The facial expression of a blind athlete after losing an important game
The facial expressions we make to show or hide our emotions are hardwired into our brains rather than learned during life, a study has concluded.
Blind and sighted athletes made the same expressions when they won and lost, US researchers found.
This, the study reported in the Journal of Personality and Social Psychology study suggests, meant the expressions were not picked up by watching others.
The researchers believe they could be remnants of evolutionary history.
The idea that facial expressions are in-built is not new - scientists have suggested it since the 1960s.
However, the study at San Francisco State University provides some of the strongest evidence yet to support it.
Professor David Matsumoto and his team compared 4,800 photographs, capturing the expressions of sighted and blind judo athletes at medal ceremonies at the 2004 Olympic and Paralympic Games.
In each case, the faces of gold and silver medal winning athletes were scrutinised.
While the winners frequently showed genuine joy at their victory, those in the lesser medal positions often produced "social smiles" - smiles involving only mouth movement, indicating that they may be artificial rather than spontaneous.
The researchers concluded that sighted and blind competitors showed or controlled their expressions in exactly the same way.
Professor Matsumoto said: "The statistical correlation between the facial expressions of sighted and blind individuals was almost perfect.
"This suggests something genetically resident within us is the source of facial expressions of emotion.
A sighted athlete shows a very similar expression after losing
"Losers pushed their lower lip up as if to control the emotion on their face and many produced social smiles - individuals blind from birth could not have learned to control their emotions in this way through visual learning, so there must be another mechanism.
"It could be that our emotions, and the systems to regulate them, are vestiges of our evolutionary history."
He said that humans might have learned that rather than express negative emotions by yelling, biting or throwing insults, they may have developed a system that automatically closes the mouth to prevent it.
Dr Bridget Waller, from the University of Portsmouth, said that primates often used a "bare teeth display" to signal to other primates that they were not going to attack them, and humans might have inherited a similar system.
"It is a way of signalling your benign intent. It may not be so much that you are stopping yourself from doing something, but actually demonstrating to somebody else that you are not going to do it.
She warned: "People can think that all communication revolves around language, when it happens on so many other levels, such as body language and facial expression."