Mascara Face. You know what it is: that slack-jawed, open-mouthed, dopey look when someone swipes the wand over their lashes again and again in an effort to get them just right.

It seems to be a worldwide and well-documented phenomenon. Nina Leykind, cofounder of British mascara giant Eyeko, tells mental_floss she knows the look well. As someone who helped create the concept of a“mascara bar,” she's seen plenty of people apply the stuff. “I love mascara face!” she says. “Bizarrely, as soon as someone lifts the wand to their eye, their mouth opens. I think if someone is not doing mascara face in front of me, it’s because it’s a conscious effort not to.”

We asked Rockefeller University neuroscientist Zeeshan Ozair for a possible explanation behind mascara face. According to Ozair, three nerves acting in conjunction—the trigeminal, the facial, and the oculomotor nerve—are likely responsible.

“The trigeminal nerve controls the movement of muscles of mastication, which open and close your jaw,” Ozair tells mental_floss. “Two other nerves, the facial nerve and the oculomotor nerve, together control the movement of eyeballs and eyelids.” Those three nerves all originate in close proximity to one another in the brainstem at a point of origin called a nucleus.

Mascara face may be a kind of physiological fluke. “In several people, connections”—called collaterals—“develop between these different brainstem nuclei,” says Ozair, who’s currently studying human-specific aspects of neural development and corticogenesis (cortical development). “As a consequence of these collaterals, when one nerve is activated, the other is as well.”

But mascara face doesn't have to happen, Ozair says. As Leykind has observed at her company's mascara bars, people aware of her presence became self-conscious of their own facial movements and were able to stop. In instances where muscles respond to both voluntary control and involuntary reflexes, as is the case with mascara face, “voluntary control almost always takes precedence,” says Ozair.

He likens it to the knee-jerk reflex. “If you were to actively think about it, you could stop the knee jerk from happening," he says. "Likewise, if one were to think about [mascara face], you could stop the mouth movement voluntarily.”

But why doesn’t the phenomenon happen when someone simply opens their eyes without applying mascara? Well, it can, explains Ozair. Think about the last time you heard truly shocking news. Did your mouth fall open? Was it "agape"? Did you call the news "jaw-dropping?" Those terms aren't metaphors. “In periods of stress, e.g., when you hear bad news, the voluntary control of the jaw is overridden, and you may open your mouth agape unconsciously,” Ozair says.

When it comes to applying mascara, the physical action stimulates the sensory part of the trigeminal nerve, which forms connections with the motor trigeminal nucleus. In other words, says Ozair, the action provides additional input for the slack-jaw reaction. In this case, you don't need to hear jaw-dropping news for your jaw to, well, drop.

Interestingly, the phenomenon doesn’t happen in reverse; for example, your eyes don’t pop open when you’re chewing. That's because the collaterals have directionality. “Reflexes in one direction don’t translate into reflexes in the opposite direction,” Ozair says.

There are exceptions. In rare instances, some people have collaterals in the opposite direction. “When they open their mouths,” he notes, “they involuntarily lift their eyelids as well.” (Which raises the question: Is the opposite of mascara mouth lipstick eyes?)

So, what percentage of the population actually possesses the collaterals responsible for mascara face? For that, Ozair says, “I don’t have an answer … This hasn’t been studied at an epidemiological level!”

Epidemiologists, looks like the (eye)ball’s in your court now.