Mona Lisa's enigmatic smile has intrigued viewers for centuries and now researchers claim they have unlocked the secret behind it.

A computer analysis of the Leonardo da Vinci masterpiece has found that she is 83 per cent happy, 9 per cent disgusted, 6 per cent fearful and 2 per cent angry.

Dutch researchers at the University of Amsterdam say the computer rated features such as the curvature of the lips and crinkles around the eyes.

In what they viewed as a fun demonstration of technology rather than a serious experiment, the researchers scanned a reproduction of the Renaissance painting and subjected it to cutting-edge "emotion recognition" software, developed in collaboration with the University of Illinois.

The program draws on a database of young female faces to derive an average "neutral" expression, then uses this expression as the standard for comparisons.

According to British journal New Scientist, the software is capable of recognizing emotions just by looking at photographs.

The software could lead to the development of computers that adjust their response depending on the user's mood, the journal says.

Lead researcher Nicu Sebe said he took the challenge as seriously as he could, using the faces of 10 women of Mediterranean ancestry to create a composite image of a neutral expression.

"Basically, it's like casting a spider web over the face to break it down into tiny segments," Stokman told the Associated Press.

"Then you look for minute differences in the flare of the nostril or depth of the wrinkles around the eyes."

Famous painting

Possibly the most famous painting of all time, the Mona Lisa has fascinated both art critics and scientists who have tried for five centuries to understand her cryptic smile.

Da Vinci began the work in 1503, and it now hangs in the Louvre Museum in Paris.

The painting, also known as "La Gioconda," is believed to have portrayed the wife of Francesco del Giocondo.

The woman in the portrait is dressed in the Florentine fashion of her day and is seated in front of a visionary landscape of mountains and gardens.

It is her alluring and aloof expression, denoted by a faint smile at the corner of her mouth, that has given the portrait universal fame.