The romantic saying "their eyes met across a crowded room" could have some scientific basis.

It turns out that eye contact with a pretty face is enough to start the brain buzzing within seconds.

British researchers have found that when someone sees an attractive face, their brain's reward centre lights up.

They believe bonding with attractive people has an evolutionary advantage and is hard-wired into the brain.

As well as the more obvious rewards of finding a suitable mate, associating with attractive people perhaps enhances someone's social status.

Knut Kampe of the Institute of Cognitive Neuroscience at University College, London, led the research.

He told BBC News Online: "From an evolutionary point of view, it is important to rate someone as attractive because this indicates health, strength etc.,

"Humans, like monkeys and apes, are animals that live in a very complex society.

"It is very important to know rapidly with whom it might be rewarding to bond with.

"In that way it makes sense to bond with attractive people, irrespective of sex."

Monkey hierachy

In monkeys, for example, bonding with an animal higher up the pecking order confers social status on an individual.

Liasons with someone deemed attractive could have similar advantages in humans, he believes. But "mere beauty" is not enough, it seems.

"Attractiveness relates to more than just the fairness of the face," Dr Kampe told BBC News Online. "That was confirmed by our study."

He said volunteers asked to rate faces as attractive looked for qualities such as radiance, empathy, cheerfulness, and even motherliness, as well as conventional beauty.

Eye contact

In the experiments, 16 volunteers (eight men and eight women) were shown colour images of 40 different unknown faces, while their brains were scanned.

They were then asked to rate the attractiveness of each face on a scale of one to 10.

Faces deemed attractive by the subject, irrespective of gender, activated a certain part of the brain.

This only happened when there was eye contact, not when the pictures showed an averted gaze.

The part of the brain activated is the ventral striatum, the brain's reward centre.

Studies in monkeys and rats have shown that this part of the brain lights up on anticipation of a reward such as food or water.

It is also involved in addiction. Asked whether it might be possible to become addicted to a pretty face, Dr Kampe said he thought not.

"Addiction is an extreme example," he told BBC News Online. "I don't think normal faces would lead to addiction.

"It must be a super face," he added.

First impressions

The research also shows that an attractive face is clocked by the brain in a matter of seconds.

This suggests it is an automatic process, perhaps hard-wired into the brain.

It could account for the finding in psychological tests that people make an instant judgement about a stranger based on first impressions.

The face research is published in the scientific journal Nature.