Why do so many women go blonde?

(Reuters/Twitter)

Teen actress and singer Miley Cyrus wowed fans this week by chopping off most of her hair and dying it platinum blonde. Afterwards, she tweeted, "LOVE my hair ♥ feel so happy, pretty, and free."

With the new 'do, Cyrus joined the throngs of women around the world who choose to go blonde.

So, what's the appeal?

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At its root, the desire to have light hair represents an urge to look different, said Peter Frost, an anthropologist at Laval University in Quebec City. Most people have dark hair, so blondes stand out.

The urge to be blonde may also be driven by deep evolutionary history beginning many millennia ago when light shades first appeared on women's manes, allowing them to turn the heads of potential mates.

"The more common a hair color becomes, the less often it is preferred," Frost said. "It's a kind of novelty effect. The moment you become ordinary, you no longer have the same appeal. There's selection for being a bit different and eye-catching."

Modern humans evolved in Africa. Even after migrating to Europe about 35,000 years ago, scientists think that all people had black hair. Then, sometime between 10,000 and 15,000 years ago in northern and eastern Europe, studies suggest, the hair-color gene MC1R developed variations that produced a diversity of hues, including red, brown and blonde.

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Eye color, which is controlled by several genes, including one called OCA2, diversified at the same time. Some researchers have speculated that lighter hair and eyes helped people better acquire vitamin D in a high-latitude environment. Frost has a different theory.

During the last Ice Age, he proposes, men had to travel longer distances through Arctic tundra to find animals to hunt. That led to higher death rates for men as well as a decreased chance for polygamy because it would have been nearly impossible to support more than one family with such a scarcity of food.

As women came to outnumber the supply of monogamous men, they had to become more competitive for male attention. In evolutionary terms, this produced strong sexual selection for novel hair and eye colors. Women with unusually bright looks were eye-catching and appealing.

Men didn't experience the same pressure, which might explain why it is still more common for women to be born blonde, and why it takes longer for blonde hair to darken on girls than it does on boys.

Even today, Frost said, the market for blonde hair dye is greater among women in places like Latin America, where naturally light locks are particularly unusual. In Sweden, where a large proportion of people are blonde, women often darken their hair. Purple, magenta and other unusual hues have also become popular.

In addition to the desire to stand out, going blonde might represent a subconscious attempt to look young and cute. That's because, along with broad foreheads and little noses, blonde hair is also more common in young children than in adults.

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All of that hair coloring may pay off for women, suggests some research. In a study published in April in The Journal of Socio-Economics, for example, French waitresses earned more money in tips from male customers if they wore blonde wigs.

Other research, which included more than 12,000 American men using a popular dating website, found that men showed a slight preference for blondes over other hair colors, said Jena Pincott, author of the book, "Do Gentlemen Really Prefer Blondes?: The Science Behind Sex, Love, and Attraction." Polish men have been shown to prefer blonde hair on women who are older than 25, a finding that supports the youthful-look theory.

For modern women, the benefits might be psychological more than anything else.

"If being a blonde makes you feel more attractive, you'll be more confident, seek more attention, and likely get it," Pincott said. "Then you'll have more fun."

Once blonde hair becomes too common, though, it may lose some of its appeal.

Some research has shown that single men prefer pictures of blonde women if embedded in a series of brunettes. But if the men see mostly blondes, brunettes become more attractive to them. Scandinavian men, who are surrounded by blondes from birth, often say they prefer women with darker hair.

"Modern men are attracted to blond hair for the same reason as their Ice Age counterparts: It's eye-catching and, much of the time, rarer," Pincott said. But, she added, "Even the most dazzling shade won't help you stand out if everyone has it."