CHAPTER SIX

THE BRAIN OF THE BEHOLDER: BEAUTY AND SEXUAL ATTRACTION

IN EARLY OCTOBER 2000, TWO TECH-SAVVY TWENTYSOMETHINGS named James Hong and Jim Young were sitting around drinking beer. Hong was an unemployed computer engineer, and Young was a Berkeley grad student getting a joint degree in electrical engineering and computer science. The moment of inspiration came “when a comment Jim made about a woman he had seen at a party made us think, wouldn’t it be cool if there was a website where you could tell if a girl was a perfect ten?’’ Hong later recalled.

It might have stopped there, but these were Bay Area computer geeks in the dot-com age. Within a few hours, they had mapped out the idea and Young began writing code for a site that let users post photos and rate other people’s looks. Before a week had passed, they took their site live, with ten photos of friends and a catchy name: Am I Hot or Not?¹

When word got out online, things went viral and the response was overwhelming. Within twenty-four hours, the site had more than 150,000 views, and Hong and Young panicked and shut it down. But realizing they had a runaway hit on their hands, they quickly leased additional servers and reopened. By December, less than two months after their beer-soaked epiphany, the site was getting nearly 15 million hits per day.¹

The concept of attractiveness ratings had clearly struck a chord. Predictably, others jumped in the game and Am I Hot or Not spawned a series of copycats, though none were as successful. In 2003 Mark Zuckerberg, a sophomore at Harvard, got into trouble for creating a “hot or not”–style site, posting students’ ID photos so that other students could rate their “hotness.” Within days, amid overwhelming traffic and complaints from fellow students, he had to close it down.² But it turned out okay for him. Three months later he tried again, launching another website from his dorm room. It was called Facebook.

What was it about judging people’s appearance that was so irresistible? As the critic David Denby noted, it may tap into our culture of “snark,” where snide invective has become the dominant idiom.³ But it may also have tapped into something more enduring about the human mind. Converging evidence from evolutionary biology, psychology, and neuroscience suggests that our brains have a biological interest in assessing the attractiveness, beauty, and sexual appeal of other people. Even without the facile clickability of a website, we’re constantly making judgments about who’s hot and who’s not.

In this chapter, we’ll explore what makes us attracted to each other and what the challenge of choosing a mate tells us about the brain and its vices. Along the way we’ll encounter some intriguing questions: What’s the difference between a beautiful face and one that only a mother could love? Could supermodels owe their careers to parasites? Why do gay men have more older brothers than heterosexual men do? Can there be disorders of desire?

HOT TOPICS

WE ALL KNOW THAT SEX SELLS, AND ADVERTISERS HAVE CREATED something like a nation-state of beautiful, sexy people who inhabit the two-dimensional terrain of magazines and television. The “beauty industry” offers eager consumers an endless supply of new solutions to the scourges of imperfection and aging. People are willing to endure an astonishing battery of physical insults—skin exfoliation, teeth bleaching, toxin injections, and surgical procedures—to defend their ideals.

Where do these ideals come from? Perhaps the most fashionable answer is to blame that ubiquitous villain, the media. But even if it were true that a cabal of media types conspire to impose their will on a helpless public, they would fail without mass collusion. How did these images become so powerful? Why exactly does sex sell? Are ideals of beauty and sexual attractiveness based in the biology of the human mind? Rather than defining standards of beauty, could advertisers and plastic surgeons be tapping into a well of aesthetic preferences that evolution prepared our brains to favor?

SKIN-DEEP OR HARDWIRED

IN 1991 NAOMI WOLF’S BESTSELLER THE BEAUTY MYTH ARGUED that modern Western conceptions of female beauty and sexuality are the creation of a male-dominated power structure determined to “mount a counteroffensive against women.” She tried to debunk the notion that biology plays a role in standards of attractiveness, calling that notion a “beauty myth” that was born with the Industrial Revolution and disseminated by the unprecedented reach of male-dominated mass media. As she put it:

The beauty myth tells a story: The quality called “beauty” objectively and universally exists. Women must want to embody it and men must want to possess women who embody it. This embodiment is imperative for women and not for men, which situation is necessary and natural because it is biological, sexual, and evolutionary: Strong men battle for beautiful women, and beautiful women are more reproductively successful. Women’s beauty must correlate to their fertility, and since this system is based on sexual selection, it is inevitable and changeless. None of this is true. (p. 12)⁴

One problem with Wolf’s argument is that it’s a bit of a false dichotomy. Like the outmoded nature vs. nurture argument, Wolf makes it seem as if it’s all biology or all politics. She is surely right that ideas about beauty or sexuality are not objective or changeless, but it’s hard to imagine a mainstream biologist making such a claim.

Clearly, ideals of beauty have varied over human history and across human societies. Even within developed Western nations, iconic images of female and male attractiveness have shifted substantially. Between 1953 and 2003, the average Playboy centerfold gradually grew thinner (although the very thinnest centerfolds were actually more common in the 1950s).⁵

Increasingly, images of beauty have become . . . well, unnatural. On the occasion of Barbie’s fiftieth birthday, the BBC News Magazine applied Barbie’s dimensions to a real-life woman. Their volunteer was a twenty-seven-year-old woman named Libby. Holding her height constant at 5’6” and giving her Barbie’s dimensions turned Libby into a tiny-waisted waif. But if her waist was held constant, the only way to achieve Barbie’s proportions was for her to sprout to an Amazonian height of 7’6”. Male action figures experienced a similar transformation. From the 1960s to the 1990s, GI Joe, Luke Skywalker, Batman, and the like have practically exploded into heaps of muscularity. Harrison Pope, the psychiatrist we met in Chapter 1, has been studying the vagaries of body image and its cultural transformations and found that if you extrapolated the dimensions of modern action figures to the size of a 5’10” man, they would have physiques “far exceeding the outer limits of actual human attainment” (p. 70).⁶

Even if there is no media conspiracy, let’s just say the weight of the evidence does support an effect of media exposure on our internalized ideals of attractiveness. Among women, exposure to mass media portrayals of thinness as a female ideal are associated with an internalization of the thin ideal and greater dissatisfaction with their own bodies.⁷

So is there any reason to believe that our responses to sexual attractiveness and beauty have a biological component? There are two lines of evidence to consider when we answer that question. One is about what scientists call “proximate” causes of behavior and the other is about “ultimate” causes. Proximate causes are the specific brain and behavioral mechanisms that control our behavior. Ultimate causation has to do with why and how these biological mechanisms might have arisen in the first place. And “ultimately,” it’s about evolution.

IS BEAUTY A THING OF THE PAST?

WHAT’S THE EVOLUTIONARY ARGUMENT FOR WHY OUR MINDS AND brains are biologically attuned to beauty and sexual attraction? It boils down to the fact that sexual attractiveness is related to reproductive success and that makes it susceptible to natural selection. Now, the field of evolutionary psychology is sometimes accused of spinning “just-so stories”: begin with an observation about human behavior and come up with a story that relates it to our Pleistocene past. And critics have challenged its assumptions about the relevance of our ancestral past to our modern minds.⁸

But let’s start with a few notions that ought to be uncontroversial (unless you’re a Creationist). Humans are the product of natural selection, and natural selection favors traits and behaviors that increase the likelihood of transmission of an animal’s genes. To reproduce, you need to find and attract a mate. If other humans of your sex are around, you may need to compete for mates. And if your goal is to produce offspring who have the best shot at reproducing themselves, you’d do well to be able to discriminate among potential mates. Basically, you want to choose a mate who maximizes the probability that your offspring will survive and reproduce.

So, from an evolutionary standpoint, the argument is that we will be attracted to features that advertise that a potential mate is a good bet for successfully transmitting our genes into the next generation. Natural selection would have promoted physical and behavioral traits that are effective signals of a high-quality mate. The more effective we are at convincing a potential mate that we have desirable traits, the more desirable and therefore reproductively successful we would be.

Ideas about the evolution of sexual attractiveness have undergone their own evolution, beginning, appropriately, with Darwin. In 1871, with the publication of The Descent of Man, and Selection in Relation to Sex, Charles Darwin “dropped the other shoe,” as the biologist E. O. Wilson put it. Twelve years after the publication of On the Origin of Species, which laid out the argument for evolution by natural selection, he introduced the concept of “sexual selection.” The idea emerged from Darwin’s recognition that many traits and behaviors in the animal kingdom do not have obvious advantages for survival. From the dramatic plumage on male peacocks and the euphonious song of the nightingale to the elaborately branched antlers of the red deer and the brilliantly colored face of the mandrill—Darwin saw these traits as puzzles to be solved. They require a lot of energy to build and maintain, creating an opportunity cost that must have been outweighed by some kind of benefit. Darwin’s solution was that these ornaments and displays were an expression of sexual competition. They were the outcome of a process of sexual selection that favors traits that improve the odds of mating.

For the most part, Darwin observed, the pressure was on males, who needed to compete with one another for access to females. That may have sparked a sexual arms race that made males bigger, meaner, and tricked-out with weaponry like big muscles, fangs, and horns. They also needed to impress the ladies, which is where peacock tails come in—they’re part of a charms race.

In 1972, evolutionary biologist Robert Trivers contributed another fundamental insight. His parental investment theory argued that mating strategies are shaped by the relative amount of investment a male or female has to make in order to increase their offspring’s chance of surviving and reproducing. He wrote: “ . . . the sex whose typical parental investment is greater than that of the opposite sex will become a limiting resource for that sex” (p. 141).⁹ In most species, including humans, females make the greater investment and males often have to compete for mating rights and guard their mates so that they’re not cuckolded. Under some circumstances, males will do best by inseminating multiple females and investing little. But females can even the score by favoring males who are willing to stick around and provide resources for their young. So sexual selection should impel females to choose mates whose appearance and behavior signal both “good genes” and “good parent.” Of course, under circumstances where offspring have a much better chance at surviving with two parents around, both males and females will have a vested interest in long-term pair bonding, also known as monogamy.¹⁰

Decades of field research have largely upheld the original predictions of sexual selection and parental investment theory, but it’s also clear that the variety of mating behavior in the animal kingdom is more complex than first thought.¹¹ Today, most evolutionary biologists believe that sexual selection has shaped male and female mating strategies—that is, how, when, and with whom we have sex. Among primates, mating strategies range from monogamy (in gibbons, for example) to polygyny (as in a male gorilla and his harem). Polyandry—that is, females with male harems—is relatively rare. Humans, of course, practice a wide range of mating behaviors—across and within cultures and even within a given lifetime. Sometimes we’re monogamous and sometimes we’re polygamous.^* Humans have a menu of mating strategies that we can and do pursue—depending on what our options are.¹² Sometimes it pays to pursue “short-term” mating and sometimes it pays to commit. And that has implications for what’s hot and what’s not.

MIND YOUR PLEASING CUES

IF WE ACCEPT THAT OUR ANCESTORS HAD REASON TO DISCRIMINATE among potential mates, the next question is what cues did they use to do it? Unfortunately, we can’t ask them, so this is where things get more theoretical (and more controversial). But the answer would have depended on what strategy they were pursuing. Since the costs of bearing a child are generally greater for females, we’d expect them to have looked for longer-term mates—males who have resources to invest in their offspring and a disposition that suggests they’re willing to be a provider and a partner. Being the physically weaker sex, females might also favor males who show signs that they can defend a family. For shorter-term trysts, women would presumably look for a male who looks like he’s got good genes—that is, a guy who’s high-status, strong, and healthy.

For males, the costs of child bearing are lower and having more sex partners could mean having more offspring. If the goal is maximizing your reproductive success, the most important cues you should respond to are those signaling fertility. That means youth, good health, and sexual characteristics that reflect reproductive hormones like estrogen. Under some circumstances, the evolutionary view predicts that males should be more likely than females to seek short-term sex and multiple partners. Of course, in difficult environments, where two-parent children do better, men would do better looking for good mothers and partners.

These are the kind of predictions that set many people on edge, in part because they are often oversimplified and taken as justification for sexist behavior. The pop culture version of evolutionary psychology’s view is that men have a biological imperative to “spread their seed” and women are innately gold-diggers who just want to seduce high-status men. But that’s a pretty poor rendering of the argument. What’s worse is that it’s often accompanied by the classic “naturalistic fallacy”: the idea that what’s natural is right. But just because we have some kind of evolved predisposition doesn’t mean that that’s how we ought to behave.

The reality is a bit more nuanced. Evolutionary theory gives us a framework for understanding how the human mind developed, what its biases are, and why it doesn’t behave like a blank slate. But nowhere does evolutionary theory claim that our mental mechanisms are independent of context or insensitive to environment or culture, or that there isn’t a range of normal individual differences. We don’t live in the Pleistocene anymore. The idea that our brains evolved in response to conditions that held in our evolutionary past doesn’t mean that they are well-adapted to the modern world or that we can’t use them differently in our enlightened present.

So let’s look at the evidence. Are men really more inclined to promiscuity? Many studies supporting the evolutionary account are based on asking people in Western cultures (often college students) about their sexual behavior, a rather narrow test of the theory.^* But if standards of attractiveness and mate choice are simply the creation of a male-dominated media culture, we’d expect to see lots of variation in different parts of the world. Yet the data are fairly consistent.

A survey of more than fourteen thousand people from forty-eight countries on six continents found that men reported more promiscuity than women in every country. In cultures where men outnumbered women, monogamous behavior was more common. This may be because in those situations women are able to dictate the terms of mating when demand for women outstrips supply.¹⁵ Monogamy was also more common in cultures where rearing offspring was precarious and the need for parental investment was higher. These were countries where rates of malnutrition, low birth weight, and infant mortality were high. In nations with the highest levels of gender equity (e.g., more equal wages, more women in government), women were more promiscuous, but they were still well behind the men. Of course, the problem with studies like these is that they are based on correlations and self-report. They can’t tell us much about why men and women report different sexual behaviors or how much of that has to do with biology or culture. But there are other reasons to think that evolution has had a hand in how we judge attractiveness and in shaping our sexual desires.

HOT GENES

PEOPLE FROM DIVERSE CULTURES SEEM TO AGREE ON CERTAIN elements of attractiveness in male and female faces, and preferences for attractive faces develop as early as infancy, well before we are exposed to the world of supermodels.¹⁵ From an evolutionary standpoint, being attracted to a face is like tasting sweetness. We taste sweetness, not because there is something inherently “sweet” about a strawberry or a candy bar, but because natural selection shaped our brains to experience something pleasing when our taste receptors encounter sugars. Animals whose brains responded in this way would have been drawn to foods that provided ready sources of energy. In this same way, there is nothing inherently beautiful about a face, but beauty acts as a cue about the value of a potential mate. So what are we responding to when we see beauty?

Each of our bodies is a vessel for the genes we inherit and transmit. They pass through us in a relentless quest to carry on and live to see another generation. Genes that enhance successful reproduction will out-compete those that don’t. For evolutionary biologists, there’s a clear implication: we’d expect animals to have developed mechanisms for evaluating the quality of potential mates.

If we accept that natural selection acts at the level of genes, desirable mates would be those that have good genes.^* So what are good genes? There might be many answers to that question, but a few seem particularly relevant to reproductive success—starting with genes that promote healthy development and disease resistance. And, in species where parental investment is an issue, the sex that has to invest more resources would also do well to pick up on signs that a potential mate is a good investor to avoid being “left holding the baby.”

Researchers have tried to test these predictions by looking at what people and other animals find attractive and seeing how well they line up with expectations. Most studies have highlighted three features that would have ranked high on a Pleistocene version of Am I Hot or Not. And each of them could be an advertisement for “good genes”: averageness, symmetry, and sexual dimorphism.

AVERAGE IN THE EXTREME

WE TEND TO EQUATE BEING “AVERAGE” WITH MEDIOCRITY. BUT when it comes to physical traits, the average can be extraordinary.

By many accounts, the greatest racehorse of all time was a chestnut-colored thoroughbred named Eclipse. Born in Berkshire, England, on April 1, 1764, the day of a great solar eclipse, he began racing in 1769. For the next seventeen months he was undefeated in race after race. He didn’t just win—he crushed the competition. In fact, in 1771, after an unblemished record of victory, he was retired, mainly because of a lack of competition. He was so revered as a champion that after his death, his skeleton was preserved in the hope that veterinarians could one day divine the secret of his perfection. In 2004 Dr. Alan Wilson, an expert in biomechanics and veterinary medicine, led a team of scientists at the Royal Veterinary College in an effort to decipher the mystery of Eclipse’s magic. Starting with precise measurements of his skeleton, they used computer modeling to reconstruct the dynamics and mechanics of his movement. And they found something remarkable.

Eclipse was perfectly average. The shape and length of his legs were at the midpoint of the range of modern horses. And “being right in the middle of normal,” Wilson surmised, may have been the secret of his success: the very averageness of his proportions gave him just the right balance of flexibility and strength. As Wilson put it, “When they all come out optimum, it looks like average.”¹⁶

But Eclipse was not only great on the track—he was also great in the sack. After retiring from racing, he began a long and legendary career as one of the top sires of all time. He fathered 344 winning horses and it’s been estimated that 80 to 95 percent of all living thoroughbreds have traces of Eclipse in their bloodline.^17, 18

The link between averageness and good genes has implications for what we find attractive in each other. The surprising fact is that beautiful people are more likely to have average than unusual features. Actually, average in this context doesn’t mean “ordinary,” but rather features that are roughly in the statistical middle of the features seen in a population.

It was Francis Galton, Darwin’s half-cousin, who first discovered the beauty of the average. In an effort to study the essential features of different “types” of people (criminals, consumptives, Englishmen), he developed a technique he called “composite portraiture” in which he aligned and superimposed photographic plates. By doing this, he was able to create a composite, statistically averaged face which he could then take as the exemplar of the group of faces he had combined. In 1881, while describing his technique before the Photographic Society, he commented on the surprising image he got when he averaged the faces of a group of consumptive men whom he had selected for their ill-appearance: “The result is a very striking face, thoroughly ideal and artistic, and singularly beautiful. It is, indeed, most notable how beautiful all composites are”(p. 272).¹⁹

In fact, Galton saw a business opportunity for the Society’s members: make a composite family portrait. People would love it. “The result is sure to be artistic in expression and flatteringly handsome. . . . Young and old, and persons of both sexes can be combined into one ideal face. I can well imagine a fashion setting in to have these pictures” (p. 273). Galton’s technique never caught on, but his insight about the beauty of averaged faces had legs. Over the past two decades, researchers have used more sophisticated digital technology to create composite faces, and the results are in line with Galton’s original observation: the average is beautiful.¹⁵

So what’s so great about being average?

From an evolutionary perspective, averageness may be a signal of good genes. For one thing, genetic mutations and chromosomal abnormalities that interfere with development are likely to create an appearance that is unusual when compared to the average features of a given population. In fact, facial averageness has been correlated with better health.¹⁵ Average features may signal what geneticists call heterozygosity, which means there is a greater diversity of genetic variants within an animal’s genome. A face that approximates the average features of a group may reflect more diversity among the variants (alleles) in a person’s genes. Inbreeding, which is known to increase the risk of genetic disease, homogenizes an animal’s genome and makes it less varied. Many mutations cause problems only when an individual carries a double dose (that is, they are recessive), so having a mixture of alleles across the genome would reduce the risk of genetic disease. And like averageness, greater genetic heterozygosity (diversity) has been associated with better health.²⁰

A blending of alleles could also reduce the risk of infection by parasites and other pathogens. The major histocompatibility complex (MHC) is a region of the genome that’s packed with genes that control our immune responses. Over evolutionary time, human genomes have accumulated a tremendous variety of MHC alleles or variations to help us combat the endless number of parasites and other pathogens that threaten us. We have to be nimble to keep up with these continually evolving pathogens, and having a variety of MHC immune genes can be lifesaving. More diversity in the MHC translates to more options for resisting disease (up to a point). If averageness is a reflection of genetic blending, it may act as an advertisement to potential mates for a more optimal set of immunity genes.

But does MHC diversity make you hot or not? To address that question, one group of researchers took photographs of men’s and women’s faces and analyzed their DNA.²¹ Then a separate group of men and women rated the faces on overall attractiveness as well as averageness and symmetry. In males, MHC heterozygosity was indeed associated with higher ratings of both averageness and attractiveness.

If part of our attraction to potential mates is driven by a search for more diverse MHC genes for our offspring, maybe we should be drawn to people whose MHC genes are different from ours. The child of parents who have dissimilar MHC genes would have more MHC diversity. When lovers say “you complete me,” maybe they’re not kidding: when it comes to resisting disease, “good genes” really means “complementary genes.” Indeed, Oxford scientists found that married couples were much more likely to have complementary (dissimilar) MHC genes than random pairs of people, although, for unclear reasons, that was only true for European-American and not West African couples.²²

I assume you don’t have a genotyping lab in your pocket when you’re out on a date, so how are you supposed to know if a potential mate has MHC genes that are different from yours? You might be able to smell it on them: it turns out that MHC genes seem to influence body odor. It’s not clear exactly how this happens, but species as diverse as fish, lizards, mice, and humans seem to be able to distinguish MHC similarity by smell. To see whether MHC similarity influences sexual attractiveness in humans, most scientists have turned to the “smelly T-shirt” test. In a typical version of the study, men are asked to wear a T-shirt for a few days to capture their body odor. Women are then asked to rate the T-shirts according to how attractive or pleasing they are. For the most part, women in these studies seem to prefer mates who have dissimilar MHC alleles.^23, 24

One study actually addressed a more direct question about the MHC and mate choice: does MHC similarity predict sexual desire among couples? Christine Garver-Apgar and her colleagues at the University of New Mexico recruited forty-eight romantically involved couples and asked them a battery of questions about their sexual feelings for each other and for other people.²⁵ Consistent with the evolutionary hypothesis, women in relationships where the couples had similar MHC genes were less sexually aroused by their male partners, and their partners also rated those women as less sexually adventurous with them. The women in MHC-similar couples also reported cheating more on the partners in their current relationship, but not in past relationships, which rules out the possibility that these women are just more promiscuous.

When it comes to immunity genes, opposites attract.

TURN THE OTHER CHIC

AVERAGENESS HAS ITS LIMITS. THE WEIGHT OF THE EVIDENCE SUGGESTS that although average faces are usually more attractive than unusual faces, they are not necessarily the most attractive. There’s another beauty trait that, like averageness, may have evolved to signal disease resistance—symmetry. In general, more symmetrical faces and bodies are more attractive.^15, 26

Our bodies are billboards that advertise our genetic idiosyncrasies and the diary of insults and injuries we’ve endured: our limps, our scars, our sun-weathered complexions. But our evolutionary ancestors were even more likely to show their cards than we are. In the days before modern medicine and cosmetics, parasites and infections like leprosy might cost you an eye, take a patch out of your skin, or a chunk out of your nose. Asymmetries in the shape of your face or body might also signal a genetic problem in the development of body parts. Biologists refer to these small random differences between the left and right side of the face and body as fluctuating asymmetries. A potential mate who has fluctuating asymmetries might be one who has less desirable genes: more mutations that interfere with normal development and an immune system that’s more susceptible to disease. If our brains read symmetry as attractive because it’s a sign of disease resistance, supermodels may owe their careers in some small way to the parasites that long-ago shaped our biological “hot or not” meters.

Still, the effect of symmetry on attractiveness is small,²⁷ and sometimes asymmetry can be beautiful. Iconic beauties like Marilyn Monroe and Cindy Crawford are well known for their “beauty spot”—a mole on one cheek that many people find attractive.

Proving that nothing is beyond the reach of scientific study, researchers set out to answer that age-old question: Where’s the best place to have a mole? They showed images of women’s faces with various configurations of beauty spots (technically melanocytic nevi) to a panel of 250 male and female judges that included physicians and artists.²⁸ When it comes to facial moles, it turns out that location matters. The most attractive moles are unilateral and off to the side. The closer it is to the midline (like on your chin or the bridge of your nose), the less attractive it is. And symmetry fared worst of all—women with a mole symmetrically placed on each side of the face were rated least attractive.

STUDS AND BETTYS

PERHAPS THE LEAST SURPRISING OF THE BEAUTY FEATURES IS sexual dimorphism, which basically means we tend to like faces that look most like the sex they represent. Males are attracted to feminine-looking females and females are attracted to masculine-looking males. Highly feminine features like full lips, high cheekbones, and a small chin, are strongly preferred by males across races and cultures.¹⁵ The same goes for hormone-related signs of female fertility: larger breasts, smaller waist-to-hip ratios. The reason may have to do with the influence of reproductive hormones—estrogen in females and testosterone in males—that cause these sex differences during puberty. A distinctly feminine face may act as a fertility signal for men, telling them that this woman has reached sexual maturity and has estrogen on board.

And, in fact, women with larger breasts and an hourglass shape have higher reproductive potential as reflected in higher estrogen and progesterone levels during their menstrual cycles.²⁹ For males, the square jaw of a masculine face signals high testosterone and, by implication, sexual potency and perhaps social dominance. Since testosterone can also suppress immune function, a very masculine face on a healthy male may also be saying: “My genes are so good, I can handle loads of testosterone and still be healthy.”

POST-MENSTRUAL SYNDROME?

ON THE OTHER HAND, RUGGED HANDSOMENESS MIGHT BE A double-edged sword. A guy who’s swimming in testosterone might be good for breeding strong, healthy offspring, but he might also be aggressive and unfaithful, which is not the kind of guy you want raising your kids. This might explain an intriguing observation about women’s preferences, namely that women change their minds about male attractiveness depending on where they are in their menstrual cycle.

In one of the first studies to show this, women were presented with a series of male faces and asked to pick the one they thought was the most physically attractive.³⁰ Unbeknownst to the women, the researchers had digitally manipulated the faces to make them more masculine or more feminine. They categorized the women based on the phase of their menstrual cycle. The group categorized as “high conception risk” were in the follicular phase (the first half of the menstrual cycle, before ovulation and therefore able to conceive during that cycle) and those categorized as “low conception risk” were in the luteal phase (after ovulation has occurred, and thus unlikely to conceive). The high-conception-risk women were more attracted to the more masculine faces while the low-conception-risk women preferred the less masculine faces. In a second experiment, the researchers allowed women to morph male faces to be more masculine or feminine and asked them to pick the face they find most attractive for either a short-term relationship or a long-term relationship. When women in the high conception risk group chose a short-term mate, they made the face more masculine. Other studies have shown that during their most fertile phase, women report being attracted to more masculine bodies and voices and having more sexual interest in men other than their partners.³¹

Why would a woman’s menstrual cycle affect how she views a man’s sex appeal? One explanation is that it’s an adaptation that allowed ancestral women to optimize their options. Recall that more masculine features are thought to be one index of “good genes” and less masculine features suggest a more cooperative (better parent) kind of guy. Women who cheat on their long-term partners are more likely to do so during the follicular phase, when they are most likely to conceive. Since men can never be certain that a child is theirs, a woman might be trading up for “good genes” by having a tryst with the hunk just before she ovulates while holding on to Mr. Mom for the long term. It’s the best of both worlds—she gets genetic benefits from her lover and material benefits from her partner.

If mate preferences change with hormonal shifts (in estrogen and progesterone) during the menstrual cycle, could you alter women’s preferences by manipulating their hormones? This sounds like the diabolical plan of an evil (male) scientist, but, in fact, tens of millions of women enroll in this experiment every month. They take oral contraceptives.

The pill works by suppressing estrogen and keeping progesterone levels high enough to block ovulation. The usual cyclic shifts in these hormones are flattened out. Could taking the pill change how women feel about men? Indeed, several studies have now reported that pill-users don’t show the preferences for more masculine or symmetrical faces, or for the sweaty T-shirts of MHC dissimilar men that other women show around the middle of their menstrual cycles. It’s as though the pill has not only flattened their hormonal cycles but their sexual desires as well.

There’s an interesting flip side to this story. These same hormone cycles also affect how men look at women. Around the time of ovulation, just as a woman is beginning to tune into more masculine men, she actually becomes more sexually attractive herself. At their peak of fertility, women’s faces, voices, and odor change in subtle ways that make them more attractive to males.^32–34 Some studies show that women dress more provocatively during these periods,^35–38 and are more likely to flirt with and fantasize about sex with men other than their primary partner.^37, 39 But the pill appears to mute this surge in sex appeal that normally happens around ovulation.⁴⁰

At this point, you’re probably thinking, that’s interesting, but there’s only one way to really study the pill’s effect on sexual attractiveness. That’s right: lap-dancing. So, as a service to science, psychologists at the University of New Mexico went to “gentleman’s clubs” in the Albuquerque area to recruit lap dancers for a study of hormonal effects on female attractiveness. As you may or may not know, lap dancers perform topless dances on the laps of men who have to remain seated and can’t touch them. The dancers make their money through the tips they get for each dance. The more alluring they are and the more dances they do per shift, the more money they make.

Every day for a two-month period, the dancers were asked to log onto a website and report on where they were in their menstrual cycle and how much they earned in tips. By measuring the dancers’ income, the researchers could get a direct, numerical estimate of how attractive each dancer was. So they could now ask: What effect do ovulatory changes have on male tipping? The results were clear: earnings shot up during the peak days of fertility (the week before ovulation). On average, the women made nearly twenty dollars an hour more during their fertility peak compared to the last ten days of their cycle (when they would have been unable to conceive).

But women taking the pill didn’t get that earnings boost. The authors concluded that the pill’s steady dose of estrogen and progesterone had quashed the subtle shifts in female attractiveness that occur across the menstrual cycle. The evidence that hormones enhance physical and behavioral cues of fertility suggests that natural selection may have promoted “come hither” cues that attract men when conception is most likely. But the pill, by artificially blocking the hormone surges that trigger ovulation, may short-circuit those cues.

These studies raise questions that might have implications beyond the economics of lap dancing. Most of the studies have been small and have limitations, but they suggest that men and women look at one another differently when a woman is ovulating, and the pill blocks this natural cycle. Could the fact that millions of women take hormones that might interfere with normal mate preferences be affecting how and with whom they form relationships?⁴⁰ Might they have chosen other mates? Would a woman who met her mate while she was on the pill find him less attractive when she comes off?

A 2011 study of more than 2,500 women suggests that the pill could indeed be affecting women’s desires and mate choices. In the study, each woman was asked about her relationship with the biological father of her first child.⁴¹ Women who met their male partners while taking the pill reported being less sexually attracted to and aroused by their mates than those who hadn’t been on the pill. On the other hand, the pill-taking women were significantly more satisfied with nonsexual aspects of the relationship, such as how successful the men were as financial providers. And, overall, women who had been using the pill were more likely to stay with their partners over the long term. In other words, women who chose their mates while on the pill went on to have relationships that were less sexually satisfying but more durable. That fits with the idea that by blunting hormonal cycles oral contraceptives could bias women to choose the stable partner over the “hot” guy.

GLOBAL AVERAGES

AS INTRIGUING AS AN EVOLUTIONARY ACCOUNT OF BEAUTY AND sexual attraction may be, there are plenty of ambiguities. No one has convincingly shown that preferences for averageness or other beauty indicators of “good genes” are actually associated with greater reproductive success in humans. The most obvious limitation is that we can’t directly study what our hominin ancestors did or how they behaved. Most of the studies I’ve discussed so far have drawn their subjects from industrialized cultures. And, as we know, standards of beauty in those cultures have been changing. The fact that other animals have developed strategies for mate choice that resonate with our own certainly supports the evolutionary theory, but other animals aren’t subjected to the powerful and shifting influence of cultural conventions that we are. They’ve never had to face the judgmental standards of religious law or fashion magazines. There is a thriving industry of animal magazines—Cat Fancy, Birdtalk, Modern Dog, Reptiles, and the like—but animals don’t read them.

The other kind of evidence we’d like to see if the evolutionary arguments hold water is some degree of cross-cultural consistency. If our standards of beauty are embedded in a universal human nature, they ought to apply beyond New York, Paris, and London. In particular, it would be helpful to know what goes on in today’s preindustrialized societies, especially hunter-gatherer groups that might more closely resemble our own ancestral roots.

One such group, the Hadza, are a nomadic hunter-gatherer society who live in remote savannah-woodland areas in North Tanzania. There are only about one thousand Hadza living today. The men hunt and collect honey while the women dig for wild tubers and gather berries, and collect baobab fruit. They are a predominantly monogamous society and less than 5 percent of Hadza women marry outside the group. They also have almost no access to “the media.”

Several years ago, while a graduate student in anthropology at Harvard, Coren Apicella traveled to Tanzania to see whether beauty signals found in industrialized countries would appeal to the Hadza.⁴² She began with forty photographs of young Hadza individuals (twenty men, twenty women) and an equal number of photographs of young, white adult British men and women. For each group, she used computer morphing software to create two types of composites with different numbers of faces in each. The “more-average” faces were made by blending all twenty (male or female) faces in each group and the “less-average” faces combined five of the twenty faces. She then asked the Hadza and the British subjects which faces they found more attractive. If natural selection has biased us to see averageness as beautiful, we’d expect the more-average faces to get higher ratings even in the media-naive Hadza society.

Have a look at the figure. Which set of faces do you think are more attractive—the top row or the bottom?

The more-average faces are in the bottom row. Both the Hadza and the British rated the twenty-face Hadza composite as more attractive than any of the five-face Hadza composites. The European and hunter-gatherer groups agreed: the more averaged, the more attractive.

Apicella’s results seem to support the hypothesis that at least one of the alleged beauty signals that media-soaked Westerners find attractive is also attractive to a society that, culturally speaking, is worlds away. But that wasn’t the whole story. When Apicella showed the Hadza the European composites, they had no preference for the twenty-face images.

I asked Apicella how she made sense of this. She noted that Westerners have been exposed to a lot of African faces, but the Hadza have not been exposed to Western faces. They have no mental template for an attractive white face. “We might have this biological or universal preference for what is average,” she theorized, “but it’s the environment and the faces that we’re exposed to that will shape our prototype of what is average and . . . which faces we find attractive.”

Perhaps there’s a basis for reconciliation here between the Darwinists and the beauty-mythers. Apicella’s view at least opens the door to the idea that cultural fashions (including the Western fashion industry) might indeed shape our standards of beauty, even if some of the basic parameters are innate. Our minds may be primed to perceive some physical features as beautiful, but they may need to be calibrated by exposure to the facts on the ground. If we are bombarded with enough images of “waifer”-thin models, our prototype of average may shift.

The environment may tweak our biological templates of attractiveness in other ways as well. In another study, Apicella along with psychologist Anthony Little and anthropologist Frank Marlowe, compared the appeal of symmetry in Hadza and British adults.⁴³ If symmetry is attractive because it signals disease resistance, the effect might be stronger in the Hadza who sleep on the ground, are exposed to wild animals and plants, and have no access to modern medical care. In other words, for the Hadza, symmetry might be a more reliable signal of good disease resistance genes. And, in fact, both the Hadza and the British rated symmetrical faces of their own group as more attractive, but the effect was stronger for the Hadza. Once again, these data suggest a degree of universality in our criteria for beauty, but the details of the environment we live in can blunt or enhance their salience.

We’ve seen that evolutionary theory and a growing body of data across cultures and species suggest that our sexual behavior and our desires have been shaped at least in part by sexual selection. But it’s important not to overstate the power of these ancestral influences. To the extent that these influences operate on us today, they merely bias our brains toward certain information and behavioral strategies. They are not determinative; instead, they provide an envelope within which we operate. How they play out depends on the local circumstances of our lives: our social and physical environment, competing needs, and even chance. And as I mentioned earlier there are a variety of facts about our sexual preferences and behavior that aren’t easily explained by an evolutionary account. There have clearly been cultural fluctuations in beauty standards (e.g., the recent Western emphasis on thinness in women) that might have overridden or distorted any biases evolution gave us in judging attractiveness. But this doesn’t mean our modern cultural preoccupations were born in a Madison Avenue meeting. If there’s a middle ground between the two poles of social constructionism and evolutionary determinism, it may be that Western culture has discovered the power of supersizing features that we are biologically prepared to find attractive.

Could our modern media culture be feeding us souped-up versions of what we are instinctively drawn to? In the late 1940s, the ethologist Niko Tinbergen noticed that he could elicit powerful instinctual responses from animals by using stimuli that exaggerate the natural triggers of innate behavior patterns. For example, herring gull hatchlings peck at their parent’s bill to beg for regurgitated food. Tinbergen found that he could get hatchlings to peck like mad—beyond their normal rate—when he presented them with a phony model of a bill that exaggerated its natural shape, angle, and color.⁴⁴ In the same way, parental birds will ignore their own eggs in favor of fake super eggs that are much bigger than anything they would encounter in nature.⁴⁵ By pushing an animal’s biological buttons, these “supernormal stimuli” essentially trick the brain into finding them irresistibly attractive.

Drawing parallels between animal and human behavior is always a tricky business, but the concept of supernormal stimuli does resonate with some familiar features of modern Western culture.⁴⁶ Consider our eating habits. Natural selection prepared us to enjoy foods high in fats and sugar because they’re energy-rich and would have had obvious survival value for our hominin forebears. Our ancestors (like millions around the world today) were much more likely to die of malnutrition than complications resulting from obesity. But in the modern industrialized world, high-calorie foods have become cheap and plentiful. We now crave combinations of fat and sugar that never existed in the natural world, including the fast-food trifecta of megaburgers, large fries, and a shake. No one in the Pleistocene ever tasted ice cream (much less a fudge sundae), but a lot of us would eat it before anything else in the four food groups. Exaggerate and they will come.

It’s tempting to speculate that something like this accounts for the potency of the increasingly extreme fashions in male and female beauty. From the unusually large breasts and thin waists of Victoria’s Secret models to the broad shoulders and washboard abs of Abercrombie and Fitch hunks, we are surrounded by images of attractiveness that may have taken our biologically prepared preferences to an unnatural extreme. We’ve gone beyond cosmetics that accentuate full, red lips and smooth skin to cosmetic surgeries and Botox and collagen injections that literally reshape our bodies to maximize signals of youth, fertility, and health. It’s as though our conceptions of attractiveness have been torn free of their biological roots by a flood of images that push our mental buttons.

BEYOND THE STRAIGHT AND NARROW

YOU MIGHT HAVE NOTICED A BIG HOLE IN OUR DISCUSSION OF sexual preferences and mate choice. After all, not everyone follows the “boy meets girl” story line. Since perhaps the beginning of human existence, same-sex behavior has been a part of our experience. If natural and sexual selection have driven human mate preferences to maximize reproductive success, homosexuality would seem to present a puzzle. Assuming most homosexual couples don’t reproduce, wouldn’t genetic variants that promote homosexuality be disadvantaged and fade away in the evolutionary contest to leave descendants?

In American life and politics, few debates are as charged as the one surrounding sexual orientation. The nature of sexual attraction and sexual orientation has always been more than a scientific issue. One of the most contentious battles in modern politics centers on whether people who are primarily attracted to individuals of their own sex should be allowed to marry. Beliefs about the origin of homosexual behavior have been used to justify social policy, religious proscriptions, and legal decisions—mostly in the direction of discrimination against gay people.

The debate has often come down to three explanations: homosexuality is a choice, homosexuality is learned behavior, or homosexuality is innate. The first two are often seen as compatible and in opposition to the third, and people usually pick sides. In a 2009 national survey of Americans, 47 percent said that homosexuality is “something people choose to be” while 34 percent said it is “something people are born with.” Only 19 percent said they didn’t know.⁴⁷ Those who believe that it’s biologically innate tend to have more favorable views toward homosexuality, but pro- and antigay activists don’t necessarily endorse a nature or nurture position. As the sad history of discrimination based on skin color makes clear, the fact that something is innate or immutable doesn’t preclude its being the target of prejudice and social exclusion. And, indeed, in the late nineteenth and early twentieth centuries, biological theories of homosexuality were used to justify persecution and bizarre attempts to “cure” the condition—including testicle transplants.⁴⁸ On the other hand, there are modern conservative groups like the Family Research Council, which emphasizes the role of choice in sexual behavior and asserts not only that “homosexual conduct is harmful” and “unnatural” but also that “there is no convincing evidence that a homosexual identity is ever something genetic or inborn”^49, 50

The complexity of talking about sexual orientation is obvious even in the question of how common it is. If you were going to do a survey to determine the prevalence of homosexuality, what would you ask people? Are you attracted to members of your own sex? Are you only attracted to members of your own sex? Have you had a same-sex sexual partner? Do you consider yourself homosexual? The answers you get would depend a lot on which questions you ask.

In a comprehensive survey of sex in the United States in the 1990s, only 1.4 percent of women and 2.8 percent of men said they thought of themselves as homosexual or bisexual, but about three times as many said they’d had a same-sex sexual partner at some point in their lives.⁵¹ A large survey by the National Center for Health Statistics in 2002 found that 49 percent of men and 65 percent of women who’d had a same-sex partner considered themselves heterosexual.⁵²

So what’s the evidence that same-sex behavior is part of the “biology of normal”? First of all, hundreds of animal species engage in same-sex sexual behavior: insects, snails, birds, dolphins, sheep, and the list goes on.⁵³ While same-sex pairing is not uncommon in the animal kingdom, the kind of exclusive homosexuality that we recognize in humans is quite rare. However, the long history of human homosexual behavior and the fact that it exists in cultures around the world certainly suggest that it’s a part of human nature and might even have an adaptive function.^* And evolution-minded scientists have proposed a variety of possible explanations.

For example, there’s the gay uncle hypothesis, a spin-off of the established evolutionary idea of kin selection. It’s obvious that reproducing is a good way to pass on your genes. But there are other, more indirect ways as well. Behaviors that increase the reproductive fitness of your close relatives (kin)—protecting them, nurturing them, and so on—could also help transmit your genes (since the more closely related you are, the more genes you share). My niece shares 25 percent of my genes, so helping her survive and reproduce helps send my own genes into the next generation.

The key implication is that genes promoting kin-directed altruism could be maintained in a population if they offset the cost of not having your own offspring. By extension, some argue, natural selection might maintain the frequency of genes predisposing to homosexuality if they also promote kin-directed altruism toward close relatives.

It’s a coherent hypothesis, but if it were true, it would require a very large effect since, for example, gay men have only a fifth the number of children that heterosexual men do.⁵⁵ To compensate for that difference, gay men would need to be some kind of superuncles. And the few studies that have tested the theory have found little evidence that gay men provide enhanced caregiving for their nieces and nephews—at least in Western cultures.^55, 56

On the other hand, Canadian psychologist Paul Vasey has been able to find some support for the gay uncle hypothesis in his studies of sexual behavior in the South Pacific islands of Independent Samoa. In addition to men and women, Samoan culture has defined a third gender referred to as fa’afafine (literally, “in the manner of a woman”).⁵⁷ The fa’afafine are biologically male but tend to be effeminate and their sexual relationships are with other males (though not other fa’afafine). In a series of studies, Vasey and his colleagues found that fa’afafine invest much more time and effort in caring for their nieces and nephews than do Samoan women and heterosexual men.⁵⁸ What’s more, their avuncular tendencies are not related to a more general altruism toward unrelated children.

Why hasn’t this been seen in Western cultures? Vasey argues it’s possible that modern industrialized cultures—where families are dispersed and homophobic attitudes are common—are too removed from the conditions of our ancestral environment and simply don’t allow the adaptation to be expressed. But that explanation is hard to prove or disprove.

But there’s another observation about the fa’afafine that does match up with Western studies. Their mothers have more children than the mothers of heterosexual Samoan men.⁵⁹ The same phenomenon has been reported in studies of Italian and British families—mothers and aunts of gay men have more children than those of straight men.^60, 61 Based on these data, the “fertile female” hypothesis claims that homosexuality in men is in part a by-product of genes that enhance the fecundity of females. But again, it’s only a hypothesis at this point.

The notion that homosexual behavior is related to natural selection presupposes that genes are involved. But none of the theories I discussed above really provide direct evidence for this. So, is there any convincing evidence that a gene affects same-sex sexual behavior? The answer is clearly yes . . . if you’re asking about fruit flies. The fruit fly Drosophila melanogaster has been a staple of genetic research for decades, and its courtship behaviors have been meticulously catalogued and dissected.

For nearly forty years, scientists have known that a single fruit fly gene—known as fruitless—controls courtship behavior in males and females.⁶² In 2005 researchers showed that fruitless can act like a genetic master switch—capable of turning male courtship and mating behavior on and off.^63, 64 Males and females express different versions of the fruitless gene, the result of a difference in how the gene’s message is spliced together after it’s transcribed from the gene itself. Using genetic engineering, females can be forced to express the male version, and when they do, something dramatic happens: they stop mating with males and start courting females. When it was reported, this discovery was stunning. Here was a single gene that could trigger a highly complex behavior pattern (what we might metaphorically call “sexual orientation”) in animals.^* Fruitless does this by encoding a transcription factor that turns on or off a whole set of other downstream genes, which in turn encode elements of sexual behavior.

The fruit fly provides a kind of “proof-of-principle” example that genes can regulate sexual behavior and mate choice. But let’s face it: people are a little more complicated than fruit flies. The flies’ entire lives last a month and their brains have one millionth the number of neurons ours have.

So what do we know about genes and human sexual orientation? For one thing, several family studies suggest that homosexuality runs in families: in those studies, siblings of gay men or women were about two to five times more likely to be gay than were the siblings of straight individuals.^65–67 But because family members share both genes and environments, family studies by themselves can’t prove or disprove that a trait is genetic. It’s conceivable that a child learns to be gay from his parents or siblings for reasons that don’t involve genes. So far, though, there’s no convincing evidence of differences in gender identity or sexual orientation between children raised by lesbian or gay parents and children of straight parents.⁶⁸

Studies of twins have consistently found that sexual orientation is heritable, meaning that variations in genes account for a proportion of differences in sexual orientation across the population.^69–72 In the largest study so far, including more than seventy-five hundred twins, the heritability of same-sex sexual behavior was higher for males (34 to 39 percent) than females (18 to 19 percent),⁷¹ suggesting that genetic differences affect sexual orientation in humans and the effect may be stronger for gay men compared to lesbian women. On the other hand, since the heritability seems to be quite a bit less than 100 percent, these data also imply that most of the difference among people in sexual orientation is not due to genetic variations.

But if genes are involved, which genes are they? The few studies that have attempted to map genes related to sexual orientation have had conflicting results.^73–77 The bottom line is that, to date, no specific genes influencing homosexual behavior in humans have been identified. Which is not to say that they don’t exist—there’s really been no large-scale effort to find them.

BROTHERLY LOVE?

ONE SURPRISING PREDICTOR OF MALE HOMOSEXUALITY HAS TO do with families but only indirectly with genes. And no one is quite sure what to make of it. It’s called the “fraternal birth order effect”: having older brothers increases the likelihood of homosexuality in men. This effect has been seen in studies from Canada, Italy, Britain, the United States, and even among the fa’afafine of Samoa.^{59, 60, 78–80} Estimates from this research indicate that each additional older brother increases the odds of male homosexuality for the younger brother by 33 percent^* and that 15 to 30 percent of men can attribute their sexual orientation to their fraternal birth order.^79, 81, 82

How could having older brothers impact your sexuality? The leading theory points to the immune system. Canadian researchers Ray Blanchard and Anthony Bogaert have proposed that while a mother is carrying a male child, she may be exposed to male-specific proteins from the fetus. Because her immune system sees these proteins or antigens as alien, she develops an immune response—essentially, antimale antibodies (possibly directed against proteins made by genes on the Y chromosome). With each successive male child, the mother’s immune response may grow stronger. In effect, her body remembers how many male offspring she has carried. When these antibodies encounter the fetal brain, they might alter the function of neural circuits involved in sexual differentiation and, as a result, sexual preferences later in life.

The idea of mothers developing antibodies to fetal proteins is not a new one. The most familiar example is Rh incompatibility. Pregnant women are routinely screened to see whether they carry Rh factor, a blood group protein that’s absent in a small proportion of women. When an Rh negative mother carries an Rh positive child, she may develop anti-Rh antibodies when she is exposed to the fetus’s Rh factor during delivery. This could be a problem if she has another Rh positive child: her antibodies may attack the fetus’s red blood cells, causing a severe anemia.

While there is no direct evidence that an immune reaction really affects sexual orientation, there is some intriguing circumstantial evidence.^{79, 82} First, the birth order effect is only seen for males with older brothers. Females with older brothers and boys with older sisters do not have an increased likelihood of being gay. Second, males who have nonbiological (adopted or step-siblings) older brothers don’t show the effect, suggesting that a younger brother’s homosexual orientation doesn’t just stem from some psychological effect of having older brothers. Rather, it seems that the brothers have to have shared the same womb. Third, it’s the number of biological older brothers—and not the amount of time reared with older brothers—that predicts sexual orientation. Finally, there are known male-specific proteins that are expressed early in development and are found on the surface of brain cells, providing a possible target for an antibody response. Clearly, more direct evidence is needed before this hypothesis can be fully evaluated, and no one has suggested that the fraternal birth order effect explains most of male homosexuality. For one thing, most gay men have no older brothers.

But if this “maternal immune hypothesis” is right, it demonstrates an important point about how nature and nurture can produce behavioral traits. As we saw, twin studies indicate that genetic variation contributes to sexual orientation, but also that most of the differences in gay vs. straight preferences are accounted for by environmental (nongenetic) factors. But even if environmental factors have a larger effect on same-sex preferences, this still doesn’t mean that a gay person would have a choice about their orientation. The maternal immune hypothesis would be an example of an environmental factor that has nothing to do with choosing or learning—in this case, it’s the environment of the womb.

The prenatal environment has been considered in other ways as a biological contributor to sexual orientation. Testosterone and estrogen have powerful “organizing” effects on brain development that begin in the womb. Testosterone “masculinizes” the fetal brain, directing development toward male sexual behavior and gender identity.⁸³ One line of evidence suggests that lesbianism might be related to exposure to excess testosterone during fetal development. And studies suggest that a simple index of how much testosterone you saw as a fetus may be readily at hand.

Try this: straighten your right hand, put your fingers together, and look at how they line up. Males and females tend to have a difference in the length of their second (index finger) and fourth fingers (ring finger)—known as the second digit to fourth digit ratio (2D:4D ratio). In males, the second finger (2D) tends to be slightly shorter than the fourth (4D), but in females the second and fourth fingers are about the same length. It’s thought that the 2D:4D ratio is pretty much determined by how much testosterone you were exposed to in the first trimester of pregnancy. The theory goes that girls with the male pattern (a smaller 2D:4D) ratio were exposed to more testosterone than girls with the more typical female pattern. A recent analysis of a large number of studies that have looked at this found that lesbian women do indeed have a smaller average 2D:4D ratio than heterosexual women, though the effect was small.⁸⁴

So what can we say at this point about the biology of same-sex sexual behavior? We know that it is widespread among animal species and that in at least one species (fruit flies), specific genes and neural circuits have been identified. The possibility that it has adaptive functions in animals and humans is certainly plausible, but so far the evidence is unconvincing. Twin studies suggest that sexual orientation is partly heritable, but there are plenty of unresolved questions. In humans, no specific genes influencing homosexuality have been identified. On the other hand, there is no credible evidence that same-sex orientation is either a choice or learned behavior for most gay or lesbian people. Like everything else we’ve discussed, sexual orientation is a complex and multidimensional part of our lives. It can’t be reduced to nature or nurture and there’s no reason to think that a single cause is necessary or sufficient.

A BEAUTIFUL MIND

REMEMBER THAT, FROM AN EVOLUTIONARY STANDPOINT, PERCEIVING beauty is kind of like tasting sweetness—an enticement to desire, an experience that became rewarding because it was tied to something that enhanced fitness. Just as a sweet taste might signal a valuable source of energy, an attractive face or body might signal a high-value mate. If that’s right, it makes a strong prediction about where we might find biological responses to beauty and sexual attractiveness: the brain’s reward system. And if the idea is that we are “wired” to respond to certain features of attractiveness, it would be reassuring to see the wiring. But now we’ve crossed over into the realm of proximate causes—the here-and-now workings of the brain.

In 2001 a group of scientists at Massachusetts General Hospital and MIT in Boston tested that hypothesis by showing pictures of four sets of faces to groups of heterosexual men: beautiful women, average-looking women, beautiful men, and average-looking men. The men gave the beautiful women and the beautiful men high attractiveness ratings and were even willing to work to keep looking at them (by pressing keys on a keypad), but their brains were more discriminating. Only the beautiful female faces activated the nucleus accumbens, a key reward center that is also turned on by all manner of guilty pleasures: cocaine, speed, nicotine, money, and, yes, sweet tastes.⁸⁵ In other words, the men appreciated the aesthetic appeal of beautiful people of both sexes, but only the beautiful female turned on the brain’s pleasure centers. Since that original study, others have found that looking at beautiful faces engages reward circuitry in the brain, particularly the nucleus accumbens and the orbitofrontal cortex (OFC),^85–88 an area involved in tracking whether experiences are rewarding or aversive.^89, 90

As you might expect, sexual preferences also matter. When asked to give attractiveness ratings, men and women generally agree on how beautiful other men and women are.⁹⁰ Even heterosexual men can appreciate the appeal of a beautiful man. But what we say and how our brains respond may be quite different. In one study, men and women, regardless of their sexual orientation, gave virtually identical answers when asked to rate the beauty of photographs of other men and women. But fMRIs of their brains revealed a hidden signature of sexual preferences: in heterosexual women and gay men, brain reward centers (including the OFC) were more strongly activated by attractive male faces, while in lesbian women and heterosexual men, these centers lit up for attractive female faces.⁹¹

Physical features beyond facial attractiveness also appear to have this rewarding effect on the brain. Men seem to be drawn to the hourglass shape of a woman’s body.^92, 93 The average waist:hip ratio of a man tends to be in the range of 0.8 to 0.95, while in women the average is in the range of 0.67 to 0.79.⁹⁴ In the 1990s psychologist Devendra Singh suggested in a number of studies that American men are most attracted to women whose waist:hip ratio hovers around 0.7.⁹⁵ He also found that while Playboy centerfolds and Miss America winners have grown slimmer over the years, their waist:hip ratios remained relatively constant within the range of .68 to .72. Studies in several other countries have largely supported the idea that men are typically attracted to women with an hourglass shape, although the favored waist:hip ratio does vary across cultures.^92, 96–98

Men seem to make these judgments in the blink of an eye. In one study, researchers tracked men’s eye movements using an infrared camera while they were shown pictures of a naked woman whose body was digitally morphed to vary her waist:hip ratio (0.7 vs. 0.9) and breast size. Within 200 milliseconds, their eyes registered her waist/hip area and they judged the low waist:hip ratio (0.7) as more attractive.⁹⁹ Singh interprets this phenomenon from an evolutionary perspective: a low waist:hip ratio has been associated with fertility, youth, and health and so, like facial cues of good genes, an optimal waist:hip ratio might have served as a signal of mate quality. It’s certainly possible that these preferences are driven by media exposure, but even men who were blind from birth rated mannequins with a waist:hip ratio of 0.7 as more attractive than those with a larger ratio when they were asked to feel and touch them.¹⁰⁰

For many years in Western cultures, women have gone to great lengths to maintain and exaggerate an hourglass figure—from corsets and girdles to tummy tucks and liposuction. Perhaps it’s all an attempt to push buttons in the brain of the beholder. In one study, Singh showed men “before and after” pictures of naked women who underwent a cosmetic procedure that involved removing belly fat by liposuction and grafting it to the buttocks, effectively creating a surgically enhanced hourglass. The men rated women who achieved a waist-hip ratio of about .70 as most attractive⁹⁴ and their reward centers (including the OFC and nucleus accumbens) lit up for the new and surgically improved waist:hip ratios.¹⁰¹

But—shock!—men and women are different: men appear to have a stronger OFC response to physical attractiveness,⁸⁸ supporting a whole lot of psychological research showing that physical appearance seems to matter more to men than women. To take one example, a massive BBC Internet survey of more than two hundred thousand men and women spanning fifty-three nations asked about traits that people desired in a mate. “Good looks” were in the top three traits for 43 percent of the men but only 17 percent of the women.¹⁰²

Of course, none of these studies really answer the question of whether our neural reactions to human beauty and sex appeal are innate or acquired. That’s because they are simply looking at patterns of brain activity among people who live in a particular (modern, Western) culture. Also, the fact that reward circuits turn on when we look at attractive people doesn’t necessarily mean that these responses are innate—it’s still possible that our brains have been culturally conditioned to find certain features rewarding.

However, the available data sketch a plausible picture about the biology of attraction that ties the ultimate and proximate mechanisms together. Evolution has primed us to recognize certain signals of mate quality and sexuality in the faces and bodies of other people and added desire by linking these perceptions to reward. In doing so, we become attracted to them.

But again, we’re talking about a mental bias here. Some things may be biologically more likely to turn us on, but our specific preferences are also experience-dependent. All kinds of experiences—the crush we had in grade school, the fashion trends of our time, and, yes, the sexual politics of our culture—get piled on top of the biological foundation that we bring to the world. Our brain circuits are undoubtedly shaped by the portfolio of associations we acquire through the particular trajectory of our social and sexual lives. There’s clearly more to being “hot or not” in the twenty-first century than signaling good genes, and one man’s (or woman’s) sexy may be another’s “yuck!”

So there is some persuasive evidence that our minds are attuned to evaluating sexual attractiveness and that our brains get a buzz from sensing hotness. But does our understanding of normal tell us about how things can go awry? Are there disorders of sexual desire?

DANGEROUS LIAISONS

PSYCHIATRIST AVIEL GOODMAN OFFERED THIS CASE OF A MAN whose sexual desires got the best of him:

An executive in his midthirties, Harold would say with a smile that his Achilles’ heel was his “weakness for the fair sex.” When an attractive woman indicated to Harold that she was interested in him sexually, he found himself unable to resist, or more accurately, he found himself unable to want to resist. He experienced himself almost as a victim, sexually drawn to women against his will. Harold’s fiancée ended their engagement after he repeatedly broke promises to her that he would stop sleeping with other women. When Harold began to use his apartment in the city for midday sexual liaisons, his lunch breaks stretched longer and longer. His formerly superior work performance began to slacken and he did not receive an expected promotion. Harold’s boss warned him that he could lose his job if he was unable to keep business and pleasure separate in his life. Harold resolved that he would turn over a new leaf and for six weeks he kept his sexual behavior in check. Then, when he was out of town on business and had just finished dinner with his work team, he commented that his neck and back were tight. His secretary offered to give him a back rub, and he accepted the offer without a moment’s thought. The back rub resulted in a sexual encounter. Upon returning to his office, Harold continued to engage in sexual activity with his secretary. Soon she began to pressure him for an exclusive relationship. When he rebuffed her, she filed a suit against him for sexual harassment. He was fired immediately.¹⁰³

Many of the conditions that fall under the heading of Sexual and Gender Identity Disorders in psychiatry’s diagnostic manual (the DSM) have less to do with sexual attraction than with sexual function: female sexual arousal disorder, male erectile disorder, premature ejaculation, and so on. But then there’s the group of conditions in a category known as paraphilias. That’s a term that few outside of the mental health (and perhaps legal) professions have probably heard, but some of the syndromes may ring a bell: exhibitionism, fetishism, voyeurism, sexual sadism, pedophilia, and frotteurism. Okay, maybe frotteurism isn’t a household word.^* The thing that ties these disorders together is what people often call deviant sexual arousal, or more specifically, a pattern of intense sexually arousing fantasies, urges, or behaviors that typically involve nonhuman objects, sexual humiliation, or nonconsenting people.

Paraphilia is one of the most interesting examples of how fuzzy the line between normal and abnormal can be. Having an odd sexual interest—say a fetish for rubber dolls—doesn’t buy you a diagnosis of a psychiatric disorder. Remember that psychiatry has a standard for when a set of symptoms or behaviors crosses over into the land of disorder: it has to “cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.”¹⁰⁴ The goal here is to avoid pathologizing normal behavior—a frequent criticism leveled at psychiatry. Without the “distress or impairment” standard, the risk is that we might too easily label someone with a disorder whether or not it causes them problems. But in the case of paraphilias, that leaves the door open to some awkward scenarios.

Imagine a man, John, who is powerfully aroused by sadomasochistic pornography. He buys pornographic literature and movies, and after his wife, Connie, goes to bed, he spends four hours per night online looking at S&M porn sites and online chat rooms. He’s not bothered by it, and his wife is unaware. Technically, we wouldn’t say that he has a disorder yet. But one night, Connie finds him cruising the Internet. She is alarmed and disgusted and a major conflict ensues. The marriage soon begins to deteriorate, and within a year, despite counseling, the couple divorce.

At this point, we’d say John has a disorder: his behavior has now caused marked distress and impairment. But it wasn’t until his wife discovered and objected to his interests that his behavior became a disorder. Had she slept soundly that night and never discovered his secret world, he would not have met the criteria for an illness. Here’s a case, then, where the diagnosis of a disorder boils down to another person’s sensitivities.

Although the precise causes of paraphilia are not well understood, there is one unequivocal genetic risk factor: carrying a Y chromosome. The proportion of paraphiliacs who are women is vanishingly small compared to men. The Canadian researcher Ray Blanchard told me, “A lot of paraphilias are so rare for females that you could probably write a case for each one you saw. And I think that speaks to the biology. I think it speaks to the fragile nature of the male developing brain compared to the female brain—it more easily goes awry.”

How common are paraphilias? We don’t really know. As Blanchard told me, “If you go knocking on doors, and say, how’s your sexual appetite for . . . let’s just say eight-year-olds, nobody is going to tell you that. They don’t tell you that after they’ve been arrested. Your average sex offender will deny paraphilic interests even when the guy has done so much of this stuff that there’s no other explanation. Trying to do an epidemiological study, I think, would be almost impossible.”

On the other hand, we know something about the relative frequency of different fetishes, one form of paraphilia. The Internet is buzzing with social networks of fetishists. There are hundreds—if not thousands—of Internet groups that like-minded people can join to discuss and share their lust for inanimate objects and body parts. In one study, researchers trying to figure out which fetishes are most popular used Yahoo to scour the Internet for fetish-related discussion groups and found nearly four hundred groups composed of thousands of members.¹⁰⁵ And there was a clear winner. Foot fetishes trampled the competition, accounting for 47 percent of those with body-related fetishes, with second place going to bodily fluids at a mere 9 percent. For those who were partial to inanimate objects, footwear accounted for nearly a third of group members.

A SEMINAL EVENT

UNFORTUNATELY, WE DON’T KNOW MUCH ABOUT THE CAUSES OR biology of paraphilia, and it’s not at all clear whether the kind of sexual attraction and arousal that occur in paraphilias involves the same mental or brain systems that we’ve discussed when it comes to regular heterosexual and homosexual attraction. However, the connection may be closer for conditions that involve the extremes of sexual interests.

In the last years of the twentieth century, an event occurred that led to an unprecedented change in human sexual experience. For the first time in history, millions of people were able to watch other people have sex. The Internet had arrived. The rise of Internet pornography is only the latest chapter in the codependent history of technology and sexual stimulation. Indeed, the very existence of some paraphilias has risen and fallen with advances in technology. Take the case of telephone scatalogia, better known as obscene phone calling. Here’s a psychiatric disorder that only became possible with the invention of the telephone. As communication technology has evolved, the telephone is becoming passé, and there are indications that telephone scatalogia is becoming less common. But now we have new media to take its place. Who knows—in the coming years it may be replaced entirely by “sexting.”

But the coevolution of sexual behavior and technology has a much longer history. The invention of the printing press in the fifteenth century enabled the spread of “obscene” books and pamphlets. In the nineteenth century photography arrived and flooded the world with a new kind of sexual imagery. And of course, twentieth-century motion pictures, television, and home videos created a full-blown pornography industry that reached into our homes. But for sheer scope, volume, and variety, the World Wide Web is unparalleled as a medium for the dissemination of porn.

Just in case you had any doubts about how mainstream Internet porn has become, consider a few numbers.¹⁰⁶ A new pornography video is made every thirty-nine minutes—more than thirteen thousand per year—and every second, more than twenty-eight thousand people are viewing porn online. There are more than four million pornographic websites and four hundred million pornographic Web pages on the Internet. By 2006 worldwide pornography revenues topped $97 billion annually (with China and South Korea accounting for the majority), a figure that exceeded the revenues of Microsoft, Google, Amazon, eBay, Yahoo, Apple, and Netflix combined. A survey of American college students published in 2008 found that 87 percent of the male students used pornography,^* almost three times the rate among the females. On the other hand, nearly 50 percent of the female college students felt that viewing pornography is an acceptable way to express one’s sexuality,¹⁰⁷ and an estimated 9 million U.S. women access porn in a given month.¹⁰⁸ Okay, okay, you get the point. People like porn.

YOUR BRAIN ON SEX

WHOSE BRAINS ARE MORE AROUSED BY EROTIC IMAGES AND porn—men’s or women’s? If you guessed men, you’d be buying into an age-old stereotype . . . and you’d be right. Brain-imaging studies suggest that even though men and women report similar levels of sexual arousal when viewing couples having sex, emotion (limbic) circuits in men’s brains are more “turned on.”¹⁰⁸ Heterosexual men are much more aroused by sexually explicit images of women than they are by images of men. While both sexes show increased activity in reward regions when they look at sexual images,^109, 110 women’s brains are less discriminating about what turns them on. They are equally aroused by sexual images of men and women, despite the fact that they say they are more aroused by looking at men. How much of these sex differences are due to innate biology or cultural learning is unclear.

The accelerating reach of the Internet has clearly made porn consumption a common practice. In recent years clinicians have seen the emergence of what has been called “Internet pornography addiction” and “compulsive cybersex.” The idea that people can be addicted to porn makes sense if we consider the evidence that sexual imagery stimulates reward circuits. These are the same circuits that become trapped in the grip of addiction to street drugs. And just like some people can experiment with cocaine or speed and not get hooked, some people can use pornography recreationally. But for others, the pull is too great.

In his confessional book Porn Nation, Michael Leahy describes the Internet as the “rocket fuel” that drove his journey from recreational user to full-blown addict. Before he encountered the bottomless well of sexual images available on the ’net, he was limited by “lack of availability (or accessibility) and anonymity. But the Internet smashed through both of those barriers.” Soon enough, Leahy realized he could get ahold of any image he wanted, “tapping into new genres or categories of porn that I never knew existed before. And I could do it all instantaneously and anonymously” (pp. 57–58).¹¹¹

TOO MUCH OF A GOOD THING?

LEAHY’S INTERNET PORN ADDICTION TURNED OUT TO BE ONLY A way-station on the road to a larger problem. In recent years, this problem has become a staple of celebrity news and scandal journalism. I’m talking about sex addiction. With the 2009 debut of Sex Rehab with Dr. Drew, sex addiction achieved the ultimate in iconic status—its own reality TV show.

With all this, you might be surprised to learn that sex addiction is not officially a disorder according to mainstream psychiatry. At least for now. Right now, sex addiction is not in the DSM, but those responsible for defining sexual disorders for the next edition (DSM–5) have proposed a diagnosis of “hypersexual disorder,” a condition that in many respects captures what people mean when they say “sex addiction.” The man who gave it that name and has spearheaded the definition of hypersexual disorder is an affable and energetic psychiatrist at McLean Hospital in Belmont, Massachusetts, named Martin Kafka.

Kafka has been studying paraphilias and sexual disorders for nearly twenty-five years. As he acknowledges, the idea that some people engage in excessive sexual behavior is not a new one. In the nineteenth and early twentieth centuries, it went by different names: Don Juanism or satyriasis (for men) and nymphomania (for women). The new diagnosis of hypersexual disorder would be made for recurrent, intense, normophilic^* sexual fantasies, arousal, urges, and behaviors that are excessive and lead to significant distress or impairment.¹¹² The diagnosis includes a variety of subtypes depending on how and where the hypersexuality is expressed: pornography, cybersex, masturbation, or—the tabloids’ favorite—“sexual behavior with consenting adults,” also known as “protracted promiscuity.” And, as you might expect from what we’ve seen about the biology of sexual attraction and mating, men are more susceptible than women.

But there’s a basic dilemma involved in defining the diagnosis: How much sexual desire is too much? Where should we draw the line between a robust sexual appetite and a disorder? Kafka’s answer has less to do with quantity than consequences: an inability to control thoughts or behaviors, repetitively engaging in fantasies and behaviors to the exclusion of other important activities or obligations, using sex or sexual thoughts to alleviate mood problems, or pursuing sexual fantasies despite risk or harm to oneself or others. And, of course, being diagnosed with hypersexual disorder would mean having significant distress or impairment. Kafka is convinced that at the extremes, an excess of sexual appetite can be harmful and worthy of treatment:

I think there’s no question that this condition exists and that it causes very significant impairment and that it’s also associated with the spread of STDs as well as very significant pair-bond impairments: separations, divorces, and so on. People get fired from work for looking at pornography. When you have this condition it causes very serious adverse consequences. So, in my opinion, this doesn’t stigmatize this group, it actually destigmatizes this group. It says, it really is a condition. There is research supporting this condition, it has criteria, and we need to explore what are the best treatments of this condition.

Why not just call it sex addiction? Kafka argues that using the word addiction would be claiming something about the causes and biology of the condition that we just don’t know yet. Is it really an addiction? Or is it more like a compulsion?

What’s at heart here is a debate among psychiatrists and others about whether a normal appetitive behavior can ever become an addiction. Appetitive behaviors are those that aim to satisfy a basic need: food, water, sleep, sex. So far, no one has proposed the idea of water addiction or sleep addiction.^* Traditionally, the idea of addiction has been reserved for out-of-control behaviors that focus on getting and consuming something we don’t normally need: illicit drugs, alcohol, and perhaps gambling. These are things that become addictive because they hijack our brain’s reward mechanisms. And those mechanisms were presumably designed to help ensure that we would be motivated to seek out the things that we do need to live and reproduce—that is, food, water, sleep, sex, and, as we saw in Chapter 5, attachment and love.

Addictions tap into the same reward circuitry as those basic needs in ways that are more direct and potent than the experiences that the circuitry was designed to find desirable in the first place. That’s why they are so powerful and dangerous. Rats will press a lever to get cocaine until they die of starvation or dehydration. And for some people, cocaine gives the reward system a direct chemical jolt that even attachment or sex can’t match. But at this point, it’s still unclear whether or not overexposure to porn or sex itself can make someone a sex “addict.”

So we’ve seen two ways that the human capacity for sexual attraction might go awry. One, paraphilia, occurs when the object of the attraction is unconventional—that is, deviant. The other, hypersexuality, is said to occur when sexual interests are excessive. But what about the opposite end of the spectrum? If there’s such a thing as too much sexual attraction and drive, can there be too little? The DSM would call for a diagnosis of hypoactive sexual desire disorder when a lack of sexual fantasy or desire causes problems for a person. Like hypersexual disorder, this diagnosis implies that there is a normal range of sexual attraction and that being outside that range can be a source of distress and impairment.

About 1 percent of the population say that they are “asexual”—that is, they have never really been sexually attracted to another person.¹¹⁵ They appear to have a higher threshold—perhaps biologically—for sexual attraction,¹¹⁶ but they don’t have a disorder. Perhaps more than any other aspect of human behavior, separating normal from abnormal sexual attraction is fraught with value judgments.

But research on the psychological and biological roots of sexual attraction has given us the outlines of an explanation for how we judge who’s hot and who’s not. From our ancestral past, natural selection has endowed us with mental biases that shape our mate choices. Those biases operate, at least in part, by tuning brain circuits involved in reward and emotion processing into beauty and sexual signals and creating desire. And yet there are clearly individual differences in what turns us on. That’s because those same systems are plastic—they can be tweaked, retuned, or even hijacked by experience, conditioning, supernormal stimuli, and even the sexual politics of our culture. But at the end of the day, these things probably draw their power from the fact that we have mental machinery and neural mechanisms—software and hardware—that care about seeking and choosing mates. There may be a beauty myth, but it’s based on a true story.

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* Polygamy is the broader term that includes polygyny (one male mating with more than one female) and polyandry (one female to several males). Polyandry, as a cultural practice, is quite rare. Less than 1 percent of preindustrial cultures practice it. Another 16 percent have monogamous marriage systems. But the clear winner is polygyny—occurring in more than 80 percent.¹²

* Though, admittedly, some of these surveys are pretty striking. In one widely cited study, male and female student researchers “of average attractiveness” asked strangers on a college campus if they would go to bed with them. None of the women asked by males agreed, but 75 percent of men asked by women said yes.¹³

* There are alternatives to the idea that attractive features signal good genes. For a more extensive account of these issues, see Matt Ridley’s marvelous book, The Red Queen.

* Actually, there is a debate among historians about just how old the concept of sexual orientation really is. Although same-sex sexual behavior is clearly ancient, social constructionists argue that the idea of classifying people as exclusively heterosexual or homosexual didn’t appear until the mid-nineteenth century.⁵⁴

* It has since become clear that the full range of male courtship behavior involves other genes in addition to fruitless, including, most importantly, a gene known as doublesex.

* This is a relative increase in the odds. So if the baseline odds of a male being gay is, say, 2 percent, this means that having an older brother would increase the odds to 2.66 percent (i.e., 2 percent + [.33 x 2 percent]).

* Frotteurism (from the French word meaning “one who rubs”) is a disorder characterized by intense sexual urges or fantasies about rubbing oneself against another person.

* One expert told me that this means that 13 percent of male students are liars.

* Normophilic refers to normal, or conventional, sexual interests as opposed to paraphilic, or deviant sexual interests.

* Food is another matter. Recent studies in rodents and humans do suggest that food can activate reward circuits in ways that are similar to that seen with drugs of abuse.^{112, 113, 114}