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.