The inscrutable facial expression of Leonardo da Vinci’s Mona Lisa has made the portrait most famous painting in history. Is she smiling? Is she mournful? Serene, mischievous or bored? Researchers claim to finally have an answer: Yes, she’s smiling; a subtle smile, but still a smile.

Scientists at the Medical Center—University of Freiburg, the Institute of Psychology of the University of Freiburg and the Institute for Frontier Areas of Psychology and Mental Health (IGPP) conducted a study to test how Mona Lisa’s expression is perceived and published results in the online journal Scientific Reports showing that subjects “almost always” perceive Mona Lisa as happy. 

Researchers presented test subjects with an image of the original painting, as well as eight images that had been slightly altered, with the corners of Mona Lisa’s mouth raised or lowered to make her appear happier or sadder. (Four happier and four sadder, with the original being the median.) Subjects indicated for each version whether the expression was happy or sad by pressing a button. The responses were added up to form percentages on a scale from happy to sad and a rating for certainty.

“We were very surprised to find out that the original Mona Lisa is almost always seen as being happy,” says Dr. Jürgen Kornmeier, a scientist at the Institute for Frontier Areas of Psychology and Mental Health in Freiburg and at the Eye Center of the Medical Center—University of Freiburg. The original image, and the four altered to increase the smile, were all identified as happy nearly 100 percent of the time.

Interestingly, the images perceived as happy were identified more quickly, and with a higher degree of certainty than those perceived as sad. “It appears as if our brain is biased to positive facial expressions,” says Emanuela Liaci, the first author of the study. A second study also showed that sadness is relative; subjects were quicker to identify sadder expressions when they were in the context of more sad images than happy.

One question raised by this study is how to account for the gap between what emotions we’re feeling and what others perceive. While Mona Lisa was perceived as smiling nearly 100 percent of the time in this study, that is not necessarily the same thing as determining that she is in fact smiling. The difficulty of reading the subtle emotional cues of other people (especially women) has been the subject of scientific research—and just about every rom com ever made.

The only way we will know for sure if the expression da Vinci painted is in fact a smile would be through the miraculous discovery of a diary or letter from the model. Such a discovery is not likely to happen, as the identity of the model in the famous painting is the subject of at least as much debate among art historians as her mysterious expression.

The possibility that the painting was based on Lisa del Giocondo, the wife of a Florentine merchant, led eager forensic experts to open a 500-year-old crypt in the hopes of recreating the woman’s face and confirming that she was in fact Mona Lisa. They were unable to come up with conclusive results. Just last year, new claims surfaced that the painting was in fact an amalgamation of the merchant’s wife and da Vinci’s gay lover. There has been previous speculation that the model was da Vinci himself imagined as a woman.

The fact that we’re still striving for answers about the origin and meaning behind this famous painting centuries later only speaks to its impact, but it’s likely to be a quest with no satisfying end. While it’s exciting to try to pin down what exactly is behind that bewitching expression, and despite the near 100-percent consistency of this study’s results, Mona Lisa still looks like she knows something we don’t—and maintains the mystery that has given her such an allure all along.