Blind and sighted athletes smile or frown in response to victory or defeat with the same facial expressions, a finding that further supports the theory that expressions of emotion are innate and not learned, U.S. researchers said.

Charles Darwin was one of the first to observe that blind people share common facial expressions with sighted people in his book Expression of the Emotions in Man and Animals (1872).

Subsequent studies have also suggested that expressions of anger and joy are hereditary, with children often sharing the telltale frown or grin of their parents whether or not they can see.

But the study, published Monday in the Journal of Personality and Social Psychology, is the first to compare the responses of blind and sighted to a specific emotional stimuli, in this case, the thrill of victory or the agony of defeat.

The researchers compared the facial expressions of blind and sighted judo athletes at the 2004 Summer Olympics and Paralympic Games, analyzing 4,800 photographs of athletes from 23 countries.

The researchers found that both blind and sighted athletes produced the same facial muscle movements in response to specific emotional stimuli. While gold medal winning athletes showed genuine joy, those with lesser medals either showed anguish or produced "social smiles," where the mouth curls into a smile without the accompanying rise of the cheeks and narrowing of eyes.

Since the blind athletes could not have learned these responses through visual observation, there must be another mechanism involved, said San Francisco State University Psychology Professor David Matsumoto, the study's author.

He suggests something in our evolutionary history is responsible for the built-in responses.

"The statistical correlation between the facial expressions of sighted and blind individuals was almost perfect," Matsumoto said in a statement. "This suggests something genetically resident within us is the source of facial expressions of emotion."

For example, the tendency of losers of athletic events to push their lower lip up could be explained as a vestige of our evolutionary ancestry.

"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," he said.