Smiling can make you appear two years older than if you wear a poker face, a study has found.

"We associate smiling with positive values and youth," said study co-author Melvyn Goodale, director of the Brain and Mind Institute at the University of Western Ontario.

"Think of all the skin-care and toothpaste companies that sell the same idea every day."

Researchers flashed images of people with smiling, neutral and surprised expressions to find out which the study participants thought were the youngest.

They found smiling faces were thought to be the oldest and surprised faces the youngest.

"The striking thing was that when we asked participants afterwards about their perceptions, they erroneously recalled that they had identified smiling faces as the youngest ones," said Goodale.

"They were completely blind to the fact they had 'aged' the happy-looking faces. Their perceptions and their beliefs were polar opposites."

Goodale said the ageing effect of a smile stems from people's inability to ignore the wrinkles that form around the eyes while grinning. A look of surprise, on the other hand, smooths any wrinkles.

"It may seem counter-intuitive, but the study shows that people can sincerely believe one thing and then behave in a completely different way," Goodale said.