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.com, 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?

Success rates for online dating are hard to measure. But eHarmony says it clearly enhances the process by catering to people who are looking for relationships leading to marriage.

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.com, 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.