Sculptors immortalize them. Poets regularly regale them. Even ordinary men pay tribute. American males, it has been calculated, spend some $3 billion a year to gaze at women with hourglass figures, those whose small waists blossom into sinuously curvy hips.

Men rate women as most attractive when they have a waist size that is 60 to 70 percent of their hip size, the late psychologist Devendra Singh found in a series of pioneering studies begun 20 years ago. And in more than a hundred other studies, men all over the world—including isolated groups unexposed to modern media—prefer a similar shape. Singh and cognitive Steven Platek found that viewing women with curvy figures stimulates a powerful internal reward system, lighting up the same pleasure centers in men’s brains that are targeted by and .

That this kind of hourglass figure is not only typical of the women men pay to look at, such as Playboy Playmates and adult film stars, but is also a preference found in many different social groups and cultural settings, suggests it has been shaped over millennia by evolutionary forces, like our tastes for sugar and fat. The preferred women are remarkably alike, and the similarity of their measurements and men’s reactions to them further suggests that there is a specific template buried deep in men’s minds.

It is likely that men who preferred curvy hourglass figures in women had more children who carried their fathers’ preferences down to the present. Still, how could an hourglass figure relate to a woman’s success as a mother? The answer is not at all obvious. But over the past several years, we have been demonstrating that it has a lot to do with . And just as much to do with what people eat and where it comes from. The evidence also suggests why American women increasingly dislike their bodies and misjudge what men like in women. It may even explain why American children fare increasingly poorly academically compared with kids in the rest of the world.

What it comes down to, in a word, is fat. But not just any fat.

What Men “Know” Without Knowing

The average playmate or adult film star is 5 feet 6 inches tall and weighs 115 pounds, giving her a body mass index (BMI) of 18.5. Her bust, waist, and hip sizes are 35-23-35, so that her waist is 66 percent of her hip size. By comparison, the typical American university undergrad has a waist that is 75 percent of her hip size. The Playmate’s hips are similar to those of students with the same BMI—53 percent of her height; it’s just that her waist size is three inches smaller. So men prefer women who are taller than average, with normal-size hips for their moderately low BMIs, and very small waists.

Her unusually small waist is only part of what makes a Playmate so curvy. The other part is having relatively normal- size hips and legs that are well endowed with fat. Even a thin woman carries an astonishing amount of fat in her legs and hips—about a third of her body weight. Men everywhere admire the fat located here. Those surveyed in 54 non-Western tribal groups almost always preferred women with large or fat hips and legs.

The total amount of fat a human female carries is seven times that of other animals, and much more than men! Only bears ready to hibernate, penguins facing a sunless winter without food, or whales swimming in arctic waters have fat percentages that approach those in normal, healthy, trim young women. Women, however, don’t regularly swim in arctic waters or hibernate for the winter. Why, then, have they been designed to store so much fat in their hips, buttocks, and legs?

One clue is that the fat stored there is protected; it doesn’t figure into weight fluctuations. Fat in the upper body is like a checking account with frequent deposits and withdrawals. But hip and leg fat is more like a certificate of deposit—it usually remains untouched by weight shifts. Only during the last few months of and while nursing do women start breaking down this lower-body fat, making it exclusively available to the rapidly growing infant. Something stored there seems very important for their children. It couldn’t be just a matter of calories, because it’s easy for the mothers to get them simply by eating more of anything.

Just as human mothers have seven times more body fat than other animals, human babies have a body part that is seven times larger than the one in other animals—an enormous brain that grows fastest in the first two years of life. Other than water, the human brain is mostly fat and has a lot of a particular kind of fat: an fat called DHA (docosahexaenoic acid). We can’t make omega-3 fats; they have to come from what we eat, and women tend to stash their DHA in the same hip and leg fat that men value.

A human baby’s need for DHA to feed its rapidly growing brain—DHA is especially incorporated into nerve cell membranes, facilitating the rapid transmission of information—is so great that a mother can’t supply enough from her everyday diet. Most of the DHA in a mother’s milk comes from what she has stored over the years in her lower-body fat. Even then, her supply is limited, as it is depleted by each child. Since a mother’s DHA levels are greatest for the first child, it may be one reason why first-born children tend to be smarter than their siblings.

That’s what men’s brains are telling us—that a woman’s figure signals the abundance of her DHA supply. Studies show that women with curvier hourglass figures have more DHA stored in their body fat. And because DHA makes brains work better, these curvier women also tend to have smarter children and, contrary to what you might expect, to be smarter themselves.

In other studies we have conducted, we find that children who live in countries where mothers have high levels of DHA in their breast milk score high on international tests of academic ability regardless of differences in income. American children rank 31st out of 64 nations. Children earn the highest scores in places with very high levels of omega-3 fat intake, like Japan, Hong Kong, and Singapore, where most women have slender hourglass figures. Japanese women have four times more DHA in their blood than do American women, with high-scoring children and low levels of .

While a woman’s hips tell men about her omega-3 stores, her waist conveys a still richer message. In addition to having more omega-3, women with smaller waists are also less likely to have been pregnant before (or currently), so their childbearing potential is still untapped. They are also less likely to die in childbirth. The same giant infant brain that mothers have to supply with DHA also makes it much more difficult for it to fit through the birth canal. Women with higher BMIs and more waist fat are more likely to have first babies that grow too large to deliver, a problem usually fatal for mother and baby unless she has a surgical birth—and one that still takes the life of one mother in 10 without access to obstetrics. Shorter women are especially likely to have this difficulty, and they also tend to have bigger waists because they have less space for their internal organs. It’s easier for women as tall as Playmates to have small waists, and it’s also easier for them to deliver their babies.

Because small waists convey so many desirable qualities, men like them to be extremely small. There is, in fact, no lower limit to waist size that men conjure. The imaginary women they depict in comic books and animated films have hips like Playmates but impossibly small waists. Jessica Rabbit’s waist is less than 40 percent of her hip size.

Men haven’t a clue about why they prefer certain body shapes or why their brains light up when they see narrow waists and well-rounded hips and thighs. But the preferences encoded in their by millennia of evolution help reveal what it is about women’s bodies that foretells success in having children.

Fat City but Skimpy on Omega-3s

Not surprisingly, the body shape that men find most attractive is actually quite similar to the one that young women typically have—correction: had. Just as evolution has designed men to prefer a shape ideal for mothering, it has moved women toward that shape. Even though Playmates are taller than average and have unusually small waists, their figures are not so different from those of young European women today—or young American women of 40 years ago. Back then, half of American women in their late teens had BMIs less than 20, and two in five had a waist-hip ratio of 70 percent or less. Unfortunately, today’s average American woman is 20 pounds heavier than in 1970. Only one young woman in six has a BMI less than 20, and only one in 20 has a waist-hip ratio less than 70 percent. What’s gone wrong?

The same innate drive that makes women store fat may also be causing them to gain extra weight now. It’s not women’s needs that have changed—it’s their ability to satisfy those needs.

Since omega-3 fats must come from our diets, the amount we can store in body fat depends on how much we can get from the food we eat. Unfortunately, American women (and men, and children) are now seriously deprived of omega-3 fat. Food companies eliminate them from products in order to extend shelf life. But as the amount of omega-3 in the American food supply has been shrinking, we’ve been getting much more of another fat, omega-6, which is cheaper, more stable—and undermines our ability to get enough omega-3.

Most omega-6 fat comes from chemically processed oils extracted from soybeans and corn. Further, feeding farm animals corn instead of allowing them to graze on their natural diet of grass (rich in omega-3) also increases omega-6 while reducing the omega-3 in meat, milk products, and eggs. Omega-6 fat is not inherently bad. Our bodies need roughly equal amounts of these fats, and for over 95 percent of human history, they were in balance. But our diets now supply 20 to 25 times more omega-6 than omega-3!

As a result, American women now have very low levels of DHA in their stored fat, as indicated by the amount of DHA in a mother’s milk; after all, providing necessary nutrients for their infants is why women store DHA in the first place. Sadly, compared with women in other countries, American mothers rank near the bottom in the DHA content of breast milk.

They also rank poorly in the DHA content of body fat. One pound of American body fat has much less DHA than a pound of fat in women consuming high omega-3/low omega-6 diets, like those in Japan. The only way an American woman can increase the amount of DHA in her fat stores is to add more pounds of fat. Since it takes a certain amount of DHA to build a human infant’s brain, a woman today needs more pounds of that low-DHA body fat. This may help explain why American women weigh 20 pounds more than they did 40 years ago, and 40 pounds more than Japanese women of the same height.

More Weight, Less Light

Because body weights are so much higher than they were 50 years ago, many American women are concerned about their weight. Back then, most women were content with their bodies: There were few books about and no weight-loss programs. Today even slender women are dissatisfied with their bodies; more than a quarter of those who weigh 110 to 114 pounds are trying to lose weight. A woman’s image of the ideal figure is not subject to the same evolutionary pressures as a man’s is; it’s much more influenced by the culture she lives in.

In the 19th century, women’s fashions seemed to reflect men’s preferences, albeit painfully. Fashionable women squeezed their waists with corsets and exaggerated their hips with bustles. In the 1920s, fashion designers like Coco Chanel sought to liberate women from such constraints and ushered in an era of simple unstructured, linear dresses. After World War II, curvier women were back in fashion, although sculpted with tailoring rather than artificial contrivances. Then the 1960s inaugurated a continuing glorification of thinness.

Today’s super-skinny fashion model is actually quite different from the women men find most attractive. At 70 inches she is taller than 99 percent of American women, and her weight of 115 pounds gives her a BMI of just 16.5, lower than that of most women in food-poor countries like Bangladesh. Two-thirds of fashion models have BMIs of less than 17, compared with just 6 percent of Playmates. While the fashion model’s waist is larger than a Playmate’s, her hip size is much smaller relative to her height, just 46 percent vs. 53 percent in Playmates.

Very tall women can afford to have relatively smaller hips (their height makes them look even smaller) because once hips are wide enough to accommodate babies fairly easily, there is little to gain from being larger. But the very narrow hips of a fashion model make her appear to be exceedingly slim—even slimmer than a typical slender teenage boy who has hips about 49 percent of his height. To look very thin, models have to be very tall in addition to having very low BMIs.

Fashion models also look skinnier because they are much less buxom than Playmates. Contrary to what you might expect, Playmates actually have smaller bust sizes than typical coeds, though they are large for their BMIs. But even though a fashion model is typically four inches taller than a Playmate, her average bust size is three inches smaller, and is just 46 percent of her height compared to the Playmate’s 54 percent. Having smaller busts and hips in relation to her height makes a fashion model much less curvy—Playmates are 36 percent more curvy than fashion models. No surprise that men aren’t lining up to look at the latest issue of Vogue or Harper’s Bazaar.

Nevertheless, super-thin women sit unchallenged atop fashion’s pedestal, although it’s not clear why current fashions idealize a super-skinny shape. Being very thin is certainly rarer today and attracts . In addition, clothing that hugs a curvy body is difficult and time-consuming to make—all those darts and tucks significantly add to the cost of making women’s clothes.

The separation of design from manufacturing operations in the fashion industry demands the patterning of clothing not by draping fabric on a figure but by sketching on paper, which not only consigns bodies to a flat two dimensions but is instantly faxable to distant factories. Untethered from real bodies, sketches are subject to distortion limited only by a designer’s imagination. There’s a higher profit margin in linear styles, although they look good only on an equally linear body.

If the fashion ascendance of super-thin women has not had a lasting impact on women’s body shape, it has certainly had a huge impact on women’s patterns of thinking: Women are much less satisfied with their own bodies after viewing images of the super-skinny models featured in women’s fashion magazines.

More significantly, this ascendence has skewed women’s perceptions of what men like. When asked to predict how men will rate women of different body shapes, women choose thinner figures than men do, figures more like fashion models than like Playmates. Here’s the kicker: The shapes that men actually choose are closer to those of the women making such predictions! In other words, women are usually more like men’s ideals than they realize, so that losing weight may not make them any more attractive to men.

Men’s minds are better than women’s at understanding women’s bodies, because there’s so much at stake for men. Not all men can win the woman of their , but their dreams drive them to compete for women with more optimal body shapes. Winning them is their best guarantee of having children with good brains, likely to be more successful in all the activities required to sustain life—and to pass on their genes.

Better Living via Better Food

While all of us are genetically programmed to enjoy calorie-rich fat, we could never have the weight-promoting, omega-imbalanced fats we consume today without the industrial chemical processing of vegetable oils and the force-feeding of corn and grains to immobilized animals meant to graze on grass. Such fats are not as satisfying as the omega-3-rich fat we evolved to eat—fat from green plants, wild fish and seafood, and the meat, milk, and eggs of animals eating their natural diet of grass. Less satisfied, we eat more. Studies show that we are hungrier after a meal high in omega-6s than after one with more omega-3s.

Fortunately, we are free to change to a diet more like the one we had when most women were content with how they looked. It’s the diet most Europeans still have—with much more of the omega-3 fats that build good brains and healthy bodies, and much less of the weight-promoting omega-6 fats. We have tried fooling Mother Nature for at least 40 years; maybe it’s time to stop.

A Very Hip Head Start

The DHA-rich fat deposits that give women curvy hips and thighs are an almost irresistible nonverbal message to men. It not only makes a woman visually attractive to males, it also signals that she has plenty of brain-building fats to confer on progeny—nature's own Head Start. And it indicates that such a woman is also likely intelligent, herself a beneficiary of those brain-buffing fatty acids.

But that's only the start of the cognitive benefits of hip fat, insists Gordon Gallup, Jr., who believes that gluteofemoral fat boosts social intelligence in women. It makes them good at understanding the mental states of others. While a low waist-hip ratio accounts for 7 percent of the variance in general intelligence, in studies he conducted at the University of Albany (New York), where he is professor of psychology, he has found that it accounts for a sizable 20 percent of variance in social intelligence.

Skill in reading the minds of others, Gallup says, is a huge mating advantage for such women—and they definitely need one. Given their attributes, "they are particularly likely to be targeted by males for dating and ." In fact, studies by others show that women with a low waist-hip ratio lose their virginity sooner than other women and have more sex partners. But their social intelligence shields them from dishonest courtship; it enables them to dope out disingenuous claims of commitment.

The same social intelligence probably boosts their mothering ability too, Gallup hastens to add. "It makes women better able to respond to their children," he says, although he has yet to put this hypothesis to a test. —Hara Estroff Marano

Shape Shifts

The narrow waist and the curvy hips and thighs of the average Playboy Playmate (center) exemplify the body shape that men the world over prefer—while women hope to look more like a fashion model (left) but increasingly embody the proportions at right.