Paris, France | They wear the world's most beautiful and expensive clothes yet their faces are the picture of blank boredom.

Why do fashion models always look so miserable? 

"You don't smile. It is just not done," said model Ty Ogunkoya as catwalk stars criss-crossed Paris for fashion week.

In his decade as a top model, the 26-year-old Nigerian-born Londoner has never once permitted himself a grin. 

"I have modelled for everyone, and no one has ever asked me to smile," he told AFP. "To be honest, it would feel weird if I did."

"When I walk I think about something sad, like when my cat died," added Klara, a 18-year-old Slovakian model. "It was run over by a bus."

But do models really need to be so glum?

"Never forget it is the clothes they are looking at and not you," Victoire Macon Dauxerre, a former model for Celine and Alexander McQueen, said she was told.

In her book, "Never Thin Enough", she tells how she was warned to "never, ever smile".

- 'It's so not done' -Her modelling agency's catwalk coach taught her how to get the perfect "haughty killer look" by slightly dropping her chin and lifting her eyes at the same time.

Rising young star Matthieu Villot told AFP the reason for the unspoken ban on smiling was clear.

"They want to show the clothes and not our faces. If we smile we focus attention on our faces and not the clothes," said the 22-year-old medical student.

Ogunkoya said he had been never told not to smile but "my whole preconception of modelling was moody guys and girls going down the runway... It is so not done they don't have to say."

Fashion historian Lydia Kamitsis said it was not always so.

The vogue for expressionless models is actually very recent, she said, dating from the rise of the Japanese designers Yohji Yamamoto and Commes des Garcon in the early 1980s.

"This was also the period of the supermodels (Cindy Crawford, Imam and Elle Macpherson) who very much had their own personalties, and it was a reaction against this," she said.

"In the 1960s, when collections were first presented as shows, models often smiled, laughed and even danced to music.

- Walking clothes hangers -"Now they are seen as walking clothes hangers. It's all about effacing their personality... the clothes are it."

Anthropologist Leyla Neri, the director of fashion at the New School Parsons Paris, agreed.

She dates the first appearance of moody, often scowling models to Brigitte Bardot and Jane Birkin in the 1960s.

It then sped up with the rise feminism and "women's need to be taken seriously in their professional lives, so you see women striking strong, unsmiling poses in Armani suits.

"Men have never smiled on the catwalk because they never have had to smile to please," Neri insisted.

"In the 1950s models smiled all the time, in fact they were kind of living dolls," she added.

"With emancipation and designers like Yves Saint Laurent you get more a androgynous look, and women became more masculine and powerful."

Contemporary designers have an "even more minimalist vision", Neri argued. "They want the most neutral faces and bodies possible to show their work.

"They do not see their models as an ideal of beauty any longer. That is something that the public has not quite understood."

Every few years, however, iconoclasts like French designer Jean Paul Gaultier send models out smiling.

Indian creator Manish Arora also cheers things up by casting his bohemian friends.

And several models ended up beaming through British designer Paul Smith's last Paris menswear show.

"I didn't tell them to smile," he told AFP afterwards. "I have nothing against smiling. If the clothes make them happy, go for it," he said. 

Villot, who took part in that show but didn't dare a smile, said models are often afraid to look too happy in case they end up looking ridiculous.

"The serious face you can do every time, but if you smile you don't know how you are going to look." 

Ogunkoya agreed. "It's easier to just walk and zone out. Smiling is definitely more of a challenge."

But would he smile if asked? "Why not? You get asked to do the most random things in this job."