And yet some questions are unpredictably predictive. One of the founders, Christian Rudder, maintains the OK Trends blog, sifting through the mountains of data and composing clever, mathematically sourced synopses of his findings. There are now nearly two hundred and eighty thousand questions on the site; OK Cupid has collected more than eight hundred million answers. (People on the site answer an average of three hundred questions.) Rudder has discovered, for example, that the answer to the question “Do you like the taste of beer?” is more predictive than any other of whether you’re willing to have sex on a first date. (That is, people on OK Cupid who have answered yes to one are likely to have answered yes to the other.) OK Cupid has also analyzed couples who have met on the site and have since left it. Of the 34,620 couples the site has analyzed, the casual first-date question whose shared answer was most likely to signal a shot at longevity (beyond the purview of OK Cupid, anyway) was “Do you like horror movies?” When I signed up for the site, some of the first things I was asked were “Are clams alive?” and “Which is bigger, the sun or the earth?” It’s hard to discern the significance.

The purpose of the blog is to attract attention: the findings, like the quizzes, are to lure you in. Rudder has written a lot about looks: whether or not it helps to show cleavage (women) or a bare midriff (men)—the answers were Yes, Especially as You Age, and Yes, If You Have Good Abs and Are Not a Congressman. He found that women generally prefer it when in photos men are looking away from the camera (hypothesis: less intimidating), and that men prefer the opposite (they want a woman’s full attention). A user can rate other people’s profiles. The matching algorithms take these ratings into account and show you people who are roughly within your range of attractiveness, according to the opinions of others. The idea behind the matching algorithms, Chris Coyne told me, is to replicate the experience you have off-line. “We tried to imagine software that would be like your friend in the real world,” Coyne said. “If I were your friend and I told you that So-and-So would be the perfect date, your response to me would be to start asking me questions. Does she like dancing? Does she smoke pot? Is she a furry? Is she tall? On the Internet, people will ask—and answer—extremely personal questions.”

OK Cupid sends all your answers to its servers, which are housed on Broad Street in New York. The algorithms find the people out there whose answers best correspond to yours—how yours fit their desires and how theirs meet yours, and according to what degree of importance. It’s a Venn diagram. And then the algorithms determine how exceptional those particular correlations are: it’s more statistically significant to share an affection for the Willies than for the Beatles. The match is expressed as a percentage. Each match search requires tens of millions of mathematical operations. To the extent that OK Cupid has any abiding faith, it is in mathematics.

There’s another layer: how to sort the matches. “You’ve got to make sure certain people don’t get all the attention,” Rudder said. “In a bar, it’s self-correcting. You see ten guys standing around one woman, maybe you don’t walk over and try to introduce yourself. Online, people have no idea how ‘surrounded’ a person is. And that creates a shitty situation. Dudes don’t get messages back. Some women get overwhelmed.” And so the attractiveness ratings, as well as the frequency of messaging, are factored in. As on Match.com, the algorithms pay attention to revealed preferences. “We watch people who don’t know they’re being watched,” Sam Yagan, the company’s C.E.O., said. “But not in a Big Brother way.” The algorithms learn as they go, changing the weighting for certain variables to adjust to the success or the failure rate of the earlier iterations. The goal is to connect you with someone with whom you have enough in common to want to strike up an e-mail correspondence and then quickly meet in person. It is not OK Cupid’s concern whether you are suited for a lifetime together.

OK Cupid winds up with a lot of data. This enables the researchers to conjure from their database the person you may not realize you have in mind. “Like that guy in high school with the Camaro and the mustache who bow-hunts on weekends,” Rudder said. “You can find that guy of the imagination by using statistics.” The database also gives them a vast pool to sell to academics. In no other milieu do so many people, from such a broad demographic swath, willingly answer so many intimate questions. It is a gold mine for social scientists. In the past nine months, OK Cupid has sold its raw data (redacted or made anonymous to protect the privacy of its customers) to half a dozen academics. Gregory Huber and Neil Malhotra, political scientists at Yale and Stanford, respectively, are sifting through OK Cupid data to determine how political opinions factor in to choosing social partners. Rudder, for his part, has determined that Republicans have more in common with Republicans than Democrats have in common with Democrats, which led him to conclude, “The Democrats are doomed.”

OK Cupid’s office occupies a single floor of an office building a block away from the Port Authority Bus Terminal, that old redoubt of pimps. It’s an open-air loft space, with the four founders at desks in the middle of a phalanx of young men (and one woman) staring at screens. The four are Sam Yagan, the C.E.O.; Chris Coyne, the president and creative director; Max Krohn, the C.T.O.; and Christian Rudder, the editorial director. As they all like to say, Sam is the business, Chris is the product, Max is the tech, and Christian is the blog.

Yagan, who is thirty-four, is also the face. A Chicagoan with the mischievous self-assurance of a renegade salesman—he can seem solicitous and scornful at once—he does appearances on “Rachael Ray” and meetings with the suits at I.A.C. He makes grandiose claims with a mixture of mirth and sincerity. As he said to me one day, “We are the most important search engine on the Web, not Google. The search for companionship is more important than the search for song lyrics.”

All four founders maintain profiles on OK Cupid, but they are all married, and they all met their wives the analogue way. Yagan met his wife, Jessica, in high school, outside Chicago, where she and their two kids now live; she works for McDonald’s, overseeing the sustainability of its supply chain. He commutes to New York every week, bunking in a hotel. Rudder, who is thirty-five and from Little Rock, met his wife, a public-relations executive from Long Island named Reshma Patel, twelve years ago through friends. They live in a modest apartment in Williamsburg, and often have friends over at night to play German board games. Coyne and his wife, Jennie Tarr Coyne, who have a toddler and a child on the way, have been together eight years, but sometimes they go out and pretend it’s their first date. She is from Manhattan and works in the education department at the Frick Collection. They were classmates at Harvard, but they met again a few years later outside a night club in New York. He had a drunken woman on each arm. “Don’t I know you?” he said.

“I was a little grossed out,” she recalled. “I decided I was done with him.”

“She decided she had to have me,” Coyne said.

Afterward, she looked him up on the Internet, and discovered that he’d come from a town in Maine near where her father, Jeff Tarr, also a Harvard graduate, grew up, and that they had gone to the same Scout camp. Chris and Jennie began e-mailing each other, and eventually went out on a date. She considers herself an excellent matchmaker, with a well-tested compatibility theory of her own—that a man and a woman should look alike. (In 2004, Evolutionary Psychology published a study of this phenomenon titled, “Narcissism guides mate selection: Humans mate assortatively, as revealed by facial resemblance, following an algorithm of ‘self seeking like.’”) She and Coyne are both blond, fair, and lean, although, because he is seventeen inches taller, she worried they’d be ill matched. They were engaged within a year. They moved into an apartment in the same building as her parents: the San Remo, on Central Park West. Jennie’s father, too, had started out in the computer-dating business; at Harvard, he’d been one of the founders of Operation Match, the inspiration for TACT.

The Coynes’ marriage has a whiff of a phantom variable that the matching algorithms don’t seem to take into account: fate. Serendipity and coincidence are the photosynthesis of romance, hinting at some kind of supernatural preordination, the sense that two people are made for each other. The Internet subverts Kismet. And yet Coyne and his wife both have a profile on the site, and the algorithms have determined that she is his No. 1 match. He is her No. 2. She struck up a correspondence with her No. 1, a man in England, who eventually, after she friended him on Facebook, stopped writing her back.

For all the fun that twenty-somethings are having hooking up with their Hornivores, their Sonnets, and their Poolboys, it turns out that the fastest-growing online-dating demographic is people over fifty—a function perhaps of expanding computer literacy and diminished opportunity. I recently got to know a woman I’ll call Mary Taft, who is seventy-six, has a doctorate in education, and has been married and divorced twice. She lives outside Boston. As a single mother, in her forties, she gave up men for a while. “When you have a kid, dating is very hard, unless you have a lot of money or you don’t give a damn,” she told me. When her son was ready to go to college, she started dating again. She was fifty-eight. Through a dating service, she met an economist, who was eight years younger than she. They lived together for a decade. Eventually, Taft told me, “he had to go to other cities to look for other jobs. I didn’t go. And that was that.” In 2000, she put an ad in Harvard Magazine. “This seemed horrible to me, but I got all kinds of responses. A nice guy from Vermont drove all the way down to see me.” And then, when she was almost seventy, she discovered Internet dating, and the frequency and variety of her assignations intensified.

She met a mathematician who lived in Amsterdam, and flew over to meet him but discovered within minutes that he suffered from full-blown O.C.D. She drove up to New Hampshire in the rain for lunch with a man with whom she’d been carrying on a promising e-mail and telephone correspondence for a few days, but he told her that he found her unattractive. She met a financier on Yahoo’s dating site. They got together for coffee at Café Pamplona, in Cambridge. He was handsome, charming, and bright. He was also, as a friend’s follow-up Google search revealed, a felon, and had served time in prison in a RICO case. “I did see him again,” she said. “And then I realized how crazy he was. He wasn’t nice, either.” For two years, she has had an off-and-on affair with a forty-seven-year-old man she met on Yahoo, and she recently met a man on Match.com who showed up for their first date wearing a woman’s sun hat, slippers, and three purses. He invited her to accompany him to Norway to meet the Queen.

“You have to learn the rules,” she said. “But there are no rules.” More often than not, she initiates contact. “At my age, I have to.” She also feels that, in her profile, she has to shave a few years from her age and leave out the fact that she has a doctoral degree, having concluded that men are often scared off by it. She has gone online as a man, just to survey the terrain, and estimates that in her age range women outnumber men ten to one. “Men my age are grabbed up immediately by friends,” she said. “Or else they believe that younger women are more interested in sex.

“I’ve learned, forget about writing,” she said. “Meet a person as soon as you can. Anyway, the profiles you read, they’re like bathtubs. There’s no variation.”

If the dating sites had a mixer, you might find OK Cupid by the bar, muttering factoids and jokes, and Match.com in the middle of the room, conspicuously dropping everyone’s first names into his sentences. The clean-shaven gentleman on the couch, with the excellent posture, the pastel golf shirt, and that strangely chaste yet fiery look in his eye? That would be eHarmony. EHarmony is the squarest of the sites, the one most overtly geared toward finding you a spouse. It was launched, in 2000, by Neil Clark Warren, a clinical psychologist who had spent three decades treating and studying married couples and working out theories about what made their marriages succeed or fail. He had noticed that he was spending most of his time negotiating exit strategies in marriages that were already irreparably broken, mainly because the couples shouldn’t have been married in the first place. From his own research, and his review of the academic and clinical literature, he concluded that two people were more likely to stay together, and stay together happily, if they shared certain psychological traits. As he has often said, opposites attract—and then they attack. He designed eHarmony to identify and align these shared traits, and to keep opposites away from each other.

Warren was also a seminarian and a devout Christian, and eHarmony started out as a predominantly Christian site. The evangelical conservative James Dobson, through his organization Focus on the Family, had published advice books that Warren had written and provided early support and publicity for eHarmony. It didn’t match gay couples (its stated reason being that it hadn’t done any research on them), and it sometimes had trouble finding matches for certain kinds of people (atheists, for example, and people who’d been divorced twice). As it has grown into the second-biggest fee-based dating service in the world, eHarmony has expanded and shed its more orthodox orientation, and severed its connections to Dobson. In 2009, under pressure from a slew of class-action lawsuits, it created a separate site specifically for homosexuals. Still, the foundational findings of Warren’s psychology practice remain in place—the so-called “29 Dimensions of Compatibility,” which have been divided into “Core Traits” and “Vital Attributes.”

These undergo constant fine-tuning in what eHarmony calls its “relationship lab,” on the ground floor of an anonymous office building in Pasadena. The director of the lab, and the senior director of research and development at eHarmony, is a psychologist named Gian Gonzaga. He and his staff bring in couples and observe them as they perform various tasks. Then they come to conclusions about the human condition, which they put to use in improving their matching algorithms and, perhaps just as important, in getting out the word that they are doing so. There is a touch of Potemkin in the enterprise.

One night in March, Gonzaga invited me to observe a session that was part of a five-year longitudinal study he is conducting of three hundred and one married couples. EHarmony had solicited them on its site, in churches, and from registration lists at bridal shows. Of the three hundred and one, fifty-five had met on eHarmony.

Gonzaga, an affable Philadelphian, introduced me to one of his colleagues, Heather Setrakian, who was running the study. She was also his wife. They’d met in the psychology department at U.C.L.A., where Gonzaga was conducting a study on married couples. Setrakian, who had a master’s in clinical psychology, was the project coördinator. To test their procedures, they needed a man and a woman to impersonate a married couple for multiple sessions. Gonzaga and Setrakian became the impersonators, and fell in love. “Some of our fake marriages had a lot more money than we have now, and a trampoline, and in-laws in Utah,” Setrakian said.

The eHarmony relationship lab consists of four windowless interview rooms, each of them furnished with a couch, easy chairs, silk flowers, and semi-hidden cameras. The walls were painted beige, to better frame telltale facial expressions and physical gestures on videotape. “With white walls, blondes wash out,” Gonzaga explained. Down the hall was the control room, with several computer screens on which Gonzaga and Setrakian and their team of researchers observe their test subjects.