Early this century, Parisian salons feared extinction because of the vogue for hair cuts rather than hairdressing. Kate Worsley reports on a pioneering history

Let's face it, no one really needs a haircut. So why do we persist in saying we do? The statement "I need to get my hair cut" usually translates as "I need to feel I am a different, more attractive person". It is you, not your hair, that needs the attention.

Unless it exploited this particular psychological quirk, hairdressing would be one of the world's most redundant professions. And also one of the poorest. In 1910 Parisian barbers worked a 90-hour week, trimming 30 beards a day at 20 centimes each to earn eight and a half francs. No wonder they had a side line in something for the weekends. Ladies' hairdressers, on the other hand, made a mint by tuning in to the well-off woman's wish to be crowned in glory.

Underlying all the twists and turns in hair fashions is the economic imperative: how do you keep them coming back to the salon? This is one of the main threads of West Virginia University professor Steve Zdatny's book, Hairstyles and Fashion: A Hairdresser's History of Paris 1910-1920.

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He has not (thankfully) attempted an earnest theoretical analysis. "I'm a historian, not a literary critic. I like real, palpable, rather than abstruse relations." He has simply collected the weekly columns of the leading Parisian hairdresser, Emile Long, who was also general secretary of the Institut des Coiffeurs de Dames, that appeared in a British trade journal during the seismic shift from hairdressing to haircutting.

Until the bob was introduced from London, Parisian stylists led the world. Women's styles had changed little for centuries, hair was piled up in varying shapes, amplified with artifical pads of real hair (postiche) and decorated with knick-knacks and ribbons. Ladies of fashion prided themselves on never washing their hair, let alone cutting it. Shampooing was thought "inutile et humiliant".

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Unwashed hair was held to be easier to dress - although the modernist hairdresser Antoine made a visit to one Belle Epoque grande dame whose long locks smelt so rancid he could not face handling them. Then, along with growing interest in hygiene and "efficiency" and "sportiness", came shorter, cleaner hair.

It sounds vaguely farcical now, but Long's columns quiver with barely suppressed alarm. Convinced the shift to cutting would put them out of business, Long and his colleagues tried to turn back time. They even formed cartels to run rigged competitions promoting their favoured style.

Long's attempts to stem the tide failed miserably. Whether or not short hair was the badge of women's emancipation, which Zdatny doubts, women wanted it and hairdressers could do nothing to stop them.

But to Long's bewilderment and relief, business boomed for hairdressers. This was Zdatny's starting point for his book. While writing a book on artisans in 20th-century France, he had observed that an unprecedented growth in the number of hairdressing salons occured at the same moment as the vogue for the garconne - the short boyish bob - exploded. France had fewer than 48,000 hairdressers in 1896, almost 62,000 in 1926 and more than 125,000 by 1936.

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During and after the first world war, women increasingly had their own money to spend, and they chose to spend it on having their hair done. Crucially, short hair needed constant cutting, just to keep it short. And although a woman had her hair washed 20 times a year at five francs a time before the war, by 1925 she was visting her coiffeur for 36 shampoos a year at eight francs a time.

Particularly lucrative were technological developments such as the Marcel wave, the first perms and, especially, artifical hair dyes, sales of which were five times greater in 1920 than they had been before the first world war. By mechanising the processes of hairdressing, technology also speeded the democratisation of styles. If the girl in the corner shop could get that same platinum wave (if a little singed) within a week, why, then your leading lady of fashion was just going to have to get herself an auburn rinse immediately. Consumerism saved the profession, almost despite itself.

Hairstyles and Fashion:A Hairdresser's History of Paris 1910-1920 is published by Berg.