My girl is eight years old. She is clever and kind and tall and strong and funny and stubborn and beautiful – well of course I think all of these things, because she’s my daughter and I love her. “Am I a girly-girl or a tomboy?” she asks sometimes, and I can never tell from the way she asks which of them she wants to be: she knows that to be a girly-girl is to belong (she has learned from experience that getting “girly” right will be rewarded and protect her from certain kinds of sanction), but she knows too that “boy” things are laudable and tomboyishness has a cachet that girliness does not. Anyway, the answer is always the same: “You’re you, darling. Whatever you like is right for you.” With her long treacle-coloured hair and her undercut, her perfect cartwheels and her graphic novels, her Taylor Swift dance routines and her speed up a climbing wall, on her rollerskates or in her football boots – yes, she is perfectly her.

Last night the conversation took a different turn. She had been watching an episode of the CBBC documentary series My Life, and the subject was a 13-year-old transboy called Leo. In an article on the production, one of the executives explains that making a documentary for children meant they couldn’t be “explicit”: in other words, if Leo has always felt profoundly that he should have a penis, the programme can’t mention that. Instead, the experience of being a boy is crystallised in an anecdote about Leo (then Lily) deciding he wanted short hair and cutting it all off, and the fact that he preferred “boys’ toys”. So I asked my daughter: “How do you know if you’re a girl or a boy?” And she said, repeating the line from the documentary: “It’s the way you feel.” Oh, my girl. How can you know “how you feel” when all your life people have been telling you what you are? When your gender has been constructed for you and your limbs and brain arranged and shaped to fit it, by family, carers and strangers, from when you were the tiniest thing?

So many things we did, none of them wicked but all of them guiding you in the same direction. As a magpie-eyed baby, women on the tram would shake their jewellery at you and then coo over you when you reached out to grab the sparkling trinkets: “Oh, she’s a real girl,” they would say, approvingly. When you toddled, you loved to play with shoes: “Just like a woman,” adults told you, indulgently. (I found a photo the other day of your big brother flapping about in a pair of his dad’s trainers. He liked shoes too – but no one told him that was the proper thing for his sex.) You loved dolls, but then so did your brother: he had to ask for his first Barbie and his baby with a buggy, though, and I felt my good liberal intent quail at the violation of the norms when I got them for him, whereas you have been given a surfeit of big-titted totems and soft-cheeked dummies to practise femininity on.

Not all the encouragement you received has been positive. You did a term of judo, then you stopped because some boys in the class began shoving the few girls who attended. They didn’t tell you this was because you were a girl, but it was because you were a girl: they decided this class was their space, so they pushed you around till they pushed you out. “Push them back, and harder,” I wanted to say – you are going to grow up tall and powerful, and right now most boys your age are smaller than you – but fighting is only going to get you in trouble, and anyway, in a few years their violence will exceed anything you can offer. It’s a fantasy, it’s not an answer. I didn’t have an answer. Dear girl, I am sorry.

The YouTubers your brother watches use “pussy” as an insult. I talk to him about this, but that doesn’t stop his friends from watching, can’t excise this disgust from your social world. You read the covers of the magazines in the supermarket, a litany of male violence and female self-loathing: all this hate for your gorgeous, compact little self, I try to keep it out and it still gets it, it gets in. A lot of it gets in through me. One day, when you were only three, you picked up my razor during bathtime, and while I was distracted, you dragged it up your peachfuzz leg just like you’d seen me do. The cut was shallow but long – your first wound of femininity, and all my fault. (It’s funny, incidentally, how shaving is supposedly masculine. In terms of area, I shave more skin than any man who isn’t a pro swimmer or cyclist. But shaving is not feminine – perhaps because it is not feminine to be hairy in the first place, so it is not feminine to shave. It is just demanded by femininity.)

So in the documentary, when Leo says, “The only difference between [my] life and a born-male boy’s life is, [I’m] trapped in this awful body and [I] have to do loads of medical stuff,” I think about all the things people have done to you because of your “awful” body, the ways they’ve (we’ve) all tamed you because we’re afraid of what your rebellion might mean. And this has been a relatively gentle process so far. Leo says, “One thing that scared me was knowing my body would soon change into a woman’s,” and I think about what the advent of hips and tits will mean for you, the pinching and the pinging and appraising eyes and comments that will fill the air around you with pins and make you squeeze yourself smaller, smaller, smaller when you should be spreading out and taking all the space you need.

“Male hormones turn boys into men, and female hormones turn girls into women,” reads Leo from his script. This is not exactly true: male hormones turn juvenile males into adult males, female hormones turn juvenile females into adult females. To become a man or a woman takes the input of a million cultural prompts, the insults and the razors and the praise and the dolls, to make you what this society has determined your sex means you must be. But they are lying to you, my girl. The documentary illustrates the brain-body mismatch theorised to explain transgenderism with a cartoon: a blue, human-shaped figure with a blue brain, and a pink, skirted figure with a pink brain. As if a skirt were a part of your female body, as if your brilliant brain was rose-tinted – and this is gender. Not the way you feel, but the fact of society allocating you to one class or the other, with no chance to escape unless you deny your own flesh. My perfect girl, this hateful world.