I spent the weekend in Ghent, Belgium, where I participated to an event called Night of the Freethinker. After giving my usual talk on Stoicism as secular philosophy (slides here), I took part in a debate on post-truth and the nature of science, together with my friend Maarten Boudry (co-editor of our forthcoming Science Unlimited? The Challenges of Scientism) and local author Coen Simon, the whole thing ably moderated by Eveline Groot. At some point, Eveline put up a slide of the Mona Lisa, Leonardo da Vinci’s famous masterpiece, challenging the panel to comment on recent “scientific findings” that allegedly settle, once and for all, the question of whether the Mona Lisa — a portrait of Lisa Gherardini — is actually smiling.
I always thought the question to be rather odd, as it is clear that Mona Lisa is, in fact, smiling, though in a (purposefully, as we shall see) unusual fashion. After all, the common phrase is “Mona Lisa’s smile,” not “Mona Lisa’s unknown facial expression.”
Be that as it may, I followed up on Eveline’s suggestion, and looked at the recently published paper to see what it was all about. Unsurprisingly, I was rather disappointed. The work appeared in Scientific Reports, co-authored by Emanuela Liaci, Andreas Fischer, Markus Heinrichs, Ludger Tebartz van Elst, and Jürgen Kornmeier. They manipulated Mona Lisa’s mouth curvature, treating it as the principal likely source of ambiguity in people’s interpretation of whether she is smiling or not. They then studied how a range of happier and sadder variants of the painting were perceived by a sample of 12 German people. As the authors write: “Stimuli were presented in random order and participants indicated the perceived emotional face expression and the confidence of their response. The probability of responding ‘happy’ to the original Mona Lisa was close to 100%. … Overall,” they concluded, “the original Mona Lisa seems to be less ambiguous than expected.”
Setting aside obvious flaws and limitations in the study — like the culturally homogeneous, very small sample size, and the confounding effect that it’s going to be hard to find a person in Western Europe who has never before been exposed to pictures of the painting — Liaci et al. very clearly did not settle the question they set out to settle. (The authors did uncover some interesting findings, though, like the fact that people interpret faces as more or less sad depending on context.) If the question is “does the Mona Lisa smile,” an opinion survey among people with no background in art history is entirely pointless. It only tells us how many regular folks perceive the Mona Lisa as smiling, but that’s clearly a different question.
One could ask art historians and Leonardo experts instead, and learn a thing or two about how smiles and facial expressions were painted by the Italian Master and his contemporaries and immediate predecessors during the Renaissance. That would be far better ground to ascertain whether Leonardo himself meant his portrait to represent a smile or not. But in fact we have access to a copious direct source of evidence: the notes and drawings of the artist, as summarized for instance in a recent book by Walter Isaacson.
The irony of attempting to “scientifically” answer a question to which everyone already knows the answer is that Leonardo’s own art was heavily informed by his scientific studies, in the true fashion of a Renaissance Man. The Last Supper, for instance, is the result of Leonardo’s studies in optics, while his deep understanding of human anatomy is in stark display in St. Jerome in the Wilderness.
The Mona Lisa was worked on by Leonardo during the last 16 years of his life, a period during which he also did dissections of cadavers to study facial expressions and their connection with human emotions. His near contemporary, Giorgio Vasari, says that “while painting [Donna Gherardini’s] portrait, he employed people to play and sing for her, and jesters to keep her merry, to put an end to the melancholy that painters often succeed in giving to their portraits.” But while Vasari attributes the result to superhuman skills originating directly from God, it was Leonardo’s genius for both art and science that gave us this magnificent example of human creativity.
Art historians know — as retold in detail by Isaacson — how Leonardo meticulously prepared the paint for the Mona Lisa, laying down an undercoat that would help create an almost tridimensional feeling for the viewer. Some paint was applied in very thin layers, over a period of years, with degrees of thickness as small as 2-5 micrometers. The way the paint is stratified and applied makes it look like the subject’s gaze is following us around the room (try it out, if you happen to be at the Louvre and you can manage to be near the painting for more than a few seconds, it is uncanny).
We learn the most about the famous smile by reading Leonardo’s notebooks during the time he spent dissecting bodies at the hospital of Santa Maria Nuova, in Florence. He wrote: “The spinal cord is the source of the nerves that give voluntary movement to the limbs. … The muscles which move the lips are more numerous in man than in any other animal. One will always find as many muscles as there are positions of the lips and many more that serve to undo these positions.” Next to one of his many drawings of faces he added: “Represent all the causes of motion possessed by the skin, flesh and muscles of the face and see if these muscles receive their motion from nerves which come from the brain or not.” He even carried out comparative anatomical studies of the face muscles of horses, which are larger and easier to dissect.
Look at this figure, which features a number of his drawings of facial expressions:
Next to it, Leonardo wrote: “Make the nostrils drawn up, causing furrows in the side of the nose, and the lips arched to disclose the upper teeth, with the teeth parted in order to shriek lamentations. … The maximum shortening of the mouth is equal to half its maximum extension, and it is equal to the greatest width of the nostrils of the nose and to the interval between the ducts of the eye.” Now look closer to the drawing I circled in the upper center of the figure. It’s a smile, very much reminiscent of the one he later developed for the Mona Lisa.
Isaacson explains yet another component of the famous smile: “Another piece of science that augments the Mona Lisa’s smile comes from Leonardo’s research on optics: he realized that light rays do not come to a single point in the eye, but instead hit the whole area of the retina. The central area of the retina, known as the fovea, has closely packed cones and is best at seeing small details; the area surrounding the fovea is best at picking up shadows and shadings of black and white. When we look at an object straight on, it appears sharper. When we look at it peripherally, glimpsing it with the corner of our eye, it is a bit blurrier, as if it were farther away. With this knowledge, Leonardo was able to create an interactive smile, one that is elusive if we are too intent on seeing it. The fine lines at the corners of Lisa’s mouth show a small downturn — just like the mouth floating atop the anatomy sheet [figure above]. If you stare directly at the mouth, the retina catches these tiny details and delineations, making her appear not to be smiling. But if you move your gaze slightly away, to look at her eyes or cheeks or some other part of the painting, you will catch sight of her mouth only peripherally. It will be a bit blurrier. The tiny delineations at the corners of the mouth become indistinct, but you will still see the shadows at her mouth’s edge. These shadows and the soft sfumato at the edge of her mouth make her lips seem to turn upward into a subtle smile. The result is a smile that twinkles brighter the less you search for it.”
Now compare this nuanced qualitative description of Leonardo’s work, informed by historical findings and knowledge of the cultural milieu the artist was working in, with the “scientific” study of a dozen random Germans’ opinions about what counts as a smile. Tell me which way you learned the most about the Mona Lisa. Then, if possible, go take another look at the painting the next time you are in Paris. Or make a point of going to see it.