After spending a decade at Match.com and Yahoo Personals writing compatibility formulas, psychologist Mark Thompson had fallen out of love with his work. "Early on, there was real enthusiasm that we were going to figure out how to bring people together," he says. "But now the industry is so competitive that it's more about what sites claim they can do than what us eggheads could actually do for people." So Thompson quit and used what he'd learned to write an analog guide to finding your libidinous match, Who Should You Have Sex With? How to Find (or Reignite) Great Sexual Chemistry (Sourcebooks, 2010). We talked to him about it, and about his booming former industry.

MC: What made you leave e-dating?

MT: I hated the way we overpromised and underdelivered. Our studies showed that the odds of meeting someone online and dating him more than a month are roughly one in 10. So it's great that all those people on the TV commercials met their spouses, but they are the exceptions, not the rule. No computer can accurately predict whom you should be with. The function of the math will make vastly more false predictions than accurate ones.

MC: But isn't blind dating always hit or miss?

MT: Yes, but you don't have to pay $30 a month to be set up by your friend. And you don't go in believing that science is behind the match. There's a different set of expectations. When diet companies show someone who lost a bunch of weight in six weeks, they have to say, "Results not typical." I think eHarmony and other sites should do the same.

MC: Do you think online dating can be fixed?

MT: It really depends on people's willingness to come back and tell us why each date didn't work out so the system could get smarter. It would be like Netflix, which learns from your preferences to make better predictions for you.

MC: Your book is about finding love. Why did you call it Who Should You Have Sex With and not Who Should You Go Out With?

MT: Because sex is a huge part of a relationship that people don't always focus on. Couples today are unwilling to settle for sexual boredom. In my studies, I've found that about half of daters say they're looking for "sexually passionate romance" and want to have a "great" sex life. Those who don't are usually worried about winding up with a partner who wants more or kinkier sex than they do. So it's a critical question.

MC: But doesn't happiness have more to do with emotional compatibility?

MT: Sexual compatibility is a keystone to a relationship's long-term success, and it requires three things: a similar emotional approach (positive, friendly, and fun, or dark and mysterious); a similar activity level (fast and active, or slow and mellow); and a complementary power dynamic (strong, confident, and powerful, or gentle and submissive). These qualities often line up with what you want interpersonally, too.

MC: How can we know if there's chemistry before we have sex?

MT: People's public personas often correspond to their sexual personas. Mr. Adventure in public tends to be Mr. Dominant in bed. He should probably stay away from Ms. Passion because they both like to be in control. Mr. Shy in public is likely to be Mr. Romance in bed. He should steer clear of Ms. Sinner because she likes darker sex than he does.

MC: With all your vast knowledge, how do you date?

MT: I'm more of an introvert, and I haven't found the Internet very effective. People who are outgoing tend to do better with Internet dating. I'm more likely to meet someone through a hiking group.

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