This is Tracy Quinn, who began to go bald at 12, a misfortune most women could not endure, including those who mimic it

"By the time I got a wig," says Tracy Quinn,"I couldn't bear to look at myself in the mirror." Now aged 34, her hair started to thin when she was 12. By the time she was 30, she had lost all the hair on her body. For her as, no doubt, for Princess Caroline of Monaco, who recently suffered dramatic hair loss, shaved the rest off and now, in headscarf and shades, must run from paparazzi in even hotter pursuit, alopecia strikes at the heart of self-esteem.

For men, hair loss can be traumatic: why else would eight million of them spend pounds 100m on "cures" each year in this country alone? But that is nothing compared with what it does to women. Despair, self-hatred and pain are its currency, suicide its troughs. "It feels desperately lonely, terrifying and humiliating," says Elizabeth Steel, who runs the Hairline self-help group for people with alopecia.

A 29-year-old woman, whose hair fell out following pregnancy, puts it graphically: "If it hadn't happened to me, I wouldn't have realised the impact. It is out of all proportion - it governs the rest of your life. To me, losing your hair after chemotherapy would be as bad as losing your breasts."

Why should hair loss be so devastating for women, when it is so common? (Alopecia is now thought to affect 50 per cent of all women at some point in their lives, though no one is sure what triggers the condition. Crash diets and nutritional deficiencies, particularly too little zinc, vitamin B12 or iron, may do it. Some women get it after childbirth, and when they go on, or come off, the pill. And it sometimes seems to be triggered by trauma or severe stress. But often it seems to occur for no good reason at all.) It has also become cool: Sinead O'Connor, Sigourney Weaver (in Alien 3), not-quite-supermodel Eve Salvail and, most recently, Demi Moore (in her forthcoming film GI Jane) have all been fashionably bald.

Outside this glamorous elite, many women with alopecia appear to feel that hair is central to looking human. "I felt like something that had landed from outer space" says Elizabeth, who was hairless for eight years. This sense is intensified for victims of alopecia universalis, when all the body hair falls out. "The alopecia areata you can get used to," says Tracy. "When it fell out all over was my crisis point. It started with my eyebrows, at the time Kate Moss had plucked hers. Then I started having nightmares about eyelashes. I said to my mum, 'I don't think I can live without them, it looks so strange'. I was frightened to take my mascara off in case another eyelash fell out. You look like a fish, a frog, bland, bald. I look like an alien - not like someone from this planet."

Hair loss is also, of course, a mark of illness, particularly cancer: the bald woman as diseased. This is the prevailing metaphor used by Tracy: "Alopecia is like cancer in your mind," she says. "A friend of my ex-husband had Hodgkin's lymphoma. I was really frightened of seeing him bald because I imagine other people feel the same way about me. If I get attacked, all I need to do is rip off my wig and they'll leave me alone."

However, men don't feel the same way; for them, baldness is associated with loss of virility. Hair loss, it seems, is heavily gendered. And for women - in contrast to men, where very short hair signifies military machismo - it has been historically associated with punishment and humiliation. "It's all very well to shave your hair and be Sinead O'Connor," says Linda, "but you have to have a very beautiful head and perfect bone structure to do that. Otherwise you look like a Belsen victim." The treatment of concentration camp victims was echoed after the Second World War, in the shaving of French women who had had sexual relationships with German soldiers.

But the devastation of alopecia goes deeper into the heart of femininity than this. For hair is clearly a potent signifier of female sexuality itself. Muslim women and nuns must cover their hair for the sake of modesty; orthodox Jewish women must shave theirs and wear wigs. And although the Western focus of male desire is the breast, for women the rituals of femininity are centred on the hair.

"The image of feminine beauty you are sold as a child," says Linda, "is a princess with long hair." It starts in the cradle, with Rapunzel and Goldilocks. Then, by the time a girl has seen the Disney versions of Cinderella and Snow White, she will have her very own Barbie doll with her very own long blonde hair. And what teenage girl's bedroom would be complete without a hairdryer and a mirror? If I think of my own adolescence, I think not of spots or breast size or weight loss, but the failings of my hair: the battle to make it "flick"; the disastrous orange bleach; the agonies of having curly hair; the ritual humiliation (by boys) of the first day in school after a haircut.

But no one suffers more acutely than women with alopecia. "It goes right through to Baywatch," says Linda. "Your body image does not fit in with the cultural norm." "I couldn't have a typical teenage life," says Tracy. "The thing that upset me most was not being able to go to the hairdresser."

By adulthood, hair has become central to female sexuality. Says Tracy, now divorced with an 11-year-old daughter, "It's awful to wear a wig in bed, but I got to the point that I couldn't make love without it. Not having pubic hair didn't worry me, because I sometimes used to shave it. But right from when I got married, I couldn't feel sexual without my hair on. You feel not like a female and not like a male - no sex at all."

Why, given all this suffering, has baldness in women become fashionable? Clearly, a woman who shaves her head is rejecting stereotypical femininity, whether she is a lesbian (like Skin, the lead singer of Skunk Anansie, or Hufty, one-time presenter of The Word) or a feminist superhero (like the cartoon Tank Girl, or Sigourney Weaver's alien-hunter, Ripley).

But now shaved heads have been detached from that simple meaning to become an attitude, an expression of individuality, as in the case of Demi Moore, Sinead O'Connor and Eve Salvail (who has a tattooed scalp). These women are marking their difference from their competitors - respectively Sharon Stone, Madonna and Cindy Crawford. At the same time, of course, it is a look which only works with an ultra-feminine face. "Both those girls have got lovely cheekbones," says Tracy of O'Connor and Skin. "But all your imperfections show." Curiously, then, in the shaved head the ultra- feminine and the ultra-unfeminine have merged.

Baldness as fashion statement is, of course, not a choice for women with alopecia in the way it is for singers and film stars. But with no sure cure (Dr David Fenton of the Hair Research Clinic at St Thomas's Hospital in London reckons he can "do something" for 50 per cent of patients), it may be the best news since the invention of the headscarf under which Princess Caroline - who has movie-star features herself - is currently hiding her shame n

For more information about alopecia, send a large stamped self-addressed envelope to Hairline, Lyons Court, 1668 High Street, Knowle, West Midlands B93 OLY.