bear to discuss anything in case I say something which clashes with her view of how things were.'

Has she forgiven them now? 'I don't understand this word forgiveness,' Rowe says sharply. 'People always go on about forgiveness without understanding what it is. You can forgive somebody for an injury when you no longer suffer from that injury, because it becomes irrelevant. So when somebody says "Do you forgive your mother?", I can only say it is irrelevant. I am still struggling with the effects of my childhood. I have been able to use the psychological effects of my childhood very effectively to build a career, but in terms of the physical, that's what I struggle with.'

Rowe begins to cough. From an early age she was diagnosed with brochiectisis, a mild form of cystic fibrosis in which damaging mucus builds up on the lungs. The condition led to constant coughing, which would send her mother into a wild rage, which often turned violent. Rowe thinks now that what prevented her from becoming a depressive herself was that she never felt guilty about disappointing her mother, largely because her primary concern was to keep herself safe from her mother's violence. Guilt, as we've learned, is a nail in the depressive's coffin. Predictably, Rowe's mother's problems started with her mother.

Rowe's illness was completely ignored by her family until, finally, after graduation, Rowe took herself off to a consultant, the beginning of a medical process which resulted in major lung surgery after the birth of her child (her mother refused to visit her in hospital). Now, Rowe spends three hours at the beginning of every day going through a process by which she can clear out her lungs, necessary in order to keep pneumonia at bay. She is under a brilliant consultant but, she says, 'the care is palliative'.

She tells me all this when I ask her if she has ever fallen in love with somebody else or experienced any degree of emotional intimacy following the breakdown of her marriage, about which she is also free of bitterness: 'That woman saved my life!' she cries of her husband's mistress. 'But no, I wouldn't inflict my illness on anybody. My husband left me for it. I think it's a romantic notion that we all find love again. I am very glad I experienced it once, but I'm better off dealing with this on my own. My son understands and my friends do, but I would be very hard to live with.'

Over lunch at a local Turkish restaurant Rowe tells me she became happy very slowly. She didn't realise it until the summer of 1975 when, living in England, she looked down and saw daisies on an ordinary stretch of grass 'as if they were floating and dancing'. She had been so busy establishing and then heading a new department of clinical psychology in Lincolnshire that she hadn't noticed she was changing.

'My great good fortune, by chance not design, was that I ended up a clinical psychologist.' She had been a teacher until her son was five, at which point she was offered the opportunity to train as an educational psychologist, which led to her working with emotionally disturbed children and gaining a diploma in clinical psychology. In 1968, three years after her marriage broke up, she accepted a post in Sheffield and began a PhD. By 1972, she was heading up a department: 'I spent hours and hours talking to depressed people,' she remembers, 'and it certainly got my family into proportion. My family certainly was not the worst family out there, by any means. But it gave me an opportunity to look at my mother from a different point of view, not just as the child of this woman but as an adult observer. Many of my patients had depressed mothers and I would tell them about mine and, if you tell a story again and again, finally all the emotion goes out of it and so what was once too terrifying to talk about becomes something really quite funny. All I was doing was finding another way of constructing it. When you laugh about somebody, you actually reduce their size. They are no longer giants looming over us; they are really quite small.'

After lunch, I watch her disappear down the street, a physically small septuagenarian swallowed up by the crowd. Later that evening, I get an email from a woman whom Rowe first treated back in Sheffield when the woman was 14. She is now in her fifties, married with children. (In 1986 Rowe gave up seeing patients to write full time.) 'I feel that if I hadn't met Dorothy my life would not be what it is now,' the woman wrote, 'indeed my parents did not think I would have a normal life and marry and have children. Dorothy has always been there for me however busy she may be and I'm sure I'm not the only former client who still keeps in touch with her. I feel very lucky. Having known her for nearly 40 years now I think of her as a great friend.'

· My Dearest Enemy, My Dangerous Friend (£7.99, Routledge) is out now