Is double better than one? In the case of Mary-Kate and Ashley Olsen, the twin thing has helped turn two once-adorable child stars into mini American style stars   

There's something weirdly intriguing about identical twins. How else can you explain the enigmatic appeal of Mary-Kate and Ashley Olsen - quite possibly the most famous and successful twins on ther planet? It seems our obsession with these two five-foot-one sized millennials says as much about the enduring fascination with identical-looking offspring as it does about our fixation with following the (often tumultuous) life paths of child stars.

We first got to know the Olsen twins via our analogue television sets, as the precocious pair shared time playing the saccharine sweet Michelle Tanner on Full House. The series ended in 1995, but the die was cast: the world was already enthralled with Mary-Kate and Ashley. Thanks to a string of '90s and '00s movies, TV shows and clever merch opportunities (hey, Mattel dolls!), the Olsens had collectively amassed a whopping US$100 million net worth (according to Forbes) in 2007. Did we mention they were aged just 21? (There's a lot to be said about the loyalty of your childhood fans - right, Selena Gomez?) 

After a slow process of rebranding themselves away from their tween audience to align themselves with the 'serious' sife of the fashion industry, the Olsen twins now boast two successful labels - The Row for elegantly minimalist couture and Elizabeth and James for lower-priced ready-to-wear. If you're a millennial yourself, you have to admit that it's been truly incredible watching these two California-bred girls morph from smiley faced tweens into bona fide fashion moguls who've won accolades such as the CFDA Womenswear Designer of the Year in 2012. 

With earnings estimated to be worth the US$1 billion (and a combined net worth of US$300 million) - you can also understand the appeal of these pint-sized former actors. Oh, and we haven't even mentioned their penchant for dating older men - which culminated in Mary-Kate marrying Olivier Sarkozy last year, in a ciggie-fuelled celebration that had everyone talking. Yep, there's that enigma thing again...

Click through the gallery above to see how the girls have evolved from from hip-slung cowboy belts and frilly blouses to a bag-lady chic sensibility they've made entirely their own.

Why the world is obsessed with Mary-Kate and Ashley Olsen