seeing Sharon Stone or Claudia Schiffer on screen or catwalk is enough to give most women a twinge of inadequacy. How much worse it must be for their sisters, who have grown up with them, seen them blossom into stars, and must know that they will never, ever measure up in the glamour stakes. Poor Kelly Stone and Caroline Schiffer: both pleasant looking women, but doomed for life to being a less perfect version of an acknowledged beauty. Perhaps the worst thing is that they resemble their siblings enough to be recognisably their sisters, but miss out by vital millimetres on their striking good looks.

Life is particularly hard for those who want to follow in famous family footsteps. Dee Dee Pfeiffer, Michelle's wannabe sister, blames her former bulimia on trying to achieve film-star slenderness. "Michelle was always the beautiful one, always the one people gave their attention to. I was fat when I was growing up, not attractive at all. And when you don't like yourself, having a sister who is just lovely doesn't help," she told one interviewer wistfully. "In Hollywood they are constantly looking at you, measuring your hips and your thighs - and comparing you to your famous sister."

Such rivalry is by no means confined to Hollywood. In many families, sisters are still split into the stereotyped "pretty one" and "clever one" when they are very young - and children tend to value looks over brains. A number of studies have shown that toddlers can distinguish between "beautiful" and "ugly" when shown pictures of different types of people.

"I was so jealous of my sister's looks. She had long fair hair and a cute little nose, and I thought she always had the prettiest clothes and the most attention. She was very fragile, while I was a heavy, galumphing child. I hated her so much that I decided to kill her. One day when we had the builders in, I dropped a brick out of a window when she was underneath. Thank God it missed," says one shame-faced near-murderess.

"I must have been about 10 at the time, and she'd have been eight. Now we're both in our twenties, and she tells me that she always felt really inadequate following me in school because I did well and she was less academic - but when you're that age, you want attention and compliments and boyfriends, and a string of A grades is no substitute."

"I hated parties and family occasions because my older sister Sally was so good in company, so chatty and witty, and I was just this lump sweating nervously in the corner," says another Ugly Sister. "I used to dread anyone talking to me because she made me look so frumpy and boring and lumpen. Looking back, I wasn't that hopeless appearance-wise, but something about Sally just paralysed me and it wasn't until we went away to different universities that I could pull myself together, make my own friends and be myself. I didn't hate her, we got on well together, but I longed desperately to be like her."

Sally, the sister in question, is astounded. "I suppose I did exaggerate myself a bit when I was round her, but it wasn't malicious, it was just a bit of showing off."

Brothers compete as well, but looks don't seem to be such a bone of contention. "For boys, prettiness is less of an issue - it's more a question of sports and careers," says Dr Arlene Vetere, family researcher and therapist at Royal Holloway London college.

"It was a question of who did things first - sporty things, physical things - and given that he was older than me and very well built, it was always him," says Tom (now six feet tall and twelve stone).

Don Swayze is more concerned with becoming as famous as his older brother than with looking as good as him: "One day I'll step on that pedestal alongside him, and they'll say these are the Swayze brothers, not that I'm just Patrick's little brother."

Social psychologists in Britain and the United States are currently concentrating on research into the reasons why siblings can turn out so differently in every way, when they are brought up in the same families and experience the same background influences. Dr Peter Stratton, director of Leeds Family Therapy and Research Centre at the University of Leeds, believes that often family attitudes rather than natural advantages are responsible.

"Once a child is labelled as attractive, it will dress more attractively, and have an `attractive' self-image," he says. "Children take their parents' word for it - they don't have any alternative. If you are a small child and your parents imply that you're ugly, how will you disprove it? There is lots of evidence to show that when you give a child a label they'll live up to it - or down to it."

Such stereotyping, though, is widespread. "Families tend to characterise someone according to a dominant characteristic," explains Royal Holloway's Dr Vetere. "In fact in all social groups there is a tendency to think of people as `pretty' or `sporty' without thinking about the context. It can be very liberating and open doors - but it can be very constraining, for example if prettiness is equated with not being very bright."

Being lumbered with the "pretty" label or the "studious" label or the "sporty" label is not necessarily a disadvantage, though. It can act as a spur to prick a child into competing ferociously against their sibling. "In a recent interview in the Face magazine, Liz Hurley confessed that her sister was always the `pretty' one and she felt at a disadvantage," observes Dr Stratton. Or else it can lead to the seeking of excellence in a field where there is no sibling competition. "In many cases, if you are told that your sister is very pretty, then you won't compete, you know she'll win in this particular contest. You'll find something she's not so good at and concentrate on that."

But forget becoming a ping-pong champion or expert dahlia grower to sublimate the fact that your sister looks like a supermodel. The best revenge is when the tables are turned in later life. "When we were in our twenties, she was always the good-looking girl-about-town, and I was the staid, dull sister who was studying to be a librarian and preferred reading to partying," says Elizabeth, 35, with barely concealed glee. "Now she's got three children, put on about 20 pounds, hasn't got time or money to do her hair, and is always covered in pureed rusk. I can afford to wear what I like and go out whenever I want. It's not that she's unhappy with her life, or that I feel particularly smug about the way things have changed. I just feel that it's my turn now."