Flappers, bohemians and screen sirens – these women made America great, says Patrick Robinson, Gap's head of design, as he previews an exhibition of Stateside style at New York's Metropolitan Museum

Until the point when America started exporting films and actresses, our fashion was really based on European style. Then it went through a switch and Hollywood brought a new voice to America.

It was sexy but also optimistic, and maybe a bit more casual than the fashion that you had seen prior to that. Around the Thirties and Forties , you see the beginning of America going into a more relaxed attitude and aesthetic which prevails today.

At the Metropolitan Museum's new exhibition, American Woman, the 'Screen Sirens' section has some of the most phenomenal dresses I've ever seen. What I love is that the show starts at the beginning, with the New York ballrooms and the society rooms; then as you walk through you understand how women in America had to go through all these stages, the Gibson Girl was sport, then the suffragettes...

I love seeing how women took on rights and areas that had been exclusively for men, such as sports and voting. It's fascinating how dress reflects women's cultural status through history.

Anna Wintour is one of the co-chairs for the Met Ball. I've known her for almost 20 years and she is still the same person as always. She is amazing to work with and quick to make decisions; she's a fascinating human being. The Ball is the highlight of the New York season; it will be very exciting to see how people will interpret the idea of the American Woman through what they wear. I'm going to wear Calvin Klein – Gap don't do tuxes.

To understand the concept of the American Woman now, you only have to look as far as Michelle Obama. She brings a casual attitude to clothes while still being very sophisticated and optimistic, individual and colourful, and there is a certain elegance in her easy approach; it never looks like the clothes are wearing her.

I can always tell Americans abroad. When I lived in France, friends would say, "It's because Americans always smile," and it's the same with clothes. American women dress with a certain optimism, an ease. You see that in Michelle, in Oprah Winfrey who is a co-host of the show with me, in the Secretary of State, with her easy business suit.

The America that I export is that nonchalant casual look that's still very sophisticated. I've always said that a European or English person in their most casual outfit is an American in their most dressed-up outfit. Americans do generalise about Europeans. They say, " Oh, that looks very European." It's usually a more elevated, sophisticated outfit that captures a European attitude, and often means blacks and charcoals. When you think of American you think of khaki and colour.

The American look differs from east to west. At Gap, we see it in the denim – the difference between women in California who like light-coloured, boot-cut distressed jeans and on the East Coast it's much more skinny-leg and darker coloured washes.

In terms of the way people shop now, I don't think people set out to buy basics. Basics come after buying something emotional – although that could be a new jean (women are always looking for a new jean). If we talk about America and about the way Americans dress, they wear denim wherever they can, from social events to dinner to performances at night.

How should you wear double denim? Make sure you don't match the denim – you can't have the same colour on the top and the bottom. You can also wear different weights, have a chambray on the top. The beauty of it is where you mix it up. That's very cool.

American Woman: Fashioning a New Identity is at the Metropolitan Museum in New York from 5 May until 15 August. Gap is exhibition sponsor and Patrick Robinson is a co-chair of the museum's Gala Benefit.

The Evolution of American Women

1890s: The Heiress

Immortalised in the novels of Henry James and Edith Wharton, the etiquette-conscious heiress acquired her specialised wardrobe for morning, afternoon and evening at Europe's most prestigious fashion houses. For her grand evening wear she favoured Rouff, Hallee, Paquin and Worth – whose founder Charles Frederick Worth had a particular fondness for American clients.

1890s: The Gibson Girl

The Gibson Girl was the first archetype of American womanhood to challenge European ideals of style and beauty. Her clothing reflected a more modern, liberated femininity: simple tailored suits and dresses for streetwear and practical shirtwaist blouses and skirts for sport. It was through sport that the Gibson Girl reflected the American woman's growing independence.

1900s: The Bohemian

Keen to express herself with strong statements, the bohemian gravitated towards artistic labels such as Poiret, Callet Soeurs and Liberty & Co. Influenced by Orientalism, medievalism and classicism, they favoured a looser, uncorseted silhouette, synonymous with greater freedom.

1910s: The Patriot and the Suffragist

One of the greatest catalysts for women's suffrage was the First World War, when over 40,000 women were mobilised to help the war effort. American women finally got the vote in 1920. Suffragists strengthened their political identity by wearing specific colours such as gold, or purple, white and green.

1920s: The Flapper

With her rouged lips, bobbed hair and penchant for smoking and gin-drinking, the Flapper rejected Victorian sexual mores. An androgynous figure was the ideal, emphasised by chemise dresses with low waists and no bust darts, such as the gold Lanvin version that appears in the show.

1930s: The Screen Siren

By the Golden Age of Hollywood, America's influence on international style was well established and the look was pure womanly, sensuous glamour. Designed for the camera, screen gowns were intended to make a dramatic impact, and draping and twisting details echoed the classical designs of Madame Grès and Madame Vionnet.