When a body language expert and voice coach encouraged Caitlyn Jenner to act feminine by giving out grins, she balked.

“Kim doesn’t smile,” Jenner told Lillian Glass, who earned a Ph.D. from the University of Minnesota. “Kim,” of course, meant stepdaughter Kim Kardashian West, figurehead of social media self-portraits.

Whether the Kardashians are to credit, more millennial women — and some men, too — are refusing to say cheese. Search any variation of #serious, #modelface or #contour, among other captions, and these stoic Instagram selfies roll in as numerous and unstoppable as ocean waves. “It’s very masculine. It’s very aggressive,” Glass said. “It’s like, ‘Look at me, darling.’ ”

Crystal Sutherland, 21, of Maple Plain, favors an unsmiling selfie to better show off one of her 42 lipsticks. The look does make her more serious, maybe more adult and composed, she muses, but “it’s definitely not that I’m not happy.”

Kelley Reierson, 26, of Minneapolis, credits “selfie culture” for letting her reclaim and love her image — “something that has been denied to [women] for a lot of their lives,” she said.

Her barista job requires constant smiles, so she favors more serious looks when she curates her online persona, where a smile would seem insincere, “especially if I’m just documenting myself out in the world, taking a selfie in a coffee shop mirror.”

Some experts say the selfies exemplify “cool posturing” and rejections of enthusiastic capitalism. The unsmiling women refuse to submit or project instability, they say, while flaunting “on-fleek” makeup.

Or maybe it’s not a trend, and media are overdramatizing the phenomenon. Like the story behind Mona Lisa’s coy half-grin, the answer lurks just out of reach.

We are born knowing how to smile. But we haven’t always flashed pearls for the camera.

The smile’s history

Smiling in photographs is a Western phenomenon, and a relatively recent one at that, said Elizabeth Otto, associate professor of modern and contemporary art at the State University of New York at Buffalo.

In photography’s earliest days, a subject couldn’t smile because the lengthy exposure made it too hard to hold the grin so rigid that the picture didn’t blur. But even as 20th-century camera developments made posing much simpler, Otto said, the gravitas of portrait sessions lingered.

For European aristocrats, smiling was a sign of losing face, said Vanessa Brown, a lecturer in fashion development at Nottingham Trent University in England. Nobility preferred stern profiles as they tried to prove they reigned above the world and its nonsense.

People began grinning for shots occasionally in the 1920s and ’30s, but smiling entered the collective American identity only after World War II. Smiling images evolved then to project happiness and complacency through television and advertising in the ’50s, Otto said, as society rolled back wartime advancements for women and they morphed back into housewife roles.

“There is a lot of attesting: ‘Yes, we are happy. We are satisfied to be wives and mothers, and we are not a threat,’ ” she said. “Also, the war was tough, too, so in some ways they were happy.”

These days, Western convention says a woman should always smile, Brown said. Mothers, grandmothers, strange men on the street tell her so.

An analysis of more than 100,000 participants led by Yale psychologist Marianne LaFrance found that women smile more and men smile less when others are watching, particularly at ages 15 to 25.

LaFrance theorized that women’s more frequent smiling emanates from lower social status — women smile more to show submission to men.

Glass disagreed. Women smile to engage others, she said. It’s the key to life.

Take your best shot

In the Midwest, smiles are queen. Commercial clients such as Target want models to appear pleased with a product. But even in this market, said Deborah Rosenberg Charloff, vice president of Twin Cities-based Caryn Models, the agency is witnessing a trend of want-to-be models submitting unsmiling or pouting photos.

Maybe they are emulating high-fashion shoots, she said, or maybe they just have bad teeth. Whatever the motivation, many of the models sport a crooked smirk during the agency’s work at the European Model Showcase.

“They don’t have the happy smile, all-American things we do here in Minneapolis,” Caryn Models President Cindy Rosenberg Nelson said.

The allure of “acting cool” propels Brown’s research, including “Cool Shades,” her book on the history and meaning of sunglasses.

Not smiling sanitizes our emotions and sends a message of inner strength, she said.

Being blasé is “like saying, ‘I haven’t gone crazy,’ ” Brown said. “ ‘I‘m not in the heat in my bedroom eating ice cream in my pajamas and being anxious all the time.’ ”

A young, unsmiling woman shows she doesn’t need — or want — to interact with someone, Brown said, and cuts off the male gaze, stopping a man from looking at her as a potential conquest.

Not buying it

Before Kim Kardashian West, there was Dorothea Lange.

People associate frowns with seriousness, said Jane Blocker, professor and chairwoman of the University of Minnesota’s art history department, because of the iconic images produced during the Depression by photographers such as Lange and Margaret Bourke-White.

Whereas the photos ingrained truth and seriousness into our visual history, advertising images require false enthusiasm: Everything is fabulous.

Society also associates women with masquerade because of the cosmetic culture, Blocker said.

“You run out of superlative adjectives, and you run out of people having happy lives. So in a vision where everyone and everything is false smiles, how do you register seriousness or truth or honesty or straightforwardness or normal reality?” Blocker said. “It may be that one mechanism for that is to not be smiling.”

Kaylen Ralph, an Anthropologie stylist and co-founder of Minneapolis-based feminist publication the Riveter Magazine, said she doesn’t do selfies, but she does follow social media’s “siren call to define ourselves.”

She said, “I’m a big proponent of the soft smile. If you are posing for a photo, chances are you’re not cracking up.”

Media outlets have mapped women’s selfies since the invention of the camera — calling out trends in “duck lips,” “sparrow faces” and “finger mouthing.” Even scientists couldn’t escape the pull of the “resting bitch face” (RBF) riptide last year.

“The obsession with figuring women out and wondering why they do things a certain way is a bit suffocating at times,” Ralph said.

Smiling will probably always reign over RBF in the land of Minnesota Nice. For every subversive, brooding stare on Instagram, there are 10,000 takes of Cheshire cat grins.

So, maybe we should all just hush up about it.

“It’s like women aren’t doing anything with their face,” Ralph said. “What do we have to say about that?”