When some climate activists threw soup at the Mona Lisa the other day, it was hard to tell through the protective glass at the Louvre if the iconic face was smiling or smirking. In fact, it’s always been hard to tell.

Five centuries after Leonardo da Vinci created the world’s most famous painting, Mona Lisa continues to transfix art lovers and just about anyone who glances at gaze that always seem to be gazing right back, no matter what angle you view from.

That includes scientists, who, like many people, have wondered if the subject in the painting has cracked a modest grin or not.

Image: Wikipedia/public domain

Back in 2017, researchers concluded the arresting, complex smirk is, in fact, a smile. But then in 2019 some other scientists argued that the lopsided expression is clearly hiding something.

If you look close, you can see the evidence:

The mouth of the subject in the Mona Lisa (which is the name of the painting, by the way, not the painted) turns up slightly on the left side — an apparent sign of happiness, both research groups agree. But the right side curves down ever so slightly, suggesting something else.

So in the 2019 study, published in the journal Cortex, scientists mixed some art and science and made two doctored images of the painting by first copying and flipping the left half of the subject’s smile over to the right side to make both sides upturned. They did the opposite to create a doctored image of the other look.

The full portrait (a), a close-up of the face (b), doctored images of the smile © and (d). Credit: Louvre Museum/University of Cincinnati

Then they showed the two images to 42 people.

Almost all of the people — 39 — saw the upturned lips as a sign of happiness. None of them thought the other version exhibited happiness, and 35 rated it neutral, while five perceived it as disgust and two called it sadness.

The scientists now say the real image does involve a smile, but it isn’t genuine.

“Asymmetric smiles are an expression of lies,” says the study’s lead author, Luca Marsili, a…