Kim Kardashian, baby North West, Kanye West and Helene Arnault attend the Givenchy show as part of the Paris Fashion Week Womenswear Spring/Summer 2015.
Kim Kardashian, baby North West, Kanye West and Helene Arnault attend the Givenchy show as part of the Paris Fashion Week Womenswear Spring/Summer 2015. - 
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Celebrities have been selling fashion for just about as long as "fashion" has been a thing. According to fashion columnist Teri Agins, author of the new book "Hijacking the Runway: How Celebrities Are Stealing the Spotlight from Fashion Designers," the line between "selling" and "creating" keeps getting more fluid.

"Celebrities have actually taken over, and they actually now have their own brands. So they're actually competing with designers," said Agins.

Agins says the celebrity takeover of the fashion industry is a great way for said celebrities to monetize their brands and expand into a demographic that major designers overlook. A large part of the success of that strategy, she says, is being able to relate to the consumer, as Jessica Simpson did back in 2004. After her new line of jeans didn't sell terribly well, she launched a shoe line with the backing of Nine West founder Vince Camuto - ultimately creating a a billion-dollar business.

"It was a confluence of a lot of things," said Agins. "Even though she wasn't famous for being a singer or an actress - she was more famous for her relationships with Tony Romo,  and she had problems with her weight - that actually made her very relatable to consumers, because all women have either had problems with weight, or with relationships, and then she had the clothes that went with her look."

The strategy worked, and is causing fashion designers to rethink the way they design their clothes, says Agins.

"Last week in Paris, the Kardashians basically took over. Nobody remembers what was on the shelves at Chanel or Lanvin, we all just remember Kimye and the little daughter sitting in the front row at all these shows. This is a modern way to market," she said.

Listen to the full interview in the audio player above.

Follow Kai Ryssdal at @kairyssdal