A symmetrical face isn't just prettier - it's healthier too

For the perfectly proportioned face is also an indication that the body it sits atop is well prepared to fight off infection. The common cold, asthma and flu are all more likely to be combated efficiently by those whose left side matches their right.

It's all to do with exposure to testosterone and oestrogen during development, say researchers whose study is published this week in the journal Evolution and Human Behaviour. Symmetry, it seems, suggests that certain men are more masculine and women more feminine. The study reveals that such faces are also an indication of a person who has evolved from a large gene pool.

The researchers, based at the University of New Mexico in the United States, measured the facial features of around 400 young people and compared them with health records over three years. Features measured included chin length, jaw width, lip width, eye width, and eye height. Further studies by scientists in the UK reach a similar conclusion. Dr Nick Neave, an evolutionary psychologist at Northumbria University, explained: "We've done research into facial symmetry and found that if a face is very asymmetrical people are turned off.

"But if a man or woman has a symmetrical face, it's a turn-on. You're attracted on a subconscious level because, throughout history, humans have chosen to breed with people they perceive to be healthy. Healthy genes mean a symmetrical face."

When the US researchers looked at rates of illnesses and antibiotic use they found direct links with looks. "The research we report in this paper provides evidence that male masculinity is a marker of resistance to infection, although, more specifically in this population, to respiratory diseases,'' they said.

It is well established that testosterone is involved in the immune system's ability to combat disease. And Professor Dave Perrett at the University of St Andrews has also suggested that women prefer symmetrical faces because this indicates healthy genes in their partner.

Similarly, women's facial femininity may signal resistance to respiratory problems.

The University of New Mexico team warned that women whose partners have mismatching ears, fingers or elbows tend to fantasise about sex with other men when they ovulate.

But the researchers caution that the link between looks and health may change as men and women age. An abundance of testosterone is not necessarily beneficial in the long term. "Testosterone has detrimental effects on longevity, as revealed by the relatively long lifespan of castrated men," researchers said.