IN AN interview near the end of his career the fashion designer Yves Saint Laurent confessed to a regret: that he had not invented blue jeans. “They have expression, modesty, sex appeal, simplicity,” sighed the owlish Frenchman. “All I hope for in my clothes.” American denim-lovers might add other attributes. As far back as the 1930s, when the popularity of cowboy films helped jeans make the leap from workwear into the wardrobes of Hollywood stars, denim has been understood to stand for something larger about the American spirit: for rugged individualism, informality and a classless respect for hard work.

“Deep down in every American’s breast…is a longing for the frontier,” enthused Vogue magazine in 1935, advising readers on how to dress with true “Western chic” (combine jeans with a Stetson hat and “a great free air of Bravado,” it counselled). Levi Strauss & Co., the San Francisco firm which invented modern blue jeans in 1873, saw sales boom after it crafted posters showing denim-clad cowboys toting saddles and kissing cowgirls.

Jump to the 1950s and 1960s, and American consumers learned the heroic history of denim from nationwide magazine and television advertising campaigns. They were told that the tough blue cloth began life as “Serge de Nîmes”, in the French town of that name, and was used by Columbus for his ships’ sails, before outfitting the pioneers who tamed the West. In a country so often riven by culture wars, jeans crossed lines of ideology, class, gender and race. Presidents from Jimmy Carter onwards have worn denim when fishing, clearing brush or playing sports to signal their everyman credentials—though Barack Obama has endured mockery for donning capacious jeans that he later conceded were “a little frumpy”.

Since the second world war, when GIs and sailors took blue jeans to the Old World and Asia, denim has carried ideas of American liberty around the globe, often leaving governments scrambling to catch up. Emma McClendon, a curator at the Fashion Institute of Technology (FIT) in New York, notes in a fine new book, “Denim: Fashion’s Frontier”, that when the Berlin Wall came down in 1989, reporters were surprised to see young East Berliners dressed exactly like their cousins from the West—in stonewashed jeans. Ms McClendon’s book accompanies a small but splendid exhibition on denim at the FIT on Seventh Avenue.

The popularity of clothing invented to survive hard labour is of topical interest in America, a country gripped by election-year debates about blue-collar, working-class voters, and whether their interests have been ignored by ruling elites. Ms McClendon argues, persuasively, that much of what Americans think they know about denim draws on a set of “origin myths”, crafted and disseminated by manufacturers over many years, both individually and in campaigns run by the Denim Council, an industry group of clothing-makers and textile mills that was active from 1955-75. The council, whose papers are now in the FIT’s archives, was formed after jeans-clad motorcycle gangs and such films as “The Wild One” and “Rebel Without a Cause” led to something like a nationwide panic about denim and its unseemly effects on young bodies and minds. Committees of denim manufacturers and advertising executives set out to combat “anxieties over juvenile delinquency”. Wholesome films about jeans appeared on over 70 television stations, and “How It All Began” cartoons ran in newspapers, tracing the origins of denim back to medieval Europe. From the late 1950s Levi Strauss & Co. ran advertisements and a letter-writing campaign urging schools to allow students to attend classes in denim. Their pitch combined images of clean-cut, studious children in jeans with such slogans as “Right for School”, explains Tracey Panek, Levi’s company historian.

Quite a lot of this marketing was hokum, or close to it. There is no evidence that Columbus crossed oceans under billowing denim sails, while the latest research is that the term “denim” may have been invented in England. Perhaps most strikingly, relatively few cowboys wore blue jeans at the height of the Wild West, Ms McClendon says: canvas and leather trousers were also common. Denim was mostly worn by small farmers, field-hands, labourers and miners—some of the oldest pieces in the archives of Levi Strauss & Co. were found in disused mines in California and Nevada (there is a whole world of denim-hunters out there, willing to endure much hardship to find a pair of 1880s Levi’s).

The best history money can buy

Ms McClendon describes economic and commercial forces at work in the 1930s. Denim sales to working-class customers slumped during the Depression. At the same time ranchers in need of extra income touted their properties as “dude ranches” at which affluent tourists could play at cowboys, apeing favourite film stars. Even Depression-era protectionism arguably played a role: Sandra Comstock, a sociologist at Reed College in Oregon, has written that tariffs on imported French clothing prodded department stores to promote domestic fashions including jeans.

Myth-making about jeans suggests a political conclusion, too: that for a supposedly classless country America takes a complicated view of work. Study denim’s history and it is hard to avoid concluding that heroic individuals roaming the land, such as cowboys, are easier to sell as fashion icons than folk who toil by the hour in a factory, garage or field, taking orders from a boss. The first gallery at the FIT exhibition shows how the earliest denim clothes were often uniforms: it includes a prison uniform, sailor’s overalls and, most tellingly, the sort of blue work-shirt made of chambray (a cousin of denim) that inspired the term “blue-collar worker” back in the 1920s. Yet, other than to a few urban hipsters in recent decades, chambray shirts have mostly lacked the “cross-over cool” of denim jeans, says Fred Dennis, senior curator at the FIT—they did not fit into a “romanticised, cool-dude weekend look”. Small wonder that blue-collar workers feel forgotten.