"Our results indicate that happiness is expressed only on the left side. According to some influential theories of emotion neuropsychology, we interpreted the Mona Lisa asymmetric smile as a none genuine smile, also thought to occur when the subject lies," said study lead author Luca Marsili from the University of Cincinnati in the US.
For the study, published in the journal Cortex, researchers asked 42 people to judge which of the six basic emotions were expressed by two chimeric images a mirror image of just one side of the left and right sides of Mona Lisa's smile.
Thirty nine or 92.8 per cent indicated that the left half of the smile displayed happiness while none indicated that the right side showed happiness.
In assessing the right side smile, 35 said the expression was neutral, five said it was disgust and two indicated sadness.
The researchers also point out that there is no upper face muscle activation in the Mona Lisa painting.
The asymmetric smile, also known as a non-Duchenne smile, "reflects a non-genuine emotion and is thought to occur when the subject lies", the researchers said.
"Considering it unlikely that a person who sits motionless for hours to be painted is able to constantly smile in genuine happiness, the simplest explanation is that the Mona Lisa asymmetric smile is the manifestation of an 'untrue enjoyment'," they added.
"While the Mona Lisa smile continues to attract the attention of its observers, the true message it conveys remains elusive and many unsolved mysteries remain to be elucidated, perhaps via the knowledge of emotion neuropsychology," the researchers said.
( With inputs from IANS )