Biological anthropologist Helen Fisher, who has spent a lifetime studying how and why we fall in love, says there is a science to the art of seduction, but it's never simple.
Fisher's research has taught her that "romantic love is not an emotion. It's a basic mating drive. A motivation system. A system to try to win life's greatest prize, which is the right mating partner."
The author of "Why Him, Why Her?" teamed up with the dating Web site Chemistry.com to try to understand that system -- to determine why we fall in love with one person rather than another.
"What I had to do," she said, "is figure out if there was any biology to your behavior."
Fisher began an exhaustive review of the scientific literature and eventually came to believe that there were four broad biological personality types associated with four specific neurotransmitters and hormones: dopamine, serotonin, testosterone and estrogen.
Fisher suspects we are all some combination of these four types, which she has named the explorer (risk-taking, associated with dopamine), the builder (calm, traditional, associated with serotonin), the director (analytical, tough-minded and decisive, associated with testosterone) and the negotiator (emotionally expressive and empathetic, associated with estrogen).
"So I looked at these four chemical systems and I thought to myself, maybe you could create a questionnaire to see to what degree we express each of these," she said. "And if I could create a questionnaire that could establish who you are, and then on a dating site, watch which biological type is drawn to which other biological type, I might come closer to understanding why him, why her."
To date, 7 million people worldwide have taken the questionnaire, including four couples that "20/20" followed as they embarked on new relationships. "20/20" would pick up the tab for their dates, but would any of them find lasting love?
Go to WhyHimWhyHer.com to take Fisher's complete questionnaire.
So which types do attract? Fisher has found that explorers go for other explorers, and builders go for other builders. But the high testosterone directors go for the high-estrogen negotiators, and vice versa.
While the four couples "20/20" profiled do not represent a large enough sample to draw scientific conclusions, the experiment represents a kind of human petri dish for Fisher to study the pitfalls that every relationship faces. Would any of these couples turn that initial meeting into lasting love?
Brandon and Maggie, both journalists, hit it off right away. True to type, Brandon, a negotiator, seemed more emotional and expressive than Maggie, a director. Their first date was at a riverside cafe, and before it was over Brandon was smitten.
"I could picture myself getting to know Maggie for the rest of my life," he said.
Jason and Nacole were also a male negotiator/female director pairing. The tall, willowy Nacole is an aspiring opera singer, and the even taller Jason used to play in rock bands but is now an Internet entrepreneur. Jason and Nacole's first date was a private French cooking class, followed by a charming candlelit dinner.
By the end of the evening, Nacole was finding Jason's easygoing negotiator style calming and reassuring. They capped the evening with a carriage ride in Central Park and a self-conscious kiss in front of "20/20's" cameras.