Are you a 'rebel' eldest child or a 'mature' youngest '? The fashionabl e birth-order stereotypes don't work, says Sarah Litvinoff
I Am A natural-born eldest child: responsible, hard-working, rather conventional. I worry about my siblings and have a tendency to cluck around everyone I know, feeling it's my duty to look after them. Only I'm not an eldest child. I'm the youngest of three. Current lore (and psychology) dictates that because I was spoilt, indulged, allowed to do my own thing - all true - I should be an adult baby: wilful and seeking to be nurtured throughout my life.

I'm not a fan of statistics, generalisations or rules - probably the only vestige of youngest child waywardness remaining. There is logic in the convention that the eldest, burdened by the expectations of excited new parents, becomes a pillar of society, that the youngest, unfettered, becomes an emotional and professional dilettante, and that the middle, caught between the two, feels a bit of an outsider and becomes an expert in compromise.

But the glorious thing about humans, versus the tidy predictability of machines, is that no one really knows how we will turn out. Birth order works well as a general rule of thumb, but there are exceptions, as Mr Ian Simpson, head of psychotherapy of the Lambeth Healthcare Trust, points out: "Life events, particularly within childhood, can alter the natural order, as can family and social dynamics."

Clifford Isaacson, who founded the Upper Des Moines Counselling Centre in Iowa has made a special study of what he calls the "new" birth order, in which he defines five personalities (only, first, second, third and fourth - the fifth exhibits characteristics of a first-born, and so on). He says, "Exceptions to birth order occur because of large age differences, parental influence or unusual circumstances."

For example, a live-in grandparent who helps with the babies can create a set of siblings who all exhibit "only child" characteristics, or a nanny or child minder can affect the order. "A child can pick up birth order from a care-giver's family, taking on the next birth order of other children as if they were siblings," he says. Isaacson's ultimate get-out clause is that "psychological birth order may not correspond to your actual birth order. However, your psychological birth order is the more significant of the two."

I'm fortunate that my sister Vida, ten years older than me, felt similarly misplaced in the birth order. "I was born at the wrong time, like people born the wrong sex. The only perk was being the one who could stay up latest. Nothing else was worth it." You only have to look at our work to see that fate got our roles mixed up. From my early twenties I've been writing earnest self-help stuff about getting your life in order and repairing your relationships. People who meet me for the first time often say I'm "different" from what they envisaged. Sometimes they mean "younger". Most people who've read me expect me to be older, by quite a bit, than themselves.

Vida, on the other hand, is an anarchic and achingly funny novelist. In Sons, Lovers, Etcetera, her heroine, Kate, a heightened and fictionalised version of herself, careers through life and love disasters as a child would - despite having a grown-up son herself - protected, as life's innocents are, by zest, optimism and a big heart. She copes with tawdry reality by dialling into her imagination and by doing the kinds of things most "grown-ups" believe are past them. As Bernice Rubens, the Booker-prize winning novelist, says, she is "audacious and funny".

Vida was the mother of two children before she spotted my potential to take over as her older sister. "One day I was telling you to go fetch my handbag, because you were the littlest, and the next you turned 18, and I saw the answer to my prayers. I'd spent my life looking for people to look after me and there you were. I could say 'these are my problems, solve them'. We swapped roles seamlessly. It only brings me up short when, by mistake, I call you 'mummy'. From the moment we had the realisation that you'd be the eldest and I'd be the baby, we didn't jostle, as some siblings do. We'd found the balance that suited us both."

There's also Sandro, a boy I've known since he was eight, the youngest of five. All his siblings and even his parents were wild. From the earliest age, he felt it his place to be the man of the family. He longed for order in his Bohemian household. He tried to keep domestic order as best he could, tidying, washing up, and trying to stabilise an unruly family. As an adult he is immaculate, bourgeois and tidy. He married young and has become a dependable father and husband of extreme rectitude. He fits Isaacson's theory that the fifth child, ignored by the fourth, becomes an eldest, or first child in personality.

Once you start looking, there seem to be more exceptions than not. Look at Princess Anne, a natural eldest, if ever there was one, who, if the laws of inheritance were different and she had been born a few years earlier, could mightily and responsibly ascend the throne when the time comes, and leave her brother Charles free to do the more interesting but less taxing things that appeal to him.

But for the swap to work there also needs to be collusion. Many feckless, flighty older siblings hang onto their positions of precedence, while the younger, staid ones fret helplessly.

Another pair of sisters who, like me and Vida, had a tacit agreement to trade roles, are Stephanie and Claire Calman, daughters of the late cartoonist Mel. Stephanie, the elder, who writes the hilarious Dressing for Breakfast on Channel 4, divides the world into "people who eat the icing off the cake immediately and people who save it for later - Claire is a saver."

We have spent a couple of evenings as a foursome, in which Claire and I have sat smiling indulgently while our "younger" sisters have competed with outrageously funny anecdotes. Under the storm of noise we try to maintain a sophisticated banter. We're the ones who surreptitiously check our watches, and use our repertoire of significant looks, veiled hints and gentle kicks under the table to keep the sisters in line.

According to Isaacson, you can tell your psychological birth order by deciding which of the following T-shirt slogans would apply to you. Only child: "Leave me alone, I'd rather do it myself". First: "I don't know, what do you think?". Second: "That won't work, It's not good enough". Third: "No problem, It doesn't bother me any". Fourth: "Life isn't easy, You have to try hard". If several seem to apply to you, he says, consider yourself an only child.

I have my own theory - current trends are upsetting the natural order. When forward-looking companies are flattening hierarchies or squashing them out of existence, junior employees are encouraged to rate their bosses' performances, and schoolchildren to take their teachers to court for their failings, something is certainly changing. Popular demand for Prince William to leapfrog his father onto the throne is reflected in my generation by parents who have to use adolescent wiles to keep their excesses from their Saffy-like children, who wail at them to "be careful", use condoms, and demand to know what time they call it when they tip-toe in at dawn.

As party leaders use youth as a campaigning card, and it is common for babies to rule in the home, it is only a matter of time before it is accepted wisdom that authority and gravitas are the preserves of the youngest, and that children behind you in the birth order liberate you from the onus of responsibility.