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Ah, the eternal question: Why is he with her? Biological anthropologist Helen Fisher thinks she has found the answer after studying academic literature on personality and poring over 40,000 responses to a questionnaire on a dating website. A Rutgers professor and paid adviser for Chemistry.com, Fisher not only believes in romantic chemistry but is zeroing in on specific chemicals. She spoke with TIME about her latest book, Why Him, Why Her: Finding Real Love by Understanding Your Personality Type. (See pictures of the 20th century's greatest romances.)

A lot of things influence who we're attracted to, but one thing that has always puzzled scientists is the role that personality plays in mate selection. Have you solved that riddle?
There are two parts of personality. There's character, which is everything you grew up to believe and do and think. And then there's temperament, which is your inherited traits. Some people are more stubborn than others, some are more curious, some are more aggressive. What I'm trying to do is add the role of biology, of temperament, to our human understanding of love. (See pictures of other species in love.)

You basically break people down into four broad temperaments, each associated with certain brain chemicals.
There was a great deal of data that people vary in terms of their expression of dopamine and norepinephrine, serotonin, estrogen and oxytocin and testosterone. I culled from the academic literature all of those data points that show that these particular brain-chemical systems are related to certain aspects of personality. And I saw constellations of temperament traits that seemed to be associated with these chemicals.

What are the four types?
People who express dopamine — I call them Explorers — tend to be risk-taking, curious, creative, impulsive, optimistic and energetic. The traits associated with the serotonin system express themselves in what I call Builders. They're cautious but not fearful, calm, traditional, community-oriented, persistent and loyal. Directors have traits associated with activity in the testosterone system. These people tend to be very analytical, decisive, tough-minded; they like to debate and can be aggressive. The fourth type is the Negotiator. Men or women who express activity in the estrogen system tend to be broadminded imaginative, compassionate, intuitive, verbal, nurturing, altruistic and idealistic.

How did you choose the four types and ascertain who they like?
They emerged out of the genetic literature. I didn't impose them. I read the literature, and I found them. And then I developed the questionnaire to make sure these people did express these four [types of temperament] and expressed them in these ways. That study was done on 40,000 people. And then on the dating site Chemistry.com, I watched who gravitated to whom. (Read about the effects of romantic movies.)

Could you actually test people for these chemicals?
We're doing that study right now at Pacific University [in Oregon]. It's taken us longer than we expected, and the problem is that it's a college population. We can get participants, but they're taking Ritalin or Wellbutrin or Prozac or anabolic steroids, or they're taking birth control pills or they're taking cocaine, or something which alters their brain chemistry. Science is not easy.

Is it possible that you could have a blood test and know what kind of partner you should look for?
It's possible. Remember with mate selection that timing is important and proximity is important. And don't forget your childhood plays a role. Let's say you're an Explorer type who grew up with Explorer parents who were just so nutty that you were constantly holding on to your high chair. Then in your college and early 20s, you went out with people who were risk-taking and novelty seekers, curious and creative like yourself and had some bad experiences. You might turn to a nice, solid, loyal, conscientious family-oriented Builder to have your babies. Even though it might not normally be the most natural type for you.

So you can use your system to see who you might be drawn to and seek them out?
There are other things too. When you're going out on a date, if you understand your primary type and the type of person you're going out with, you can better reach them and create more intimacy with them. Because the four types define intimacy differently and look for different things in a partner. They even use different words. In one of my studies on Chemistry.com, I looked at what words people used and, sure enough, the four primary types used very different kinds of words. If you use the words that ring true to this person's temperament type, with your lover or even your children or your lover's parents, and behave in ways that click with this temperament type, you can reach people more effectively.

If we're to some extent directed toward certain people by our neurochemicals, does that mean if we take Ritalin or Prozac or are on the Pill that we're likely to make bad dating decisions?
Yeah, that's a problem. You're going to marry a different kind of person. As long as you stay on the drugs, it might be O.K. What I'd do is get off drugs before you fall in love — and marry the person after that very early intense stage of love has worn off. I've always maintained that it's adaptive to marry after that stage. I think all over the world people are doing this, because they're living with their partners and even having children first.

Explorers are drawn to Explorers, Builders are drawn to Builders, and Negotiators and Directors are drawn to each other. But what if you're already married? Is your study useful then?
Yes. You can understand where the pitfalls are going to be and how to avoid them. In a long relationship, you have to pick your battles. There are some you're just never going to win. And there are others you'll win more effectively if you profoundly understand who your partner is.

According to your theory, I'm an Explorer-Director married to a Negotiator-Explorer. Is there any hope?
How long have you been married?

Eighteen years.
Do you have any children?

Two.
Well, from a Darwinian perspective, you've already won.

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• Belinda Luscombe