IF finding true love were an exact science, we wouldn’t need matchmakers, singles bars or, of course, online dating services

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Like job seekers who take the Myers-Briggs personality test to help steer them to suitable professions, we’d simply take a relationship test, whose results would identify our most compatible types of mates and rule out the frogs. Problem solved.

Of course, Cosmopolitan magazine has been running pop psychology love quizzes ? �Which Bachelor Is Right for You?,� �Is He Naughty or Nice?� ? for decades, prompting young women the world over to assess how sexually or socially compatible they might be with their objects of desire.

Now, a handful of dating Web sites are competing to impose some science, or at least some structure, on the quest for love by using different kinds of tests to winnow the selection process. In short, each of these sites is aiming to be the Netflix of love.

Instead of using a proprietary algorithm to recommend movies you might enjoy, based on your past choices, however, these dating sites offer you a list of romantic candidates whose selection is based on proprietary analyses of personality characteristics or biological markers.

Consider ScientificMatch, founded about two years ago, which aims to create romantic chemistry via genetic testing. The site, which matches people based on certain genetic markers for the immune system, takes its cue from studies showing that women are more attracted to the smell of men who have very different immune systems from their own. The site charges $1, for a lifetime membership ? the lofty fee includes a cheek swabbing kit, DNA processing, a criminal and bankruptcy background check, as well as verification of age and marital status, the site says.

Indeed, at least from the point of view of evolutionary science, you’d be better off spending $50 ? and more likely to find a mate ? by using a premium dating site than by dropping $50 on drinks in the uncertain waters of singles bars

Then there’s Chemistry, started in 2006 by the dating giant Match. Helen Fisher, the biological anthropologist who developed Chemistry’s questionnaire, says the site is designed to predict compatibility based on traits of temperament like adventurousness, decisiveness or empathy. And it charges a premium for its services: about $50 for a one-month membership, compared with about $35 for Match.

Founded in 2000 by a psychologist with experience in marriage counseling, eHarmony focuses on singles willing to invest time and to pay premium prices to find a long-term partner. People who register with the site fill out a long questionnaire that is intended to match people based on similarities in sociological variables like values, family background and social styles. Membership can cost up to $ a month.

Online dating is a $976 million annual industry in the United States, according to estimates from Marketdata Enterprises, a research firm. So, to stand out among hundreds of mass-market, open-community sites that attract everyone from people trolling for quick hookups to those headed for holy matrimony, a few services offer more elaborate mate-finding methods.

They build brand identity when they �target people who are looking for relationships rather than just dating,� says John LaRosa, the research director at Marketdata Enterprises. That means matchmaking sites with fewer users can charge more per subscriber than larger sites that list online personals.

Match, with an estimated 1.2 million paid subscribers, had revenue of about $365 million in 2008, Mr. LaRosa estimates. EHarmony, meanwhile, with about 656,000 paid members, had estimated revenue of $216 million that year, he says. But do partner-prediction sites do better at matching people than less-structured dating sites where people seek, sort and select others on their own?

People tend to be adept at heeding that first spark of attraction but may be less dexterous at recognizing the commonalities that are the foundations of good marriages, says Gian Gonzaga, eHarmony’s senior director of research and development. The site suggests potential matches based on areas of compatibility ?like values, beliefs and important experiences ? that are predictors of relationship success, he says.

�In the long haul, you want to be able to manage conflicts, celebrate positives and get through the day-to-day relationship,� Dr. Gonzaga said. �Our system is there to take care of that so you can now focus on who you find really attractive, that you feel really passionate about.�

Chemistry, meanwhile, uses answers to a detailed questionnaire to suggest potential partners based on their brain chemistry, says Dr. Fisher, a research professor in the anthropology department at Rutgers University. Based on a review of scientific studies on neurotransmitters and chemicals like dopamine in the brain, she determined that humans tend to express one of four dominant temperaments.

But eHarmony says it clearly enhances the process by catering to people who are looking for relationships leading to marriage

Since the site’s introduction in 2006, more than eight million people have answered Dr. Fisher’s questionnaire, and she has used their answers to pinpoint traits that attract people to one another. She says people of ent, whom she calls �directors,� tend to be attracted to empathetic, intuitive types she calls �negotiators.� Spontaneous types (�explorers�) tend to be attracted to their own kind, while traditional pillars of society (�builders�) also tend to seek out partners that resemble themselves.

�If Helen Fisher can give you right off the bat individuals that your brain is more likely to be attracted to,� she says, �so much the better.�

At the end of the day, however, it may be that the success of such sites is attributable not so much to their proprietary methods as to their choosy, self-selected members who don’t want to wink at and woo the first person whose profile they read online. The sites attract cohorts of people interested in slowing down the online dating and mating process, in finding out more information about potential partners ? or in ruling out unlikely suitors ? before they graduate to the meet-and-greet stage.

THE more advanced the partner prediction sites, the more they may actually serve a more old-fashioned role. The sites provide background details on a person’s family, education, aspirations, character, genetic traits and general health of the type that was once public information in farming or immigrant communities or even in hunter-gatherer societies, Dr. Fisher says.