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.
Getting to Know You: 'It Takes Time'
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.
Rudy and Lizzie were both explorers, and at the Chemistry.com party, investment banker Rudy, with his booming laugh and bonhomie, was a big hit. The quieter Lizzie, who works in the home textile business, said, "He was easy to talk to."
Rudy and Lizzie's first date began with an intimate dinner at an Italian restaurant, but right away they began to find differences, not common ground.
Rudy is into sports, while Lizzie describes herself as a "cultural woman." To Fisher, this was not a good sign. "These are two explorers and two explorers are naturally drawn to each other generally. But they have to have the same interest or want to learn about each other's interests."
But halfway through the date, Lizzie seemed relaxed about how things were going: "He's a cute guy, and you know, we'll see, we'll see. It takes time to really get to know someone."
Chemistry: It's There or It's Not
Tim and Mary, the fourth couple, seemed more at ease on their first date. He's a negotiator who works as a recruiter for a medical market research company; she's a director who works as a tax manager in the mergers and acquisitions field.
Their first date began at an outdoor cafe, and by the time they had taken an evening pottery class, they were laughing heartily and trading zingers.
"Don't you mess with my piece!" Tim jokingly warned Mary, while she teased him about his slow cleanup efforts afterward. "I'll help you out," she said, adding "I don't want to be here all night."
But despite all the laughter, at night's end, reviews of the date were mixed.
"It's just sort of that je ne sais quoi, if you'll excuse the term," Tim said. "It's that unsaid chemistry, umm, that's either there, or it's not, or it's somewhere in between. I think it's somewhere in between."
Tim and Mary never made it beyond that first date, so Tim found another match online with Stephanie, another negotiator. Tim and Stephanie's relationship took off from the start, and five months later was still going strong.
Whether strolling in the park or riding in a pedicab, Tim and Stephanie could barely keep their hands off one another. In both a dark bistro and a trendy restaurant, they engaged in endless "courtship feeding" -- serving one another food and drink.
But Fisher was not convinced that these two negotiators would maintain their relationship in the long term. "What's interesting about two negotiators," said Fisher, "is they can sort of implode."
Five months later, Maggie and Brandon were still together. "20/20" caught up with them when they were clearly having a good time at a rock-climbing lesson, followed by drinks at a dark bar. But as the evening wore on, Brandon's insecurity emerged.
"You kind of intimidate me," he said. "I feel like I'm not smart enough, funny enough."
Fisher said this is not atypical for Brandon's personality type: "They undermine themselves -- negotiators do."
After 10 dates, Maggie called it quits. For her, it was another failure that underscored the complexity of love. "I want someone who I can be with," she said, "but I don't know how to find it."
Still Looking for Love
After several months of dating, Jason and Nacole also ended their relationship, but on a happier note. "I don't think there was anything I didn't like about her, "Jason said.
Nacole echoed that sentiment: "I really couldn't think of anything that I really didn't like about Jason. … we, you know, just worked in different areas of the world."
"Chemically, it's a very good match," Fisher said. "But we also have to deal with their upbringing, their ambitions, their interests."
Fisher watched as cameras rolled on Lizzie and Rudy's date at a rowdy quiz night at a local bar.
"She's turning away from him," Fisher said of Lizzie. "When you turn the shoulder away and the head away, it's not good."
After 45 minutes, Lizzie asked to go home. Rudy seemed dejected, but Fisher predicted he would bounce back.
"Explorers are flexible," she said. "Rudy will go on to somebody else who is more charming and charismatic and works for him. And it'll be an explorer."
All of the people "20/20" followed remain hopeful that they will eventually find a life partner. And despite years of study, Fisher says that the science of seduction is still evolving.
"If everybody could match people easily, we wouldn't have been talking about romantic love for the last 4 million years."