But new research appears to turn this theory on its head. Scientists now believe that people of mixed race, particularly Eurasians, possess certain genetic advantages that lead to greater health and, as a result, increased attractiveness.

In the first study of its kind, Caucasians and Japanese people rated Eurasian faces as more attractive than faces of either race. Researchers developed a series of faces, ranging from those with exaggerated Caucasian features to those with exaggerated Japanese features. When Caucasian and Japanese volunteers looked at photographs of Caucasian, Japanese and Eurasian faces, both groups rated the Eurasian faces the most attractive and healthiest. People from other racial backgrounds will, of course, have their own preferred blends.

One researcher said the results proved that "our preferences are shaped by evolution". Humans would have encountered few individuals of mixed race when they first evolved. Only with the West's colonisation of Africa, the Americas and the Far East, as well as the trade links that were then established around the world, did different races mix more readily.

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Evolutionary psychologists argue that the findings indicate not only individual preferences for physical beauty. They also suggest that Eurasians and other mixed race individuals appear healthier. In the search for a reproductive partner, humans look for markers of good genetic health.

Dr George Fieldman, a leading British psychology expert at Buckinghamshire Chilterns University College, said facial symmetry also played a part: "It is conceivable that mixed races are ironing out asymmetries and differences of the kind that may make you more attractive."

The number of mixed race people in Britain grew by 75 per cent during the 1990s. In the 2001 census, roughly 1.5 per cent of the population classed itself as of mixed race.

New research by British scientists also suggests that a link may exist between the gene diversity and beauty. Craig Roberts, a biologist at Liverpool University, conducted an experiment into the attractiveness of individuals and compared their "beauty" to the diversity of their major histocompatability complex (MHC) genes. MHC genes have a big impact on the strength of immune systems.

Dr Roberts found that photographs of people with a greater MHC diversity were seen as more attractive than those with less. He concluded that the perceptions of attractiveness are linked to the health of a potential mate.

Not only does the research by Gillian Rhodes, a psychologist at the University of Western Australia, raise interesting questions about beauty, it also undermines centuries-old arguments by the far right that a "pure" race is healthier.

The idea of racial purity as the highest form of perfection was taken to its extreme by Adolf Hitler, whose warped beliefs led to the death of six million Jews. Dr Fieldman said: "In-breeding is not great news. All that bollocks about blue blood is just nonsense. The Nazis were terrible scientists. At a biological level it was just nonsense."

This view is backed up by Randy Thornhill, an evolutionary psychologist at the University of New Mexico. Genetic diversity can decrease the chances of contracting disease, he said. "If you hybridise two genetically diverse populations - ie cross races - then you create more genetic diversity in the offspring."

But while science may be able to prove the attractiveness of mixed-race individuals, relationship experts said cultural barriers still remain when considering mixed race partnerships. Dr Petra Boynton, a lecturer in health services research at University College London, said: "In some ways it is more acceptable now than it was 50 years ago. But this is a lab-based piece of research and, well- meaning and liberal though it is, you only need to look outside the window to see it is not that simple."