Why do people fall in love with one person rather than another?
There's no simple reason why Brad Pitt ditched Jennifer Aniston and later shacked up with Angelina Jolie.
But anthropologist Helen Fisher reckons it depends on what kind of lover you are -- an explorer, builder, negotiator or a director.
Helen's fifth book on the subject -- "Why Him? Why Her?" -- looks at our chemical make-up. It follows four others including "Why We Love" and "The Anatomy Of Love."
Helen explains: "I know what happens in the brain when you fall in love but why would that brain system become activated by some people and not others? Nobody knows. People often say 'we had chemistry or we didn't have any chemistry.' So I started toread more widely about genetics.
"There are all kinds of chemicals in the brain but most of them work physical aspects of the body -- only a few are associated with traits of personality and they are dopamine, serotonin, testosterone and estrogen. I created a questionnaire to find out to what degree you express these four types.
"I then watched my new dating site to see who is drawn to who. I thought maybe certain types would be drawn to certain types."
Helen's dating website, Chemistry.com, now has five million people registered in the States with a further two million people registered from 39 other countries. Her questionnaire, on the Match.com website, looked at a selection of 28,000 people to findout if opposites attract or if like attracts like and she found it's a mixture of both.
Helen said: "The director and negotiator really go for each other.
They are pooling very different resources to raise their babies. They complement each other. The builders are really going to build home, family and community together but the hardest to explain is two explorers being together.
"Who's going to look after the baby when you are racing off to Timbuktu? But I think these people are likely to have more than one partner and are more likely to have a series of marriages." There are also secondary types. Helen suggests Brad Pitt is an explorer/negotiator while Angelina Jolie is an explorer/director.
She says: "Angelina has tattooed a window on her body, she needs freedom and she's with a man who likes to ride around on motorcycles and who gives her that freedom.
"I suspect his secondary might be the negotiator and her secondary is the director. They could be drawn to each other's novelty seeking."
Helen has discovered her book is not just for people looking for love. She says: "I handed in a rough draft and my editor called me up and said 'I finally understand my son.'"
The book has also left a lasting impact on Helen's personal life as she admits: "It was like finding a missing piece of the puzzle. You see these patterns around you. I understand my mother better, I always understood my twin sister and I certainly understand certain things I will never get from my explorer boyfriend."
The Explorer
This adventurous type is associated with raised dopamine levels. They live for today, are novelty seeking, creative and curious. Independence is important to them.
Negatives: Unpredictable, unreliable, reckless.
Ideal partner: They look for a play mate.
Famous Explorers: Brad Pitt, Angelina Jolie, George Clooney, Barack Obama
Likely to go for: Another Explorer.
The Builder
Loyal and linked to the chemical serotonin which induces calmness. They follow rules and are traditional. They are often cautious and orderly. Positions of authority may suit them.
Negatives: Set in their ways, can be stubborn.
Ideal partner: They look for a help mate.
Famous Builders: Gordon Brown, Sarah Brown, Jennifer Aniston.
Likely to go for: Another Builder.
The Negotiator
Negotiators are empathisers and linked to levels of estrogen. They are imaginative and good with words. They like to please, are compassionate, intuitive and emotionally expressive.
Negatives: Can be indecisive and gullible.
Ideal Partner: They look for a soul mate.
Famous Negotiators: Bill Clinton, Oprah Winfrey, Darwin, Ghandi.
Likely to go for: The Director.
The Director
Directors are systemising linked to raised testosterone. They are direct, competitive, inventive and logical.
Negatives: Can be blunt and rigid.
Ideal partner: They look for a mind mate.
Famous Directors: Bill Gates, Einstein.
Likely to go for: The Negotiator.