Depression is in the news these days. We love to read and write about it and yet, whenever we discuss it, we offer the same routine disclaimer. Sufferers from depression, we are told, are too ashamed or embarrassed to admit to their condition. They are reluctant to seek help, although we all piously agree there is nothing to be ashamed of in mental illness. And we keep on repeating this to ourselves, despite the glaring examples of distinguished depressives who have been far from reticent about their state. These include Alastair Campbell, Stephen Fry, Professor Lewis Wolpert and, most recently, the highly successful novelist Marian Keyes, whose outburst of near-despair last week has provoked a good deal of commentary, both sympathetic and snide. She says she can't eat, sleep, write, read or talk to people and that she doesn't know when she will ever emerge from this darkness. She has told the world about this on her website, so she won't mind my repeating it.

So, is depression fashionable or is it unmentionable? Is it a taboo or a mark of distinction? Is a confession an act of exhibitionistic self-indulgence? It's not clear whether we think we run risks by ignoring it or whether we talk about it too much. It is obviously unwise for politicians to admit openly to depression, at least while they are in or seeking office, and dentists and doctors don't often own up to it, although many suffer from it. I was told by a reliable source that the suicide rate for dentists in the United States is higher than that in any other profession, yet most dentists appear to be quite jolly. It's all right for actors, performers, writers, artists and women to admit to weakness, because they aren't responsible and they don't count.

Fashions in illness come and go. Appendixes, adenoids and tonsils are no longer as smart or as suspect as they were and it appears that the vogue for ulcers has given way to the label of acid reflux. There are fashions in mental health, too, and the recent proliferation of dramatic eating disorders such as anorexia and bulimia begins to make depression look seriously old-fashioned. Feminist scholars Elaine Showalter and Lisa Appignanesi have written about the changing history of women's afflictions, noting that the 19th-century diagnosis of hysteria (etymologically, a womb-related disorder) more or less disappeared and was overtaken by a blanket diagnosis of depression. And it would appear, from such inevitably unreliable statistics as we have, that women are more liable to depression than men, despite the exceptions cited above. More women take anti-depressants and more women are succumbing to depression year by year, or so recent surveys suggest.

In the old days, the days of my mother and grandmother, female depression was often linked to the menopause and took the form of low spirits, mood swings, a sense of worthlessness and redundancy and odd acts of eccentricity (kleptomania was often excused as menopausal). These states were very different from the florid hysteria from which some of Freud's patients suffered. Hormonal changes were blamed for what was seen as a not very welcome but inevitable rite of passage: you expected this kind of thing to happen to you at a certain age. The woman, no longer fertile and made uncomfortably aware of her diminishing biological attractions, sank into a period of gloom from which she would emerge when the hormones settled and she meekly accepted her reduced role. The word "menopausal" was flung around as a routine insult, but I suspect that, like the words "cretin" and "spastic", it may be less acceptable now. Men didn't have to go through this middle-aged valley, because they remained potent and didn't have to confront bodily change so bleakly. They went under later, at retirement.

Female expectations changed dramatically with the availability of hormone replacement therapy (HRT), which delayed the menopause and some of its associated ills. My generation eagerly swallowed those little reddish brown pellets of mare's urine, or whatever they were made of, as though they were the elixir of life, just as we had swallowed the contraceptive pills that had given us our freedom. We didn't care about the long-term effects, we just wanted to stay young a bit longer. We didn't want to go down into the vale just yet. I remember having serious doubts about all of this when I met a 68-year-old woman at a charity luncheon, who confided to me over the soup that she was still menstruating and proud of it. I was taken aback. I think it occurred to me then, with justified foreboding, that the depression traditionally associated with the menopause might not have been avoided by HRT, it might just have been postponed, and that it would kick in later, with all the more force because the body and spirit would be older and weaker and future prospects less bright.

The chronological curve of women's expectations has changed spectacularly during Marian Keyes's writing life. She is 46 years old, but nowadays a 46-year-old is not even seen as middle-aged. She is in the prime of life and is more likely to be accused of suffering from the hangover of celebrity and success than from the once-inevitable "change of life". I read one or two of her novels 10 years ago when I was writing an essay about chick lit and the courtship novel, inspired by the emergence of Bridget Jones. The heroines of chick lit are 10 years older than the heroines of Jane Austen, in their thirties rather than their early twenties. They are financially independent and enjoy unlimited freedom of speech and movement; Bridget Jones is noticeably more confident about sexual intercourse than she is about fancy cooking. This is a very different social world from Austen and even from early Drabble.

Women's lives have in one generation changed almost out of recognition. Keyes's Sushi for Beginners (2000), set largely in the offices of a women's magazine, is a characteristic example of high-spirited chick lit with a subplot of mum lit and contrasts the fates of three women, one ambitious and separated from her husband, one single and caring and one married to her "dream man", but hampered and exhausted by small children. These are pioneer lives, in a rapidly changing society, and they clearly mirror the aspirations and experiences of millions of readers, who made their creator a bestseller.

Some mental health experts argue that women are unhappier now than they used to be because their expectations are too high. They fail to achieve eternal youth and beauty, but are forced to live in a consumer culture that celebrates youth. This, these experts suggest, may be even more painful than the fear that, at the age of 46, they would be sinking into menopausal gloom and thence descending rapidly into old age.

I doubt it. All change brings risks. Women are less passive than they used to be, live longer and have more resilience, even though they encounter new hazards as they age. They are still at work on the shape of the future. Marian Keyes, in speaking out about her current desperate state, is already moving on. She is a writer and she will probably write her way out of it. That's what writers do.