If you think it's because artists were always trying to hide Jesus' secret face tattoo, you should know that this phenomenon comes up in most paintings, and nobody was ever sure why. It turns out it's due to brain weirdness.

Sam Kean, author of The Tale of the Dueling Neurosurgeons, studied this weird phenomenon. He says that if the choice of which side to depict was totally random, we would find that gallery collections have roughly a third of all portraits facing straight forward, a third facing to the left, and a third facing to the right (obviously). However, studies show that this isn't the case -- about 60 percent of subjects sit with their left cheek facing the viewer, their left eye practically in the middle of the painting. It's twice as common as it should be.

Gilbert Stuart, Leonardo da Vinci, Vincent van Gogh, Jacques-Louis David, Johannes Vermeer, Elias Garcia Martinez

So ... why? You could chalk it up to generations of artists copycatting their masters, but Keane points out the fact that children (aka "not trained artists") also show this preference when drawing people. Hell, even students posing for yearbook photos seem to have a habit of turning their faces to the right. The reason, apparently, is that the left side of the face shows more emotion than the right -- you can do an experiment by covering half of a face, and the left side of an angry face (their left, your right) will look angrier than the right half. Note which half of Batman's Two-Face gets the crazy scowl:

Warner Brothers
Or the crazy-awful lipstick job, depending on which version you're watching.

Now, these old-time artists couldn't have known the science behind this, but what they presumably did know was that left-faced paintings just "worked" better -- they somehow conveyed the emotion more effectively. It probably was nothing more than sitting down with the model they were painting and realizing one pose looked cooler than the other, resulting in centuries of art based on a neurological bias they weren't even aware of. And speaking of weird brain glitches ...

Depending on where you focus your eyes, she's smiling, smirking, or pressing her lips together in annoyance. How the painting accomplishes this has been a mystery for its 500-year existence. For example, if you look at the image above and meet her eyes, you'll get the impression of a faint, friendly smile. But cover the top half of her face, and you get something else entirely:

Leonardo da Vinci
She sort of looks like she's in the middle of chewing something.

So how is she smiling without involving her mouth at all? Dr. Margaret Livingstone, a neuroscientist at Harvard, has solved the mother of all of Lisa's mysteries. As Livingstone explains, it has to do with how your eye works.

The central part of your vision, called the fovea, captures the small details, like the minuscule print on a bottle of pills describing the myriad ways they could horribly murder you. The part surrounding the fovea makes up your peripheral vision -- it's a lot less accurate in distinguishing colors and objects, but it's nonetheless just as important, tasked as it is with detecting shadows and approaching threats, like that junkie who's about to jump you for your Adderall.

When we look at people, we tend to look them in the eye. When the central part of our vision looks into "Mona Lisa"'s eyes, the peripheral broadly notices the area around her mouth. Since it can't make out details very clearly, instead of noticing her lips, it captures the shadowing between her lips and her cheekbones. These shadows:

Leonardo da Vinci

In your peripheral vision, they become part of her mouth, giving it an upturned look and the impression of a smile. When your eyes switch focus to her mouth, however, they see the contours of her lips and realize the shadows are just shadows. It's not just Leonardo's famous painting that displays this shape-shifting smile, either. Apparently, real-life people with prominent cheekbones give off this impression without even knowing it. Yes, a neuroscientist simultaneously solved the "Mona Lisa" and Benedict Cumberbatch's career in one shot.

For once, those questions have easy answers, because Munch himself answered them in his journal:

"I was walking along the road with two friends -- then the sun set -- all at once the sky became blood red -- and I felt overcome with melancholy. I stood still and leaned against the railing, dead tired -- clouds like blood and tongues of fire hung above the blue-black fjord and the city. My friends went on, and I stood alone, trembling with anxiety. I felt a great, unending scream piercing through nature."

So there you have it, folks: "The Scream" was inspired by a sunset. Case closed, cue next entry.

In case your natural-catastrophe knowledge is somewhat less than encyclopedic, on August 27, 1883, one of the most epic pimples on the Earth's crust exploded with the force of a gazillion billion ruined prom nights. The serial eruptions of Mount Krakatoa were such an Earth-shattering event that they lowered the atmosphere's temperature by a degree. The copious amounts of dust and ash that the lava-spewing wound chundered into the air spread to all parts of the globe, causing a phenomenon that made the sky look like it was catching fire during sunset.

via washingtonpost.com/Discovery Channel
Or it would have, if the world hadn't been black and white back then.

All this earthly sputum crept its way from Indonesia up to Norway by the end of 1883. The ensuing sunsets looked like God had embraced his inner pyromaniac, inspiring local newspapers to print front-page stories about them ... and a certain expressionist to understandably shit his pants.

Leonardo da Vinci's drawing the "Vitruvian Man" is unquestionably the most famous work celebrating the manly pastime of making snow angels while "rocking out."

Leonardo da Vinci
"Baby, it's cold outside."

The sketch was Leonardo's attempt to draw a human body in accordance with the laws of proportion laid out by the Roman architect Vitruvius. In addition to using math to define how the ideal male body should look (much as the porn industry does today), Vitruvius also claimed that these rules reflected a sort of sacred geometry in nature and should be applied to man-made buildings. Many artists have tried to put Vitruvius' laws to paper, but Leo's version continues to be considered the closest to what Vitruvius claimed was physical perfection.

But apparently no one ever took the time to check out what ol' Vitruvy was packing -- or at least no one admitted to it until Dr. Hutan Ashrafian came along. You see, Ashrafian is a clinical researcher, and when he took it upon himself to closely examine Leonardo's 500-year-old pen-and-ink dong, he noticed something incongruous with the idea of perfection: the discernible bulge just above it.

Leonardo da Vinci
"Baby, come on -- it's not contagious."

The good doctor made the diagnosis that the model Leonardo used for the drawing suffered from an inguinal hernia (basically, it's when your intestines play peek-a-boo through the doorway your balls used to enter their wrinkly abode). These days, about a third of men will get one during their lives. Women are luckier -- since their balls never dropped, so to speak, only about 3 percent of them will have to deal with the painful condition.

Today, it's not such a big deal to rip your guts open, shove that shit back where it belongs, and Velcro you back together, but this was half a millennium ago, when people like da Vinci were still figuring out what the hell a spleen was. Back then, such a condition was likely a case of slow, agonizing crotch-death.

Leonardo da Vinci
"Baby, I'm dying ... hand job?"

It's unknown whether Leonardo used a hot live model for the drawing, or one of the cold dead variety (as was common practice back then). According to Ashrafian, if the model was deceased, it was probably complications from the hernia that did him in. If he was alive, on the other hand, then he'd likely be on the receiving end of a crotch-punch from the Grim Reaper sometime after posing as Leonardo's muse. Such was the tragic life of a Renaissance-era male model. Speaking of which ...

Michelangelo happened to be working on the Sistine Chapel when Raphael painted his famed fresco, and Raphael chose to celebrate his artistic buddy down the hall by casting him as Heraclitus. Five hundred years on, the portrait elucidates something about Mikey that he couldn't possibly have known in his lifetime: According to Dr. Carlos Hugo Espinel, Michelangelo-as-Heraclitus' strangely angular knee indicates that he suffered from gout, a kind of inflammatory arthritis with a bonus helping of kidney and bladder stones, because sometimes God says unbearable joint pain just isn't quite enough.

Raphael
To cope, he wrote awful emo poetry about his pee. (Not a joke.)

It seems like a long jump to this conclusion from a single knobby knee, but in fact, Michelangelo's living conditions and work habits support the hypothesis. The man was a consummate workaholic, and he would often spend days on end concentrating on being a superhuman artist without so much as a second thought about all that mortal "sustaining oneself" bullshit. He would subsist on minimal intake of bread and wine during these work sprees, and as such, he'd be exposed to lead -- a contributor to gout -- from the lead-based paint, and because wine was made using lead containers in those days. Add to that Michelangelo's numerous diary entries bemoaning all his bladder and kidney problems, and ... yeah, dude had the gout.

So the next time you feel like bragging to some Internet comment section about your accomplishments in life, just remember -- Michelangelo painted the fucking Sistine Chapel while this was happening to him:

James Gillray
Sometimes perspective is a tiny foot-chomping demon.

Rembrandt's contributions to the art world are a major reason that the era in which he lived and worked has come to be known as the Dutch Golden Age.

Abraham Storck
It's like the Renaissance, but with windmills.

He left behind a staggering body of work, a hefty chunk of which was self-portraits, because making someone else sit their ass still is hard. If said portraits' level of realism is to be trusted, then they reveal a physical trait of Rembrandt's that, on first thought, would seem like a hindrance to becoming a great artist, but in fact likely helped nudge him along that path. That trait, as evidenced below, was a lazy eye.

National Gallery Publications
"One of my eyes gazes straight into your soul. The other gazes into your left ear hole."

Dr. Margaret Livingstone -- she of the neurological explanation behind the "Mona Lisa"'s smile -- studied Rembrandt's self-portraits and discovered that in all but one, the eyes stare out in different directions. And this isn't just a case of Rembrandt getting sloppy with his brush -- in the portraits he painted of other people, the eyes are straight as an arrow. So what does this tell us?

Rembrandt Harmenszoon van Rijn
It tells us that sex with the missus always felt like a threesome.

It tells us that Rembrandt was better at seeing the world in two dimensions than regular-sighted folk, and that this "disability" may well have led to him developing his incredible artistic talents. That's because the brains of people who are cross-eyed or wall-eyed tend to learn to discard the input from the offending eye, resulting in stereoblindness -- that is, the inability to see the world in 3D. It takes input from both eyes to construct a three-dimensional image. If you want proof, conduct this little experiment the next time you go to a 3D movie: Cover one eye and measure the amount of time it takes you to puke. Repeat until you stop questioning what we tell you.

People tend to develop stereoblindness in early childhood, so if Rembrandt saw the world in two dimensions, he possessed a natural ability -- from a very young age -- to translate what he saw in front of him onto a two-dimensional canvas. And Rembrandt may not have been the only one to be gifted with a disability that gave him artistic superpowers -- a further study showed that a disproportionate amount of famous artists and current art students suffer from stereoblindness as well. So there you have it: The incongruous key to becoming a successful visual artist might just lie in having a major visual deficiency.

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Related: Did you know whenever you see art depicting the Virgin Mary, chances are good she's squirting breast milk? Or that your favorite piece might have a glaring mistake you never noticed? This is why art is cooler than you thought. Every work is like a Where's Waldo? book.