Scientists have discovered that symmetrical women, those whose left sides are pretty much identical to their right, have 30% higher levels of female hormones, increasing the chances of conception.

"Our results suggest that symmetry is related to higher levels of oestradiol and, thus, higher potential fertility. As a consequence, men attracted to more symmetrical women may achieve higher reproductive success, " say the researchers.

Symmetry is known to play a major part in attractiveness, and both men and women who are more symmetrical are considered more attractive. But why symmetrical people are more attractive has puzzled researchers. Research is now showing that there may be evolutionary reasons for being attracted to someone whose feet, ankles, hands, fingers, eyes, breasts, arms and ears are the same on each side. It's suggested that such symmetry is a visible marker of good health, good genes and high fertility, which is why men are drawn to a well-balanced female anatomy.

To test the theory, researchers at Harvard University and other centres in Norway and Poland carried out investigations on around 200 women aged 24 to 36 with regular menstrual cycles, no fertility problems, and who were not using hormonal contraception.

The researchers used measurements of the women's fingers as markers of symmetry.

The second and fourth digits of each hand were measured to the nearest millimetre. If they differed by up to one millimetre, the woman was classed as symmetrical, while those with differences over two millimetres were asymmetrical.

Saliva samples were taken and tested for female hormones. The results show considerable differences between the two groups of women.

Symmetrical women had 21% higher mid-cycle female hormones than asymmetrical women. At other times the differences was as high as 28%.

Researchers at the University of Western Australia have also found a link between symmetry in men and sperm quality. They say the finding may explain why women are unconsciously attracted to symmetrical male faces.