A couple of decades ago, I began noticing that the leading lady in a movie was almost always fairer-skinned than her leading man.

It appears filmmakers and their audiences subconsciously associate lightness of complexion with femininity. Yet, nobody ever seems to talk about it.

Medieval Europeans referred to women as "the fair sex," but in contemporary discourse, skin color is associated only with race, not with sex.

We don't behave like that, however. You may have wondered, for example, why Nicole Kidman seems to have a film coming out every few months. She starred with Sean Penn in The Interpreter in April, will be in Bewitched with Will Ferrell on June 24th, and is slated to be in five more movies scheduled to come out over the next year and a half. Yet, the only film she was ever in that earned $100 million at the domestic box office was Batman Forever a decade ago.

Kidman is a perfectly adequate actress. But one reason she works so much is because of her extraordinarily light complexion. A producer can hire her knowing that no matter which actor he signs to play opposite her, she will be fairer than him.

In 21st Century Hollywood, surprisingly enough, skin tone seems to matter more than height in pairing romantic leads. Kidman is 5'10.5," which you might think would cause casting problems, but that doesn't keep her from working nonstop. She is considerably taller than many popular actors, including her ex-husband Tom Cruise, with whom she made three movies.

Audiences famously want their leading men to look "tall, dark, and handsome" (a phrase first applied to that epitome of male glamour, Cary Grant) when they embrace their leading ladies. But, apparently, "dark" is even more important than "tall."

My impression is that female fans are more insistent than male fans that their favorite actresses be fair. Conversely, male fans don't much like pale actors, as Jude Law's problems shedding the dreaded "pretty boy" tag demonstrate.

While black actors like Will Smith can reach superstar status, it's much harder for black actresses, especially ones darker than the half-white Halle Berry, to win massive popularity. For example, in his hit romantic comedy Hitch, Smith was teamed with a fairly obscure Latina actress rather than a black one.

When the Internet came along in the 1990s, I discovered that an anthropologist at Université Laval in Quebec named Peter Frost had been researching for years this question of why actresses were so fair, and much else besides.

His findings are quite extraordinary.

He's finally published a lucidly written and wide-ranging book entitled Fair Women, Dark Men: The Forgotten Roots of Color Prejudice. It proves well worth the wait, shedding light on a broad array of contemporary social issues.

It turns out that this favoritism toward lighter skinned women is not an invention of Hollywood. You'll note that conventional "social constructionist" thinking can't explain this phenomenon. The standard academic's logic would predict that, because whites rule and men rule, therefore the whitest men would be the most popular.  But pallid blonde actors of the James Spader ilk typically play evil preppie-yuppie villains, not heroes.  Conversely, the movie industry is responding to a fondness for fairer females found in almost all cultures across almost all eras.

In his foreword to Fair Women, Dark Men, U. of Washington sociologist Pierre L. van den Berghe, author of one of my favorite books, The Ethnic Phenomenon, summarizes:

"Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker."

Frost reports that out of 51 different cultures in the anthropology profession's famous Human Relations Area Files, 44 cultures favored lighter complexions on either only women (30) or on both sexes (14). In only 3 cultures was fair skin preferred on men only, and in just 4 cultures was darker skin desired.

Lighter ladies were favored in many countries with little exposure to Western beauty standards, such as medieval Japan, Ethiopia, Aztec Mexico, and Moorish Spain, where the dominant culture was darker skinned than the conquered natives.

Frost discovered that the reason women were called "the fair sex" is because women are indeed fairer on average after puberty. He notes that 50 out of 54 anthropometric studies from around the world have shown that women's untanned skin, such as under the upper arm, reflects more light than men's. Women have more subcutaneous fat, which gives them a lighter look.

The gender difference in color is not large, but before Europeans came into frequent contact with sub-Saharan Africans and others of highly different hues, it was noticeable. Frost writes:

"When one's social horizon takes in a limited range of observable skin tones, small gradations of color take on more importance…. A 'white' person was simply a fair-complexioned individual; a 'black' person, a dark-complexioned one. This old way of seeing things persists today in surnames that once referred to the normal range of skin color in Europe, [in] surnames like White, Brown, and Black among the English…"

Don't believe me that we all carry unconscious assumptions about women having paler skin? Here's a test provided found in Frost's book, provided by Richard Russell of the Sinha Laboratory for Vision Research at MIT.

Which one of these faces is a woman and which is a man?

I bet you assumed the one on the left is a woman and the one on the right a man.

In truth, a computer generated these images by averaging male and female faces. The only difference between them is in complexion. (The lips look more attractively feminine on the left face because of the greater contrast with the skin tone.)

Not surprisingly, men and women often behave in ways that exaggerate the sex difference in color. For example, although sun-tanning was fashionable among Hollywood starlets in the 1960s and 1970s, the current generation seldom tans.

Could it all just be social class prejudice? Traditionally, wealthier women who didn't have to work outdoors could avoid tanning more than poor women who had to slop the hogs. That plays a definite role in maintaining the bias, but the cultural fondness for fairer women is even found among hunter-gatherer tribes where all women have to be outdoors every day finding food.

As I pointed out in my recent review of Thomas Sowell's new book Black Rednecks and White Liberals, women do most of the work in the tropical agricultural cultures of Africa and New Guinea. Accordingly, the predilection for fairer women was traditionally least apparent there. Men looking for wives might well see darker women as physically stronger and thus better providers. Frost notes:

"There is some ambivalence in societies where women do most of the agricultural labor. In such a context, wives tend to be chosen for their ability to work outdoors, especially in the sun, and less weight is given to other criteria, like physical beauty. This is true in most agricultural societies of sub-Saharan Africa and in New Guinea."

However, the fairness fad has been growing in Africa as more affluent women get office jobs, leading to a rapid increase in the sales of often-dangerous whitening creams.

Frost also points out a corollary of this sexual selection for lightness:

"Since higher-ranking men marry the more attractive women, the upper classes tend to lighten in color with each passing generation, as in India."

This seems particularly true in Latin America, where the elites remain quite white-looking despite almost 500 years of intermarriage. The trick is that the most successful short, dark men often wed tall, blonde women and have more European-looking offspring, thus replenishing the caste system.

Frost's short but mind-expanding book shows once again how much more fascinating the study of humanity can be—when freed of the boring old prejudice that biology has nothing to do with human beings.

[Steve Sailer [email him] is founder of the Human Biodiversity Institute and movie critic for The American Conservative. His website www.iSteve.blogspot.com features his daily blog.]