[As promised. I would like to acknowledge my debt here to West and Zimmerman's Doing Gender, to Natalie Reed, and to the valiant people who have attempted to explain to me what the fuck Judith Butler is going on about. Also, I'm not saying any of this is how it should work; I'm simply describing how, as far as I can tell, it does work.]
Our culture transmits to us a certain idea of How Gender Is Supposed to Work. Or, well, ideas. People of different races and classes and abilities and sexualities and religions and body types and personalities get different messages about how gender is supposed to work. You get different messages based on where you grew up, what your family’s like, where you went to school, what media you watched, who your friends were, whom you date, where you work. In fact, you can say that no two people get exactly the same gender socialization.
People use the cultural ideas of how gender works as a toolkit to express their own identities and to communicate them to other people. (For some people, gender is not a relevant axis on which they construct their identities. Thus you get cis by default people.) Ways that people express their gender can include clothing, grooming, hygiene, mannerisms, word choice and syntax, the way they pitch their voice, their lifestyle, sexual choices, how they interact with or treat other people, and probably half a dozen other things I can’t think of.
Because gender is a means of communicating facts about one’s identity, people “read” each other’s genders. People slot each other into “male” or “female,” and then into a multitude of gendered subcategories– butch, femme, good girl, slut, dandy, bro, geek, fag, et cetera I’m sure you can think of more. Of course, just because you’re trying to communicate something does not mean other people are able to understand what you’re trying to communicate (the jargon I hear is “illegible gender”); other people bring their own biases and interpretations and Mounds of Gendered Shit to the table.
People end up treating other people differently based on what gender they read the person as having. For instance, I mysteriously get much more respect in intellectual conversations when people read me as male or at least masculine. All this gets Really Important in relationship to sexuality, because for a lot of people what sex or gender they read their partner as is a fundamentally important aspect of attraction. And in turn what gendered subcategories they read their partner as fitting into affects whether they want a sexual relationship and what kind of sexual relationship they want.
People feel more comfortable expressing their genders in some ways than in other ways. In some ways, this is shaped by how people treat them: after all, there are plenty of women who look feminine to get guys without having much connection to femininity at all. But for a lot of people the act of expressing yourself in a certain gendered way feels right: putting on a skirt just feels ineffably better than sweatpants; it just feels right to swish; when you slick back your hair you look in the mirror and see you. I think that’s a lot of what people are getting at with “makeup is empowering!”– it’s not that makeup is empowering, it’s that expressing your gender is empowering, and for some people that involves makeup. (I predict about half my readers are nodding along going “yep” at this point and the other half are like “what the fuck is zie even talking about?” Sorry, other half. Please take it on faith that this Is A Thing I Promise.)
—As for the “hey, if that’s true, why can’t trans women just be men in makeup?” Because they feel comfortable as women, not men. Duh?
Why do people have feelings of gender comfort? Some kind of biopsychosocial nonsense, probably. I’d argue that there’s Probably Some Kind Of Neurobiological Thing Or Things That Affect One’s Gender Somehow, because both gender roles and trans people tend to pop up a lot cross-culturally, and because certainly from the inside it doesn’t feel like my bodily gender dysphoria is a product of the culture I grew up in. But I’m not going to say “it’s the prenatal hormones!” or “we have the brains of the opposite gender!” or anything specific. Wait for neuroscience to become more advanced.
As to the psychological and social factors… I only have one navel to gaze at, my own, so I’m not going to generalize about other people’s experiences. But I use gender-neutral pronouns and call myself “genderqueer” or “nonbinary” and wear a binder because I happened to encounter the trans community and fit my experiences into their framework of what genderqueer people do. I paint my nails and wear makeup because, shit, man, so did David Bowie and Billie Joe Armstrong. I cut my hair to look like Joan Jett and grew it out to look like every gamer guy who can’t be bothered to get a haircut. I’m perfectly fine fucking boys because of my formative experiences reading slash fic and lesbian feminist theory. (Which is kind of the opposite of what lesbian feminism was supposed to do, but c’est la vie.) I choose my outfits based on whether strangers “sir” me or sexually harass me. I joke that my gender identity is bishonen! Either “nature” or “nurture” is an inadequate explanation of how I got my gender: the answer is both.
(Also I really like this list from Natalie Reed’s Twitter of assorted factors that have to do with gender somehow. Gender is coooooomplicated.)