I was thinking about breasts as I watched The Substance. Coralie Fargeat’s body-horror fable features Demi Moore as a newly 50, supposedly fading fitness star who makes a pharmaceutical Faustian pact allowing her to create a nubile 20-year-old (played by Margaret Qualley) to replace her half the time. Breasts aren’t Fargeat’s main focus – it’s an ass more than a tit movie – but there are plenty on show. One (minor spoiler alert?) plops bloodily to the floor at a climactic moment and if that – miles from the most harrowing bit – sounds too revolting, it’s not the film for you.

I was thinking about breasts, because I had just read about the 64% increase in reductions in the US since 2019 (not including post-surgical reconstructions or gender-affirming top surgery). Many are on women under 30, and under-19s “represent a small but fast-growing part of the market”, the New York Times reported. Women, apparently, want “yoga boobs” or the girlish “coquette” look – a braless life.

Moore’s breasts, which make a brief cameo in The Substance, looked great, incidentally. She’s 61, playing 50, presumably because an actual 50-year-old actor would show no sign of ageing whatsoever, destroying the film’s premise. Moore’s casting leaves it shakier still: she looks incredible, far too good to need the titular substance.

But plenty of us opt for Substance-adjacent solutions to body dissatisfaction, including cosmetic surgery. Bodily autonomy is a right I’m fairly het up about defending and there’s no single motive for having “work” done: breast reductions often resolve decades of physical pain and unhappiness; smaller breasts attract less unwanted attention; and no one should underestimate the anguish of a big-boobed adolescence, feeling you can never wear what you’d like, or escape the leering.

But how much of our surgically addressed dissatisfaction (augmentations are still even more popular than reductions) is intrinsic to the adipose tissue and how much is culturally constructed? A “breast size satisfaction survey” in 2020 reported that 70% of women worldwide dislike the size of their breasts. “Commodification and scrutinisation of breasts can influence how women feel about their own bodies,” the same study notes, and well, duh.

Breasts of all sizes have been freighted with cultural, erotic baggage for centuries; since medieval Madonnas depicted with a chaste, single exposed one ceded to sexy secular pairs, and pornography spread with the printing press. They still are, of course: in one scene in The Substance, a casting team joke they wish a woman had breasts on her face instead of “that nose”.

You can frame surgery as empowerment or emancipation, a “fuck you” to anyone else’s opinion. But it’s also about body parts seeming or feeling too big or too small, or the wrong shape. Women under 30 are, increasingly, keen consumers of all kinds of cosmetic procedures. Describing the gen-Z “mainstreaming of plastic surgery”, the Washington Post covered two TikTok-documented breast augmentations and one reduction. How quickly will that trickle down to the Sephora tweens when serums no longer cut it?

It’s a bleak thought, beautiful bodies not being beautiful to their owners. So is Fargeat’s statement: “I don’t know a single woman who doesn’t have a troubled relationship to her body.” Her film really doesn’t help, though, with its lingering, lascivious gaze on dewy youth and portrayal of ageing as explosively, grotesquely repulsive. The intent is satirical, but does satire work when it reinforces what it’s satirising?

I started thinking about breasts, and ended thinking about bottoms. Partly because Qualley’s perfect one gyrated in my face repeatedly over the hour and 40 minutes I sat on mine in the multiplex. Partly also because I read an interview with Moore promoting the film in which, despite preaching the hollow Hollywood gospel of self-love, she twice said she didn’t entirely like how hers looked on screen. “It’s not like there weren’t shots in it where I go, ‘Ugh, my ass looks awful.’” Then again: “Ugh, I didn’t love my butt.” (Though she does acknowledge that these were kneejerk reactions: “It’s not that I look that bad.”)

I also watched The Substance in the week in which a woman reportedly died after a “liquid” filler Brazilian butt lift. Its popular surgical cousin is the most dangerous cosmetic procedure; another woman died after one in Turkey this August. That is the real body horror.

• Emma Beddington is a Guardian columnist

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