Will you marry me?
It's a simple question made difficult because a lifetime of commitment rides on the answer. And how we arrive at this sought-after moment is one of the most complicated mysteries of our lives. How do we find love? Or more important, what exactly is love? Why do we feel it for one person and not another?
Biological anthropologist Helen Fisher has devoted a lifetime of research to these questions, and now she believes she has found some of the answers.
"I feel as if I've stumbled into Mother Nature's kitchen and found some of her recipes," Fisher said.
Watch the story Friday on a special edition of "20/20" at 9 p.m. ET
A seminal moment in Fisher's research occurred when she and her colleagues, Arthur Aron and Lucy Brown, started putting young lovers in an MRI machine. When they showed these subjects photos of their partners, a small region of the brain called the ventral tegmental area lit up on the scan. The ventral tegmental area produces the stimulant dopamine, and sends it to the brain's reward centers -- the centers for wanting, craving, motivation, focused attention and ecstasy.
For Fisher, it was a revelation.
"This made me realize 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," she said.
But while Fisher thought she knew what love was, it took a call from an Internet dating service to get her thinking about why we fall in love.
"They asked me a question: Why do you fall in love with one person rather than another? And I said, 'I don't know. Nobody knows.'"
Fisher found the question intriguing, and she began to suspect that the answer must lie somewhere in the brain.
"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, looking, for example, at studies that have associated specific genes with certain behavior patterns, at studies that have examined the impact of recreational drugs on the brain and at studies that have examined the change in personality brought about by drugs like L-Dopa, which drives up dopamine and is given to people with Parkinson's Disease.
The Road to Romance
In the end, Fisher came to believe that there were four broad biological personality types associated with four specific neurotransmitters and hormones: dopamine, serotonin, testosterone and estrogen.
Fisher believes we are all some combination of these four types, which she has named the explorer (dopamine), the builder (serotonin), the director (testosterone) and the negotiator (estrogen).
Good examples of the risk-taking explorers, Fisher said, would be John F. Kennedy, Ernest Hemingway and Angelina Jolie. Colin Powell, she said, exemplifies the calm, traditional builder. Directors are analytical, tough-minded and decisive, and Fisher believes Bill Gates is a perfect example. Negotiators, on the other hand, are emotionally expressive and empathetic. A good example? Bill Clinton. "He says, 'I feel your pain,'" Fisher laughed.
"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."
Fisher teamed with Internet dating site Chemistry.com, and together they developed a questionnaire designed to elicit which personality type a member most resembles. To date, 7 million people worldwide have taken the questionnaire, and Fisher has been able to see who matched up with whom. She has put the results of her work into a new book titled "Why Him Why Her?"
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.
To get an up-close look at the science of love, Fisher also asked Chemistry.com to host a party for 150 singles in New York. Everyone was assigned a personality type based on Fisher's questionnaire, and everyone was given a task: meet a list of potential partners based on Fisher's insights about romantic compatibility. When the party was over, Fisher chose five couples for "20/20" to follow. Watch this Friday as we follow them on the road to romance.
Are you an explorer, a builder, a director or a negotiator? Answer the questions on the next page to find out. Or go to WhyHimWhyHer.com to take Fisher's complete questionnaire.
Answer the following questions from Fisher's test to find out more about your personality:
What do your doodles look like?
Doodles are used to determine if you are empathetic, assertive, risk taking, traditional, competitive, curious, or have a need for order.
Hearts: Negotiator, Geometric Shapes: Director, Repetitive Squiggle: Builder, Abstract Form: Explorer
If you were the publisher and had to choose a title for this book based on the cover below, what would it be?
A. Adventures on the Rhine
B. Anatomy of Friendship
C. Power Plays
D. Things Left Unsaid
Explanation: The title you select reflects if you seek intimate connections, build stable long-term ties, are logical and analytical or a thrill seeker. A. Explorer; B. Builder; C. Director; and D. Negotiator.
How does the length of the index finger on your right hand compare to your other fingers? Which hand in this photo most resembles your right hand?
Explanation: Testosterone builds the length of the ring finger in the womb. The longer your ring finger in relation to your pointing finger, the more likely you are to have mathematical, mechanical and/or musical skills.