Leonardo da Vinci’s famous portrait of Lisa Gherardini, titled Mona Lisa, is one of the most-discussed works of art ever, partly thanks to the subject’s ambiguous look.

According to a recent study by the University of Freiburg, though, the answer to whether she is “sad” or “happy” is simple: her expression is unequivocally “happy”.

Twelve participants were shown nine black and white photos of the Mona Lisa, eight of which had been digitally manipulated at the mouth; four made the model look happier, the other four sadder.

After shuffling the photos and showing them to each participant 30 times, the team found that the original photo was thought to be “happy” 97 per cent of the time.

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“We really were astonished,” neuroscientist Juergen Kornmeier a that German University told AFP. “Given the descriptions from art and art history, we thought that the original would be the most ambiguous,” he said.

A second experiment was also conducted, involving eight “sadder” versions of the portrait shown to participants with more nuanced changes. While the original was still seen as happy, they found the manipulated photos were proclaimed ever sadder than before.

“We don’t have an absolute fixed scale of happiness and sadness in our brain,” Kornmeier added. “Our brain manages to very, very quickly scan the field. We notice the total range, and then we adapt our estimates.”