Leonardo da Vinci's painting, Mona Lisa, has intrigued and befuddled scholars for centuries. Traditionally, it's been thought that the subject, Mona Lisa, was gleefully hiding a secret from those around her, a small smile on her lips.

Recently, however, a study by the University of Freiburg challenges that long-held notion in art history, instead proposing that the bias of our brains towards positive facial expressions was what created that opinion. Scientists at the university's center of psychiatry and psychotherapy created eight versions of the original Mona Lisa with very gradual, subtle changes in the curvature of her mouth: four with a sadder face, the lips turned slightly downwards and four with a happier one, the corners of her lips turning up instead.

Naturally, when unknowing observers saw the original photo, it was perceived as happy. Surprisingly however, when participants were asked to look at a set of photos--which named the original as the happiest, despite having happier photos in the range of 8 in the set--they reported happier images as "sad" ones.

These results allowed scientists to conclude that sadness and happiness--emotions which we perceive as clearly distinct from one another--are actually relative depending on setting. What we feel and what we're able to deduce from our environment is incredibly limited--we're boxed in by the information provided and the background stimuli we receive, nothing else.

Thus, this being the case, it turns out that we don't actually know what Mona Lisa was thinking at all. Contrary to popular belief, she might not be holding a juicy secret from us. She might not be mischievously or contentedly seeing anything at all. Since we possess a bias that naturally skews us towards perceiving her face as happy, we have to keep in mind that--in reality--we have no idea what Mona Lisa might have been thinking at all.

And recognizing where our cognitive biases lie will help us better understand and take in the sensory information we receive from the world around us. So, don't jump to conclusions, or judge any book by its cover. Turns out, sometimes we don't know what we don't know.