The simple story of pleasure is that animals evolve to enjoy what’s good for them; pleasure is the carrot that drives them toward reproductively useful activities. (Pain is the stick.) It feels good to drink when thirsty and eat when hungry because animals that were inclined to feel such joys left more offspring than those that weren’t.
This logic easily applies to sex. If one animal seeks out opportunities for mating and the other is indifferent, then, all else being equal, the first will have more offspring. From an evolutionary perspective, chastity is genetic suicide: You can’t have offspring without sex, and sex, like food, is the sort of thing that you usually have to work to get. We have therefore evolved a motivation to seek it out. The pleasure of sexual intimacy poses no puzzle at all.
This evolutionary analysis is simple, but it provides a starting point for exploring the specific nature of sexual desire: Why do certain people make us wild with desire and others leave us cold?
The cynical argument goes that our sexual and aesthetic responses are triggered entirely by certain perceptual features. For example, we all like to look at pretty faces. This is not just sexual. Straight men and women enjoy looking at attractive same-sex faces. Regardless of their gender, good-looking faces trigger neural circuits devoted to pleasure. Even babies are suckers for a pretty face and prefer to look at one from the very start.
This baby result would have surprised Darwin, who believed that standards of beauty are culturally arbitrary and so have to be learned. However, there are features that everyone everywhere finds appealing: Unblemished skin. Symmetry. Clear eyes. Intact teeth. Luxuriant hair. Averageness. This last one might seem surprising, but if one picks out 10 faces at random, either 10 men or 10 women, and morphs them together, the result would be good looking. When shown this composite face, babies would probably rather look at it than at any one of the individuals. So would you.
Why would these considerations matter? Factors such as smoothness of complexion, symmetry, clear eyes, intact teeth, and good hair are overt cues of health and youth, which are good things for everyone to attend to when looking for a mate. This is particularly the case for symmetry; it is hard to be symmetrical, and bad things such as poor nutrition, parasites, and simply the ravages of time eat away at it. It is a mark of success.
It is less clear why averageness is good. It might be that it reflects health, on the logic that most deviations from normal are bad. Averageness also corresponds to heterozygosity, or genetic diversity, which is another good thing. A very different possibility is that average faces are in a literal sense easy on the eyes; they require less visual processing than nonaverage faces, and we tend to prefer visual images that are easier to process. One wrinkle here is that while average faces look good, they don’t look terrific—the most attractive faces are not the average ones. (When you do these morphs, you get a fine face, but not one with movie-star good looks.) Perhaps it isn’t that average faces are positively attractive; it’s that nonaverage faces run more of a risk of being unattractive.
Yet looks aren’t everything when it comes to desire. The logic of adaptation says that we are attracted to those who have certain relevant traits—and some of these are not visible on the face or the body.
One consideration is familiarity. In one study, researchers got a team of women to attend different classes at the University of Pittsburgh. These women never spoke during the lectures and never interacted with the students. But the number of classes they attended varied—15, 10, 5, or none. At the end of the course, students were shown pictures of the women and asked what they thought of them. The women judged as most attractive were those who had attended class 15 times; judged least attractive were those the students had never seen before. This is a small study, but it fits a voluminous literature in social psychology on the “mere exposure” effect: People like what they are familiar with, which is a rational way for the mind to work given that, other things being equal, something you are familiar with is likely to be safe. Mere exposure applies to attractiveness, then, explaining some of the appeal of the girl (or boy) next door.
In another study, experimenters had people rate the photographs of classmates in their high school yearbooks according to how much they liked them and how attractive they felt they were. Strangers of the same age also ranked the photographs for attractiveness. The ratings by classmates and the ratings by the strangers should have matched, but they didn’t. The classmates’ attractiveness ratings were swayed by how much they liked each person, further evidence that there is more to being good-looking than looking good.
Even when you rate the faces of strangers, appearance isn’t all that matters. One study found that a main factor in attractiveness has nothing to do with averageness, symmetry, sexual dimorphism, or anything like that; it’s whether the person is smiling.
What else determines our sexual and romantic response to another person? Even when you narrow down possible candidates to those of the right sex, family relation, and history, still, it is hard to choose a long-term mate. You can read youth and health from the face and body, but one also looks for qualities like intelligence and kindness. Smart and kind people do well in the world, and so do their children. You also want someone who will faithfully take care of the children, and someone who will help and support you. It is not surprising that in the largest study ever of human mate preferences, looking at people in 37 cultures, the most important factor for both men and women was found to be kindness. All of us are on the lookout for partners who are smart and faithful and kind. The problem is figuring out who they are.
This brings us to what biologists describe as sexual selection. Consider the flamboyant tails of peacocks. These are worse than useless; they are unwieldy and heavy, slowing the bird down, hard to keep clean, a “Kick Me” sign for predators. Before developing the theory of sexual selection, Darwin wrote that the sight of the peacock feather made him sick—it was a humiliating refutation of the logic of natural selection.
The solution that he arrived at is that these tails don’t directly help with survival; they don’t avoid predators or kill prey or provide warmth or anything else that helps the peacock better deal with the physical world. But they are appealing to peahens. If peahens prefer to mate with peacocks with a little bit of color, then the next generation will include both more colorful males and peahens with a similar taste in flamboyance, and then, over the course of evolutionary history, you end up with the peacock’s tail.
Geoffrey Miller, a psychologist at the University of New Mexico, has argued that many of the more interesting and ostentatious aspects of human nature have evolved through sexual selection as a way for people to advertise their worthiness to one another. Miller would include dance here, and much of sports, art, charitable activities, and humor. For him, the brain is a “magnificent sexual ornament.”
I am not going to discuss Miller’s grand theory in detail here, but he has two insights about sexual attraction that are worth exploring. The first is costly signaling. The idea is that displays of personal quality are taken seriously only if they involve some cost, some level of difficulty or sacrifice. If anyone can easily do the display, then it is worthless, because it is trivially easy to fake. Costly signaling shows up in the gifts we give to one another, particularly during courtship. Miller asks, rhetorically, “Why should a man give a woman a useless diamond engagement ring when he could buy her a nice, big potato, which she could at least eat?” His answer is that the expense and uselessness of the gift is its very point. A diamond is understood as a sign of love in a way that a potato isn’t, because most people would only give one to someone they care about, and so the giving signals some combination of wealth and commitment.
Miller’s second neat idea is that of a “hot chooser.” The idea is that when we choose mates, we are looking for people who give us pleasure. This might seem obvious at a personal level, but Miller explores it from an adaptationist perspective, as a force for the evolution of certain traits.
A simple physiological example is the penis. There are all sorts of oddities about the human body relative to other primates—males grow beards, females have enlarged breasts and buttocks and narrow waists—but the most striking difference has to do with the male genitals. Some primates have genitals that are more visually interesting than the human one. The mandrill has a bright purple-pink scrotum and red penis, vervets have a blue scrotum and red penis, and so on. But the human penis has a clear tactile advantage, being longer, thicker, and more flexible—very different from the small, pencilthin penises of other primates, which are about two to three inches long and made rigid by a penis bone. Miller makes the controversial claim that this is the product of female sexual selection: Females were drawn to males who gave them sexual pleasure, leading to the evolution of a better penis.
The brain, for Miller, has evolved much like the penis. People are on the lookout for entertaining mates. We prefer to be with, and mate with, those who make us happy. This puts evolution in a new light. Evolutionary psychologists typically see the mind as either a scientific data-cruncher, constructing theories of the natural environment, or as a Machiavellian schemer, trying to outfox others in a zero-sum game of social dominance. Maybe the mind is also an entertainment center, shaped by the forces of sexual selection to give pleasure to others, to possess the capacity for storytelling, charm, and humor.
The argument so far is that sexual desire can be smart. While we’ve evolved to be sensitive to the shape of the face and the curve of the hips, we also look at deeper factors, including sexual history, signs of commitment, and wit, warmth, and kindness.
I want to emphasize a further aspect of this depth, which is that we are not exclusively attracted to faces or bodies, or even to personality or intelligence. We are attracted to specific people who so happen to have these certain properties. We fall in love, after all, with individuals, not with aspects of people. As George Bernard Shaw put it, “Love is a gross exaggeration of the difference between one person and everybody else.”
As an illustration of this fact, think about the person you love the most. Now imagine that there is someone else in the world, someone who looks virtually identical to your special someone, so much so that most people cannot tell the two apart. Indeed, imagine that he or she is a genetic clone of your partner and has been raised in the same house by the very same parents.
In other words, imagine that your partner has an identical twin. If you were attracted to the properties of a person, rather than the person him- or herself, then your attraction should extend to a considerable degree to the twin. Interestingly, studies of people who are married to twins find that this doesn’t happen. The romantic attraction is to the person you’re married to, not to his or her superficial qualities.
Sexual desire is similarly calibrated to individuals, not properties. Online porn sites boast about pictures of naked celebrities captured from movie clips or, in some cases, from telephoto lenses. What presumably makes these pictures arousing isn’t the visual experience by itself (sometimes blurry and unrecognizable); it is the knowledge of who the person is. If you were told that the picture was of someone else, the arousal would fade. Magazines will pay fortunes for a naked picture of an attractive famous person, and nothing at all for a naked picture of someone who looks like that person, even if, on a physical level, it is the very same picture.
Another illustration of the essentialist nature of desire comes from a rare disorder called Capgras syndrome, in which people come to believe that those close to them, including their spouses, have been replaced with exact duplicates. One theory is that it results from damage to the brain areas responsible for the emotional reaction we get when we encounter those we love. A sufferer might then see someone who looks just like his wife, but it just doesn’t feel like her. There is the gut feeling that she is a stranger, and so this is resolved by seeing her as somehow an impostor—perhaps a clone, or alien, or robot.
The typical response is fear and rage, and sufferers have sometimes murdered close family members. But there is one exception that I know of. This is a case study from 1931 of a woman who had complained about her sexual dud of a lover; he was poorly endowed and unskilled. But after suffering brain damage, she met someone “new.” He looked exactly like the man she had known, but this one was “rich, virile, handsome, and aristocratic.” Sexual and romantic feelings are deep, and her brain damage allowed her to start over, thinking of her lover as a different individual, a better one. This is a vivid example of the essentialist nature of attraction. As Shakespeare put it, “Love looks not with the eyes, but with the mind.”
Adapted from How Pleasure Works, by Paul Bloom. Copyright 2010 by Paul Bloom. With the permission of the publisher, W. W. Norton & Co. The author is a professor of psychology at Yale University.