Close-up of young woman's faceIt's true that you can't judge a book by its cover and it can often be said that beauty is in the eye of the beholder.

But scientists now believe that they have proved that beauty really is only skin deep.According to researchers from the University of Gottingen in Germany, the evenness and glow of a woman's skin is the key factor in the attraction stakes.

They photographed 170 women's faces, aged from 10 to 70, and applied their skin to an idealised computer model.

Scientists then asked 430 volunteers to score the images for attractiveness, the only changing factor being the quality of the skin.

Dr Paul Matts, a skin researcher at Procter & Gamble who worked on the project, told The Sunday Times: "The highest scores went to the models with the most even skin tone. It seems to be more important than almost any other factor."

In fact, where once it was thought that the symmetry of the face was top of our beauty perceptions, the study revealed that people were much more likely to avert their eyes from the dreaded uneven tone.

The researchers believe that humans may have developed this subconscious skin analysis as an indicator of good health.

Dr Bernhard Fink, an expert in evolutionary psychology, said: "Skin tone, especially in women, says a great deal about reproductive potential.

"Humans are highly evolved to pick up on cues like that because at one time in our past it would have been crucial for reproductive success."

But as many of us aren't lucky enough to have flawless skin, it's no wonder that sales of foundation are soaring.