FACIAL expressions of emotion are innate, not learned, research that compared faces of sighted and blind athletes as they weathered defeat shows.

Researchers in the US say they have demonstrated that facial expressions conveying emotion are hardwired into our genes, not learned through observation.

Even the "social smile" - displayed stoically by the silver medallist at the medal ceremony, for example - is instinctive, with sighted and blind athletes using the same facial muscle movements to deliver a forced smile in difficult circumstances.

Using more than 4800 photographs of athletes from 23 countries, the researchers compared the facial expressions of sighted and blind judo athletes who took part in the 2004 Olympics and Paralympics.

"The statistical correlation between the facial expressions of sighted and blind individuals was almost perfect," said David Matsumoto, a psychologist at San Francisco State University.

However, he said it was not possible that the athletes who were blind from birth could have learned the expressions, which are used to mask their true emotions according to the occasion or environment.

At the medal ceremony, 85 per cent of silver medallists produced "social smiles", which engage only the mouth muscles. A genuine - or Duchenne - smile uses more muscles and cause the eyes to twinkle and narrow and the cheeks to rise.

The Duchenne smile is named after the 19th-century French neurologist Guillaume Benjamin Amand Duchenne, who discovered that a smile resulting from happiness involved the eyes and the mouth.

This genuine smile was not what Professor Matsumoto and his colleague Bob Willingham of the Centre for Psychological Studies observed among the silver medallists.

"Losers pushed their lower lip up as if to control the emotion on their face and many produced social smiles," Professor Matsumoto said. "It's possible that in response to negative emotions, humans have developed a system that closes the mouth so that they are prevented from yelling, biting or throwing insults."

Like their sighted peers, the blind athletes expressed sadness with a downturned mouth and the raising of the inner eyebrows. Similarly, anger was displayed using the same facial muscles as the sighted athletes. The research was published in The Journal of Personality and Social Psychology .