Ever wondered what is the "chemistry" that binds couples together? Shared intelligence, looks, and values count for a lot, but biology has a role to play too, according to experts.

"Nobody really knows why we fall in love with one person and not another. Factors like intelligence and shared values are only half of the puzzle. The other half is how your biology influences who you fall in love with," Helen Fisher of Rutgers University in New Jersey told the 'New Scientist'.

In fact, according to her, temperament - that innate, biological element of personality all people are born with and which stays with them through life, could well be involved in romantic attachments.

Fisher started with her own model of temperament that she says is more firmly rooted in biology than previous ones. Drawing on a large body of work on the genetics and neurochemistry of personality, she divided people into four basic temperament types - explorers, builders, directors and negotiators.

Each type is associated with elevated activity in one or two neurochemical systems in the brain which lead to a set of stereotypical personality traits.

"We all express all four temperaments to some degree, but we express some more than others. I am a builder and an explorer," she said.

In their study, Fisher and her colleagues drew up a personality questionnaire designed to work out whether you are an explorer, builder, director or negotiator. They gave the questionnaire to more than 28,000 people and then tracked who was hooking up with whom.

They found a clear pattern. In a nutshell, explorers are more likely to go for explorers, directors for directors, but builders go for negotiators and vice versa.

Up to now she has only looked at the first flush of love, but Fisher expects the pattern to apply in the long term too, "I am confident that looking at initial attraction tells you something about long-term attraction."

This isn't a hard-and-fast rule, says Fisher, but it is the strongest evidence yet that what we call chemistry is partly down to our biology.