Back in April the comedy website Funny or Die created a PSA video about a dire condition afflicting a subset of women: Bitchy Resting Face. “That’s just my face” is the cri de coeur of the BRF sufferer, who looks pissed even when she’s not judging your outfit or dreaming up schemes against unsuspecting colleagues. “We’ll face it together,” the commercial promises BRFers worldwide. Scan the actors’ visages and that invitation seems less than reassuring.

The video, of course, is a joke. And it is evenhanded enough to describe an equivalent male syndrome, Asshole Resting Face, occurring in certain well-intentioned men. Still, the gag recalls the commotion over a Publishers Weekly story in which an interviewer asked author Claire Messud whether she’d want to befriend her female antihero. “For heaven’s sake, what kind of question is that,” Messud shot back. “Would you want to be friends with Humbert Humbert? Would you want to be friends with Mickey Sabbath? Saleem Sinai? Hamlet? Krapp? Oedipus?” Messud was scorchingly indignant, and many felt she overreacted. But could she have been right to intuit something insidious in the question? We do inspect every last outward manifestation of a woman’s inner life—the characters she pens, the facial expressions she makes—for evidence of “niceness.”

Women have to be nice. “Show me a smile” is a staple of street harassment (the ur-creep of the genre being Heath Ledger’s Joker, a lank-haired greaseball leering, “Why so serious?”) And when they are not, when they are merely impassive or thoughtful, they can be held up for mockery and branded as rude.

To ask why is to step into the laser grid of unspoken rules governing the arrangement of male and female faces—the gendered ways we police social performance. (If you’re a woman who thinks this sort of policing doesn’t happen in real life, consider whether a friend has ever yanked you from an introspective haze by asking “Are you mad at me?” She probably meant: Why aren’t you smiling?) We’ve tangled up so many notions of gender in our smiles that the presence or absence of a grin has come to imply a distinction between male and female. In one study, babies dressed in green and yellow were paraded before a group of onlookers. When the infants cooed, gurgled and smiled, the observers tagged them as girls; fretters and criers were assumed to be boys. The effect persisted when a different group of participants was presented with images of cheerful or angry adult faces. People readily identified smiling women as female and wrathful men as male, but they took longer and stumbled more often when confronted with furious female countenances or beaming male ones.

In a raft of studies, women report smiling more than men (and men report smiling less than women). They speak of grinning on the job, with strangers, with relatives, in a dazzlingly diverse array of situations. An unscientific scan of high school yearbook photos, newspaper clippings, Facebook pics, and advertisements backs up those studies: Women flash their pearly whites far more frequently than men, at least when someone is taking their picture. And in simulated job interviews, female participants salt their speech with smiles, while male test subjects are more likely to adopt neutral (read: alluringly strong and stoic) expressions.

Here’s the thing, though: All this feminine smiling does not mean that women are happier. Back in the 19th century, the French physiologist G.B.A. Duchenne distinguished the spontaneous facial sunrise now known as the Duchenne smile (the “true” smile) from the imposters. While a performed or deliberate smile requires just the zygomaticus major, the muscle around the mouth, “real” smiles involve both the lips and the muscles ringing the eyes, the orbicularis oculi. Or as Duchenne put it, one type of grin “obeys the will, but the second is only put into play by the sweet emotions of the soul.” (Or as Tyra put it, “smile with your eyes.”)