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  LIKE EVERYTHING ELSE, breast size and shape, as well as areola size and color, can vary widely in humans. The color of the areola is especially variable, ranging from quite dark to very pink in some fair-skinned individuals. Large areolae are important visual cues, creating the illusion of a larger breast. Breast size may send important visual signals about a woman’s potential fertility. Anything that appears to enhance breast size may make a woman more attractive; hence, the importance of the areola.

  The average size of breasts actually seems to be getting bigger: one recent British report indicated that the average breast had gone from a 34B to a 36C in just ten years. Pregnancy and breastfeeding have a significant, yet somewhat reversible, impact on breast size as the mammary glands expand and then fill with milk. In the average breast of a woman who is not producing milk, the ratio of glandular to fatty tissue is about 1:1; in a lactating woman it’s more like 2:1. And areolae often get considerably darker and somewhat larger during pregnancy and can stay that way after delivery, which may help babies find their mother’s nipples. During our history, when clothing was more optional and polygamy the norm, larger and darker areolae may have been a badge of fertility, signaling the possibility of past pregnancy to interested onlookers.

  So why are the breasts of human females so different from those of all other female primates? Since it’s the fatty tissue that gives them their distinctive roundedness, we should look to the fatty tissue for an explanation. All kinds of theories have been offered—the fatty tissue protects the mammary glands and keeps milk warm; it provides an anchor, a substitute for maternal fur that other primate babies cling to when feeding; large, round breasts are a signal to males that their owner is fertile and has the biological resources (in reserve) to be a mother. There is also likely a connection to sexual attraction, since we know that breasts, including the nipples, can swell by as much as 25 percent when a woman is aroused.

  Zoologist and bestselling author Desmond Morris believes that female breasts are actually a mimic of the buttocks. Among most primates, the male mounts the female from behind; the bright red coloring on some female buttocks around the genitals acts as a sexual signal. Morris theorizes that as humans became upright and bipedal, the optimum sexual position became face-to-face and females evolved twin globular breasts to mimic the twin globular cheeks of the buttocks.

  When it comes to breasts, there are lots of theories. But it may also be that the best answer is the simplest—the fat stores are like an insurance policy, they’re there to provide energy to potentially pregnant or nursing women when food is scarce. Ample breasts may also send a signal, to anyone who might be interested, that a woman has sufficient fat to support having and nursing a baby.

  Men also have breasts and nipples, of course, but what you may not know is that they have mammary glands too. Although it’s rare, as compared to women, having breast tissue means that men can also get breast cancer. Under most circumstances, male mammary glands are essentially dormant and men do not lactate, but under certain conditions, men’s breasts have been known to produce milk. For example, some prostate cancer patients have received female sex hormones as part of their treatment to slow the growth of their cancer, and those hormones have sometimes triggered male lactation. And transsexual men on high doses of estrogen may also respond to nipple stimulation with lactation.

  Men experiencing extreme starvation have also been known to lactate. It is thought that starvation triggers prolactin secretion from the anterior pituitary (located at the base of the brain), causing the male mammary glands to produce milk.

  Although it has yet to be fully studied (medical ethics thankfully don’t easily allow us to deliberately starve men just to test the hypothesis), male lactation may just be an evolved response that allows men to produce milk to feed their babies in times of extreme starvation. It is also not uncommon for newborn boys and girls to produce breast milk for a week or two after they’re born. Their infant mammary glands produce milk because their bodies are still flooded with hormones from their mothers, which they were exposed to in utero. These are the very hormones their mothers’ bodies produce to fill their own breasts with milk. Lactation in newborns, which is perfectly harmless, is sometimes called witch’s milk. Myth has it that witches looking to feed their familiars were stealing it from helpless babies.

  When it comes to breasts and nipples, two of each is the norm, but this is by no means an ironclad rule. Why two? It’s all about litter size: humans tend to have one or two babies at a time, so two breasts, with two nipples, usually does the trick.

  But there is at least a 5 percent chance that an extra nipple will occur—in men as well as in women. Former rapper turned actor Mark Wahlberg has a third nipple; so does British singer and talk-show host Lily Allen. Technically, they’re called supernumerary or accessory nipples, and they usually occur along the “milk line,” which runs from the armpit, through the normal nipple, down through the groin, and ends at the inner thigh. “Usually” is the operative word—they’ve been documented as far away from the chest as the bottom of the foot!

  Supernumerary nipples can range from a patch that looks like a mole to a complete third breast, with nipple, areola, and milk-bearing mammary glands. In 2005, researchers from the UK discovered the Scaramanga gene, aptly named after the villain from the James Bond novel and film, The Man with the Golden Gun, who had three nipples. The Scaramanga gene, now called Neuregulin 3, was initially reported to be involved in breast development in mice. A third nipple is not only a physiological curiosity. A case report in the New England Journal of Medicine in 2005 described a forty-two-year-old woman with what appeared to be a mass near her breast. A biopsy later revealed that it was an adenocarcinoma, a type of cancer that originates from glandular tissue. In this case the cancer most likely arose from the woman’s third nipple.

  Nipples in both men and women are filled with many nerve endings that can be a source of sexual pleasure when stimulated, ranging from mild to intense. Some women have such sensitive nipples that they can experience orgasm from nipple stimulation alone.

  BREASTS AREN’T THE only way girls’ bodies change over the course of puberty. The increased volume of estrogen coursing through their bodies causes their pelvis and hips to widen. It also dramatically alters the relative amount and distribution of body fat, depositing fat on the hips, buttocks, thighs, and mons veneris or mons pubis (Latin for pubic mound, the pad of fat underneath a woman’s pubic hair in the area above her genitals). Before puberty, the average girl has 6 percent more body fat than the average boy her age; by the time puberty is completed, she has almost 50 percent more.

  The extra fat stores on the hips, buttocks, and thighs probably serve the same purpose as the fat stores in the breasts. As we’ve discussed, fat tissue is a form of portable energy storage, and fertile females are likely to need additional energy for pregnancy and nursing, especially if they are migrating long distances. And, of course, the reason for a woman’s wider hips and pelvis (without regard to fat accumulation) is pretty straightforward—it makes childbirth more reasonable, by increasing the size of the birth canal.

  But here’s where it gets interesting: despite the current obsession with supermodel waifs and androgynous shapes, it seems most gentlemen prefer hips. Across cultures and throughout history, the classic hourglass figure—relatively narrow waist, wide hips—is considered the standard for female attractiveness. Researchers at the University of Texas at Austin worked with colleagues in China and India to examine thousands of works of American, British, Chinese, and Indian literature, some dating as far back as the first century A.D. to the present. And without exception, when waists came up romantically, they were narrow; when hips came up, they were wide; when breasts were discussed, they tended to be large, although there were certainly some exceptions.

  In the early seventeenth century, the British poet John Harrington described a beautiful woman this way:

  Her skin, and teeth, must be clear, bright,
and neat…

  Large breasts, large hips, large space between the browes,

  A narrow mouth, small waste[sic]…

  Other social research has produced a similar result. In Why Sex Matters: A Darwinian Look at Human Behavior, Bobbi Low, a professor at the University of Michigan, writes: “Across all sorts of cultures with quite different specific ideas about beauty, both men and women see as most attractive a female waist-to-hip ratio of about 7/10 to 8/10.”

  Why?

  Well, here’s one thing we know for sure: women with hourglass figures are more fertile. In 1996, Harvard researchers Peter Ellison and Susan Lipson linked higher levels of the hormone estradiol at the right time to higher fertility. A 2004 Polish study that included Ellison and Lipson concluded that women with large breasts, narrow waists, and noticeably larger hips had 30 percent higher levels of estradiol overall and mid-cycle, at the time of peak fertility, than other women. Grazyna Jasienska, the leader of the Polish team stated: “If there are 30 percent higher levels, it means they are roughly three times more likely to get pregnant.” Not everyone might agree with Dr. Jasienska’s conclusions, but if hourglass figures go hand in hand with higher estradiol, and higher estradiol means higher fertility, then women with hourglass figures are more likely to conceive and pass their genes on—which means evolution will favor hourglasses too.

  Devendra Singh, the psychologist behind a University of Texas study of romantic literature, has another theory. Medical research today shows that abdominal fat poses a very different level of health risk than hip and buttocks fat. People with large amounts of belly fat—so-called apple shapes—have a higher risk of heart disease, diabetes, and various cancers than those who carry their fat on their buttocks, hips, and thighs—so-called pear shapes. Dr. Singh thinks people may be programmed to prefer narrow waists because they know they’re healthier. Of course, it may be social programming, not genetic programming—we may tend to choose thinner partners today because we equate thinness with health. And, of course, in some cultures, where food is historically scarce, a preference for visibly larger women may have developed because a bigger size is a better indicator of good nutrition and thus fecundity (increased level of fertility). Anyone who’s ever been to an art museum or paged through an art history book with a section on Renaissance or Baroque art has seen portraits of women who certainly seemed to have had healthy appetites—and the wealth to sate them.

  Even more fascinating, a recent study suggests that curvy moms have more clever kids. William Lassek of the University of Pittsburgh and Steven Gaulin of the University of California, Santa Barbara, used data from the National Center for Health Statistics to show that children whose mothers had wide hips and a waist-to-hip ratio of 7 or 8 to 10 routinely scored higher on intelligence tests. It turns out there’s a possible explanation—hip fat contains specific fatty acids acquired through the mother’s diet that are critical to development of the brain in fetuses.

  IN THE HIERARCHY of attraction, emerging research suggests that one physical characteristic trumps all others—symmetry. By symmetry, I mean exactly that—eyes the same shape, dimples on both cheeks, legs the same length, hands the same size—you name it, left and right sides the same. Across the animal kingdom, males and females find the opposite sex more attractive when their left and right sides match, and humans are no exception.

  While some people may find a man born with a crooked nose ruggedly handsome and while Marilyn Monroe’s beauty mark may be the asymmetrical exception that proves the rule of her otherwise symmetrically beautiful face, in study after study, the more symmetrical a face, the more attractive members of the opposite sex find it. When pairs of body parts don’t match exactly—a right foot bigger than a left foot, a grin that curls up only on one side—it’s called fluctuating asymmetry. More on the sexuality of symmetry later, but for now, let’s consider one more interesting connection between symmetry, sexual attraction, and success in the evolutionary endgame—reproduction.

  One of the more noticeable examples of fluctuating asymmetry occurs with female breasts. Many women have breasts that are not the same size or exact shape; in fact, it’s actually quite common. Having asymmetrical breasts doesn’t affect a woman’s ability to nurse, but it may affect her overall health. A British study published in the journal Breast Cancer Research in 2006 found that women with significantly asymmetrical breasts are at much greater risk of developing breast cancer. According to the authors, “Asymmetrical breasts could prove to be reliable indicators of future breast disease in women, and this factor should be considered in a woman’s risk profile.” At the same time, women with evenly matched breasts also appear to be significantly more fertile than women with asymmetrical breasts.

  So, if men are more attracted to women with symmetrical breasts, they’re also more attracted to women who tend to be more fertile and possibly healthier. To be clear, this is about fertility, not lactation. Large breasts and small breasts are equally capable of producing plenty of milk, even if the same woman has one of each. The mammary glands inside the breasts do the work; it’s the fat that surrounds those glands that gives the breasts their size and shape.

  Breasts are the most prominent female secondary sex characteristic in humans. But as we know from other primates, they don’t need to be large and prominent to succeed at their apparent primary purpose, the feeding of infants. And as I’ve mentioned, the fat stores are likely there to provide backup energy for pregnant and nursing women. But maybe those fat stores serve another role too.

  In many species, males or females (and sometimes both) compete for the sexual attention of the other. Different characteristics make individuals of a given species more or less attractive; everything from behavior, like aggression and dominance, to physical characteristics, like the color of an animal’s rump, can come into play. Individuals with the more attractive traits are more likely to mate, which means they are more likely to pass their genes on to the next generation, including the genes for whatever traits make them more attractive. The evolutionary process that selects for those sexually appealing traits is called sexual selection. Many of these traits are secondary sex characteristics that serve to advertise the individual’s wares—for instance, a peacock’s tail feathers or a stag’s antlers. And just as corporations allocate enormous sums of money to advertise their products, the body has to consume food and spend energy to create these physical advertisements and keep them at peak appearance. Despite their cost in terms of resources or energy to create and maintain, large, symmetrical breasts are very valuable because they give the individual who possesses them a better chance of attracting a good mate. So it may be that, for humans, breasts function as a kind of cost-effective signal, an easy visual shorthand, flashing a possible projection of future fertility. If you have large breasts, you may have the physical resources, in fat stores and energy, to successfully get pregnant and nurture a child. In some ways, a large buttocks can be just as useful.

  BEFORE WE PROCEED any further—a quick note on terms. The main female sex organ is commonly referred to as the vagina, but technically speaking, the vagina is all on the inside; it’s the passage that extends from the outside of the body to the uterus. The vulva is the part of the female sexual anatomy that appears on the outside. Female genitals have had many names and nicknames throughout history, of course—you’re almost certainly familiar with some of them. From the late sixteenth century to sometime in the eighteenth century the vulva was referred to as the “hey nonny-no.” In the nineteenth century it was the “upright grin.” In the 1960s, it was affectionately referred to as a “furburger.” We’re going to stick with proper names, though—vulva on the outside, vagina on the inside.

  The vulva itself has multiple components that have distinct, but related, and sometimes overlapping, functions. At the top is the soft area of fatty tissue usually covered with pubic hair called the mons pubis or the mons veneris, the “mound of Venus,” after the Roman goddess of love. A version of the m
ons also exists in men. This fat pad is no accident: it actually develops at puberty after hormonal stimulation of the area. Why? Cushioning! It acts like a pillow between partners, protecting the pubic bone during intercourse. Without it, the banging of pubic bone on pubic bone could be a seriously bruising distraction. And, in one of the many ways that humans have evolved to encourage reproduction and make sex more fun, our bodies protect the mons. Even after extreme weight loss, the fat pad of the mons is still there. As Dr. Elizabeth G. Stewart notes in her guide, The V Book, “even the skinniest Hollywood actress is as comfortably upholstered for intercourse as her size-12-and-up counterparts in the real world.”

  Around the time of thelarche, when a young girl’s breasts begin to bud, a girl may notice pubic hair, emerging around the labial lips and moving up to cover the mons. This is called pubarche, and lasts for about six to twelve months. Most women typically end up with a triangle shaped pubic hair distribution. Boys also go through pubarche and grow pubic hair, but instead of triangle shaped, most men’s distribution is diamond shaped.

  So why do we have pubic hair? Well, one of the possibilities is that it serves as a human perfume factory. Here’s how.

  Besides the tops of our heads, the two other prominent places that tend to be covered with hair in men and women, after puberty, are our underarms and our genital areas. These areas are home to numerous specialized apocrine sweat glands. These glands secrete sweat that contains fats and proteins. When those compounds are broken down by the microorganisms that naturally make their home in the warm environments under our arms and between our legs (and the rest of our bodies in fact), they produce the distinctive odor that is our very own scent signature. And that scent is a key player in the chemistry of attraction.