What does desire truly look like? Science hasn’t come up with an answer, because most of us won’t let curious researchers watch us tumbling between the sheets, and surveys aren’t necessarily reliable. Are you willing to jot down answers to questions like “Have you ever felt attracted to your pet schnauzer?”—even if the unshaven young grad student quizzing you insists, “Trust me—your answers are completely anonymous”?

Only one scientist managed to survey a large number of people on a broad range of sexual interests: Alfred Kinsey. In the late 1940s and early 1950s, Kinsey and his team interviewed thousands of subjects, asking questions about a tremendous variety of turn-ons, including bondage, bestiality, and silk stockings. But the Kinsey reports are now more than a half century old, and the findings were limited: The subjects were primarily educated, middle-class Caucasians; they were not selected randomly or systematically; and the data consisted of only recollections the subjects chose to share.

Today, a wide variety of scientists—neuroscientists, psych­ologists, anthropologists, biologists, pharmacologists—study desire, and one of their most basic questions remains: Why do we like the things we like? To answer that, we must first determine what people like, and stealing a look at men and women’s true interests has been far from easy.

Until the arrival of the Internet.

In 1991, the year the World Wide Web went online, there were fewer than 90 different adult magazines published in America. Just six years later, there were about 900 pornography sites on the web. Today, there are 2.5 million adult websites. It’s hard to imagine a more revolutionary development in the history of human sexuality. With a visit to an adult video site like PornHub, you can see more naked bodies in a single minute than the most promiscuous Victorian would have seen in an entire lifetime.

By examining raw search data, we can finally view an unfiltered snapshot of human desire. Take a look at the following list. Each phrase is an actual search entered into Dogpile (a popular “meta-engine” combining results from sources like Google and Bing) in May 2010: shemales in prom dresses, Twilight slash Edward and Jacob, black meat on white street, wives caught cheating on cam, best romance novels with alpha heroes, kendra wilkinson sex tape, spanking stories, free gay video tube, Jake Gyllenhaal without shirt, girls gone wild orgies. What immediately jumps out is the remarkable diversity of people’s sexual interests.

In 2010 we conducted the world’s largest experiment: We sifted through a billion different web searches, including a half million personal histories. We analyzed hundreds of thousands of online erotic stories and thousands of romance e-novels. We looked at the 40,000 most trafficked adult websites. We examined more than 5 million sexual solicitations posted on online classifieds. We listened to thousands of people discussing their desires on message boards.

The goal? To understand the specific cues that trigger human desire.

Wolfgang likes to look at images of female derrieres. He prefers certain poses: bent over, legs splayed, leaning on her knuckles. He likes these images so much that he is willing to pay for them—sometimes several times a day. This might seem excessive, though not exactly remarkable, except for one fact: Wolfgang is a monkey.

Rhesus macaques studied at Duke University are able to trade fruit juice for peeks at photos of female perinea. Researchers have consistently found that males are willing to trade juice to view these images and will trade more juice to look at monkey erotica than any other image.

Men aren’t the only primates willing to spend money just to look at females, but they’re the only ones to develop it into an industry. The most popular paysites featuring adult videos typically attract an audience that is around 75 percent men, and when it comes to actually paying for porn, the gender gap widens into an abyss. On the web, women prefer stories and men prefer images. So what exactly are all these men so driven to look at?

The most influential male cue is age, which dominates sexual searches, adult website content, and pornographic videos. On Dogpile, terms describing age—such as teen, young, and mature—are the most frequent type of adjective in sexual searches, appearing in one out of every six.

While the data show that youth dominates male desire, and there is a rather shocking number of searches for underage women, there is significant interest in older women as well. More than a quarter of all men report that their first sexual fantasy was triggered by a sexy older person. And what is the single most popular word users enter into the PornHub search engine? Mom. MILFs (Mothers I’d Like to Fuck) are one of the most profitable genres of male-targeted pornography.

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Men’s interest in women’s bodies is well known, but the next visual cue may come as a surprise. Men are more interested in penises than women are. An eye-tracking study found that, when viewing nonerotic images, men consistently direct their gaze to the male crotch, through women rarely do. In porn, the penis is always under the spotlight. On the adult website Fantasti.cc, the predominantly male users rate more than

1 million images and videos. Out of the 100 top-rated images, 21 feature close-up shots of a penis. And on all of the major adult video sites, “Big Dick” is a popular porn category.

But men aren’t satisfied by checking out other men’s penises. They also like to flaunt their own. Chat Roulette is a website that allows users to randomly connect to other people around the world. Once you enter Chat Roulette, you see whatever other people have chosen to place in front of their webcams—a party, a cute kitten, an old man with a beard. One blogger recorded what he saw on 1,276 consecutive sessions: 298 webcams (about one in four) were aimed at a penis. Perhaps men are tapping into an ancient display mechanism we share with other primates.

While straight men have a deep-rooted fascination with penises, gay men are positively obsessed with them. Feet, butts, and chests are also highly popular in both gay and straight porn, as are domination, submission, group sex, amateurs, and numerous other interests. With so many parallels, Internet porn suggests that gay men share the same visual cues as straight men.

Forbidden acts have a very special power to arouse. Unlike anatomical cues, transgression is a psychological stimulus. Both sexes can get wildly turned on by situations that are immoral or dangerous, because of their immorality or dangerousness.

Consider the enormous popularity of cuckold porn—in which a man’s wife has sex with another man. Cuckold porn is the second most popular heterosexual interest on English-language search engines. Only youth is more popular. On PornHub, men who search for “cheating wife” view the greatest number of videos.

In cuckold porn, the boyfriend or husband almost always watches from the sidelines, usually with a look of frustration and dismay. Frequently, the wife calls out to her husband as she’s being serviced, touting the superior skills or better equipment of the bull—a common term for the cuckolder. Why would a straight man get turned on by watching a dominant, masculine man have sex with his wife? What makes a man’s sexual desire overcome his sexual jealousy? The science of biology offers one intriguing answer to these questions. Sperm competition.

Sperm competition refers to a variety of physiological and behavioral adaptations that enable a male’s sperm to compete with other males’ sperm to impregnate a female’s egg. If a man believes that his sexual partner may have been with a rival, he is driven to have sex with her as quickly and as vigorously as possible. In many species, the more dominant the potential rival, the stronger the sperm competition cue and the more intense the arousal.

Female pleasure is also one of the most potent psychological cues for male arousal. On Fantasti.cc, we analyzed 10,000 comments on 100 top-rated videos. The third most common type of comment is acknowledgment of the woman’s pleasure. For example, “She loves it!” and “Look at how excited she is!” Why are men so interested in a woman’s sexual pleasure? Perhaps for the same reason that the male brain is designed for sexual jealousy: to ensure a woman’s fidelity. The more pleasure a man provides a woman, the more likely she’ll want to have sex with him again.

Cravability is the food industry’s term for dishes that dupe the mind in order to make diners want more and more. The manufactured cravability of Chili’s Texas Cheese Fries brings together combinations of tastes that never existed before. When they hit our tongue, our brain swoons with a pleasure more intense and thrilling than when we bite into a mere fried potato.

Certain kinds of sexual stimuli combine cues in a similar way—a kind of trickery we call erotical illusions. With modern technology and human creativity, ancient sexual cues are spliced together in ways that can hyperstimulate our sexual perception, giving rise to curious new erotic cravings.

When men search for porn on the Internet, they seek out the perfect combination of cues. They hope to find a body that maximizes their desire by activating as many cues as possible. Many thumbnail sites make it easy, displaying rows of photographs featuring a wide variety of female bodies. But once in a while, a different kind of body pops out.

“I call it the ‘trannie peek,’” explains one industry veteran. “Adult webmasters figured out that straight guys will click on shemales out of curiosity and take a look. It grabs about 5 percent of the clicks on straight thumbnail galleries.”

The terms trannie, shemale, and T-girl are frequently used as slang within the adult industry for a transsexual woman who has been treated with hormones so that she possesses breasts and a female figure but still has a penis. The main audience for T-girl porn, which has exploded in popularity over the past decade, is heterosexual men.

What drives straight men’s interest in T-girls? The T-girl is an erotical illusion that juxtaposes two kinds of male visual cues. First is a set of cues for femininity: breasts, butts, curvy figures, and feminine facial features and mannerisms. But there is another vivid cue: the penis. As we’ve learned, the penis has a special power to activate the male sexual brain. When you superimpose these two cues, the result is powerful.

In Japanese anime, transsexual characters are known as futanari. Futanari porn reveals exactly what appeals to straight men about T-girls. Typical futanari features schoolgirls with giant protrusions beneath their plaid skirts, teenage girls with pink hair and a bulge in their jeans, ballerinas in tutus sporting erections as long as their slender legs.

If Japanese anime offers the greatest creative freedom for erotical illusions that titillate the male brain, then the paranormal romance is its match for the female brain.

Women respond to a truly astonishing range of cues across many domains. The physical appearance of a man, his social status, personality, commitment level, confidence, authenticity of emotions, family, attitude toward children, kindness, height, and smell are all important. Unlike men, women need to experience enough simultaneous emotional and psychological cues to cross an ever-varying threshold.

Over the past decade, sexy vampires and lusty werewolves have replaced mortals as the most popular romance heroes for women. Stephenie Meyer leads the pack of paranormal authors with her Twilight series of novels.

The rapid rise of the paranormal romance is largely due to an extraordinary variety of erotical illusions. The paranormal takes the psychological cues inherent to the genre and twists them into variations that satisfy women in deliciously new ways.

Supernatural males are alphas among alphas, turbocharging cues of masculinity. They know how to fight and are willing to annihilate the competition. They are fully capable of protecting the ones they love from a range of mundane and otherworldly dangers. But the erotical illusions are complete only when these invincible heroes are brought to their knees by the irresistibility of an ordinary woman.

Erotical illusions—including T-girl porn and paranormal romance—reveal a hidden fact about all erotic experiences: What ultimately binds sexual cues together into a single experience is our imagination.

Many believe that by reducing our desires into a set of narrow biological cues, we eliminate the magic of sex. Instead, by identifying those cues, we can see the magic more clearly. A penis and a female body combine within the sorcery of the male sexual imagination to produce an entirely new creation. Dominant men and irresistible women are magnified by the erotic artistry of the female sexual imagination to produce thrilling tales of vampires and demons.

By investigating the software of our sexual brain, we can finally appreciate the true nature of human desire. There is no such thing as an absolute “male sexuality” or “female sexuality,” but instead a number of gender-specific components, subject to the vagaries of biology and experience. Cues can flip, change, or transform, resulting in endless variations of sexual identity that defy easy labeling. But it is our sexual cues—our finite, identifiable, biological cues—that grant us all the pleasures of sex.

Our cues release us, even as they bind us.

Ogi Ogas and Sai Gaddam hold PhDs from the Department of Cognitive and Neural Systems at Boston University. Excerpted from A Billion Wicked Thoughts by Ogi Ogas and Sai Gaddam (Dutton/Penguin Group, 2011). www.us.penguingroup.com

Have something to say? Send a letter to editor@utne.com. This article first appeared in the September-October 2011 issue of Utne Reader.