Western society hasn't always preferred its women smiley. In fact, the idea that women should be constantly smiling is a pretty recent one — in the past, smiling women were often considered suspicious or sinful. In fact, during the medieval period, female laughter was seen as a sign of opening the body to the possibilities of sin. A 13th century German book asked "Why should a woman, greeted by a man, smile at him with frolicking eyes?" Why should she, indeed.

During this time period, the only women who were permitted to smile in public were those who were widely known to be chaste; for example, 13th century images of the Virgin Mary smiling at the baby Jesus were common. Beyond that, though, smiling was considered dangerous and degrading for women.

This wasn't just due to sexist thinking about women's sinful sexuality; it was also due to sexist thinking about women's supposed inconstancy and irrationality. "Man has one smile, woman a thousand," wrote Ernest Legouvé in the early 19th century, echoing what people had believed for centuries.

Many of the era's male prejudices about female smiles are on display in the ferocious criticism that Da Vinci's painting, "The Mona Lisa," attracted. One art critic wrote that her smile bore the "instincts of conquest, of ferocity, of all the heredity of the species, the will to seduce and ensnare, the charm of deceit, the kindness that conceals a cruel purpose," while another claimed that "the smile is full of attraction, but it is the treacherous attraction of a sick soul that renders sickness." Since smiling women were seen at the time as openly seductive, deceitful, drunk, boorish, or attempting to gain power, to be seen smiling in a portrait was thought of as a signal of some or all of these qualities; it was seen as a sign of low class or voluptuousness.

The female smile only really began to become more acceptable from the late 18th century onwards, as portraits began to depict slightly-smiling aristocratic women. It was the female artist Élisabeth Louise Vigée Lebrun, writes Colin Jones in The Smile Revolution, who made a truly remarkable step forward: in 1787, she published a self-portrait of herself smiling with her teeth showing. Lebrun's toothy grin looks demure to us today, but it attracted "universal condemnation" when it was unveiled, according to a disapproving writer of the time.

And despite some steps forward, the female smile still genuinely continued to shock; one of the first films to be shown publicly, "The Kiss" of 1896, caused a scandal partly because it showed a smiling woman kissing her boyfriend. The tie between smiling and scandalous female behavior was still considered a strong one.

Smiling in images became more popular for people of all genders as the 20th century wore on. But ad campaigns played a particular role in making female smiles seem less taboo.

According to a Washington Post article, "The photographic smile...was a byproduct of an increasingly sophisticated advertising culture focused on telling cheerful stories about products." This atmosphere finally created the right conditions for female smiles to become more acceptable. The "Kodak Girl" — a smiling girl or woman holding a camera — was one of the first, turning up in the early 20th century just as women's independence began to be seen as somewhat trendy; variations on the image would sell the company's products in various forms for decades.

Women's grins in advertising images from the late 19th century were used to sell everything from catalogues to soap, cameras to rouge. And by the 1950s, the tie between smiling female images and domestic products in particular was firmly embedded in the public imagination — now, instead of being thought of as deviant, smiling women were commonly thought to be safe, happy, subservient, and exemplars of capitalism.

Our culture definitely still brings that baggage to bear on smiling women today: we're taught to smile in public to show that we're "agreeable." Marianne LaFrance, in Why Smile?, calls smiling an act that "produces an emotional current": something that opens you up to the interactions and demands of others, and makes you look pleasantly available."

We teach women to smile to convey that they embody all of these qualities — and we teach it quite early: a 2009 study on gender bias in schools noted that girls in school "quickly learn to smile, work quietly, be neat, defer to boys, and speak only when spoken to."

But smiling isn't only thought to signify friendliness and agreeability among women — according to some observers, it's also socially required to show that women are not aggressive or threatening. Londa Schiebinger, of Stanford's Clayman Institute for Gender Research, has written that "Women, expected to exude politeness in both speech and manner, are required, more than men, to smile. When listening, a woman may nod and smile to express attentiveness. If a woman does not smile, she may be perceived as angry." Katy Waldman, writing for Slate in 2013, explained that it was part of the many ways in which we "police social performance" using gender.

Anecdotal interviews I conducted with women aged 27 to 35 seemed to bear all of these theories out. While getting off a train, Becky said, "A middle-aged guy waiting to get on looked at me, made eye contact and said 'SMILE!' What struck me was that when I reacted with visible anger, he looked really smug and pleased with himself, as tough he'd got exactly the reaction he wanted. Anyway, I told him 'It's incredibly rude to ask people to smile, f*ck you' and stomped off.'" Keisha, meanwhile, said it went back to childhood: "My primary school bus driver used to ask all the girls where their smile was as they got on the bus."

And it's not all explicit external pressure — sometimes, we're taught to internalize the pressure to smile. Claire told me, "Random men would compliment my smile. And this just reinforced that I was doing my job well. I thought I was making the world a happier place. No matter what I felt like inside. It was my job to make the world happier. That was on my shoulders — well, between my cheeks."

And while we may think of men as exerting the most pressure on women to smile, the social pressures are a bit more complex than that; women, it's been noted in studies, smile the most when observed, particularly by other women.

"When I'm doing my big grin at a work meeting, an (inevitably male, middle aged client) will say 'Oh yeah, you've got a beautiful smile.'...It says 'I see your confidence, and I'm going to knock it down. I own this space, and you are decoration.'"

When it comes to the workplace, the picture becomes even more complicated. On the job, women feel the pressure to smile to look accessible — and yet also face concerns that it makes them look less "aggressive" or capable of doing harder work. The subtlety of this issue is captured in the idea of "smile work," defined by one text as "a culturally imposed strategy women use to fit into departments with a tradition of male dominance. Smile work entails the symbolic management of behavior to present oneself as pleasant and agreeable." The double standard of professional women and professional men has been well-examined by experts: while women's assertiveness is often seen as threatening or gender-challenging aggressiveness, men's is seen as showing that they are leadership material.

Somewhat troublingly, a 2009 study of women's and men's faces found that thinking of smiles as "womanly" may be a culturally inbuilt prejudice in the West: smiling faces are read automatically as more feminine, while blank or aggressive faces are read as male. Women's faces are thought of as more innately "smiley" — and deviating from that can cause confusion and push-back, particularly in professional situations.

But smiling at work can also be seen as too assertive on its own. Lois told me, "Smiling with agency and confidence is all about owning your space, owning the way you project yourself into your environment. When I'm doing my big grin... at a work meeting, an (inevitably male, middle aged client) will say 'Oh yeah, you've got a beautiful smile.' It's a fascinating and aggressive counter-move. It says 'I see your agency, I see your confidence, and I'm going to knock it down. I own this space, and you are decoration.'"

Another friend, flying back early from a trip because of the death of her grandmother, asked a transport worker for help. "And this man told me, 'First, you need to smile for me.'"

While women who don't smile in a professional environment can be seen as threatening, there's another issue to be examined when it comes to female smiles and work: many women are employed in jobs that demand "service with a smile." As Schiebinger wrote, "Women are overrepresented in professions that require smiling, such as nursing, teaching, daycare, serving as flight attendants or secretaries." The smile, in these industries, is a symbol of openness to the emotional care and tending of others as a profession — something called "emotional labor" that can create intense exhaustion.

On the flip side, many men feel that being at their job entitles them to demand that women smile. Many women interviewed for this had experiences with men who attempted to use female smiles as a form of control in work-related situations. Jennifer called it "men using their position of whatever form of power/control/authority to 'playfully' extort behaviors/aesthetic stylings from women, i.e. humiliate them."

Rosanna told me about the time she was having her bag checked by a supermarket guard, who "made me wait while he proceeded to point out all of the women entering and leaving the store," and told her about his policy "that he would ask to check their bags because they either weren't smiling or he didn't find them attractive."

Another friend, flying back early from a trip because of the death of her grandmother, asked a transport worker for help. "And this man told me, 'First, you need to smile for me.'"

She went on, "I had just got off a last-minute flight from Málaga, had been traveling for god knows how long at this point, and was just trying to get my way to my next flight without crying in public, and this asshole demanded that I smile before he would tell me which train I needed to take. Which is his JOB, and which I had asked him VERY POLITELY to do, with a totally neutral (if tired) expression. So I f*cking smiled and then cried on the subway, because I was so angry and humiliated. I KNOW I had visible tear marks and puffiness around my eyes, because I had spent literal days crying at that point. But this man's concern was that I wasn't asking him prettily enough."

Women of color are subject to more than just sexist smiling-related baggage in U.S. culture; they are also subject to a long history of institutionalized racism that demanded smiles from people of color.

The philosopher Franz Fanon wrote at length about how white people have historically felt entitled to see black people smiling, so that they could perpetuate the myth that white oppression is not hurtful, damaging or dangerous. This belief can be seen in centuries of racist iconography depicting smiling black people, many of whom are depicted in the act of servitude. At the same time, racist culture has also long held that black women are "angry." For all these reasons and more, demands that black women smile are often expressions of both racism and sexism.

The demands on women to smile are diverse, and usually signal broader demands on their energy and position: be docile, be acceptable, be accommodating to male desire, be polite, be subservient. Various women interviewed for this article talked about how they have to consciously think about "taking their smile back," and not reflexively producing it to smooth over situations or make others happy. "Obviously I am naturally one of life's smilers," Jennifer told me, "and take joy in smiling and having the smile returned (yes, even with strangers) — but the idea of Mandatory Cheeriness makes me feel I am seen as an ornament to the backdrop of a man's life story."

Some, like Lois, are using aggressive smiling on their own terms to stake out their position and witness male pushback. "The kind of smile that's so big it's a bit like a sunny punch in the face," she said, has "something territorial" about it. Others, like Claire, who smiled because it felt like her social function, have found power in only smiling when they wish to, and ignoring the demands of others. "I have been told and encouraged a lot by strangers to smile," she told me. "And until recently, until I understood the politics behind it, I obliged. Now I don't. I own my smile now."