I thought I knew about feminism. I had the word “FEMINIST” written in black marker pen across the front of my homework diary aged 15, along with an anti-war sticker that incongruously involved a cupcake. I had graduated from the “girl power” of my primary school years to reading Germaine Greer on a beanbag in the college library. I felt sorry for the girls in sixth form getting Brazilians, who, unlike my enlightened self, clearly hadn’t clocked that waxing was a tool of patriarchal oppression. I studied feminist theory, went to feminist gatherings and listened to feminist podcasts. I had spent several evenings sitting cross-legged at a “collective” organised by other middle-class, university-educated women talking about intersectionality and Frida Kahlo. By the time I graduated from university, I had firmly absorbed a list of the correct ideas and words that I needed to be a “proper feminist” (but was probably not someone you wanted to invite to a dinner party).

In 2015, two years after graduating, I began a job working in a high-security women’s prison. I had read enough statistics and policy reports before I started to know that women in prisons were in desperate need of a little female empowerment. But what I quickly learned was that my feminist education had a thick wedge of information missing: namely, the part where it connected to actual women being very fundamentally oppressed because of their gender. Confronted by someone whose cervix had been plugged with four egg-sized capsules of crack cocaine on the behest of a controlling boyfriend who would reap the profits, I found it difficult to work out quite how my Frida Kahlo T-shirt and mansplaining radar were going to help things.

I quickly learned that my feminist education had a thick wedge of information missing

Women in prison are a group statistically likely to be on the wrong side of almost any curve: to have grown up in areas of deprivation and be victims of childhood sexual exploitation, sexual and domestic violence, domestic homicide and homelessness. Half of women in prison are there for committing a crime to support someone else’s drug habit – almost always a man’s. For some, this crime is selling sex from pavements, or soliciting; for plenty of others it is professional shoplifting, known as “grafting”. This means pocketing anything from fillet steak to mascara to be resold round the houses. In the months leading up to Christmas, shopping lists are collected in pubs for toys and gifts to be stolen on request and sold cut-price. Women are sent out on these grafting missions because they are seen as less conspicuous than men.

Needless to say, I was very green in my first few months in prison. I worked between the education and chaplaincy departments and the majority of my time was spent running art classes to help women with their personal development and self-expression.

I had been helping to run a soup kitchen, and lived at a community house where we gave homeless people emergency shelter in our spare room, so I had naively thought the problems would feel familiar. It was so different, though, when those issues – unstable housing, addiction and abuse – were condensed in the prison, packed into classrooms and three metre by two metre cells.

In one session, we asked the women to make an A3 map of their lives from torn-up magazines. The collage would show a road that meandered from their past experiences to future goals. Almost every road began with bottles of vodka, syringes and shadowy characters, and almost every one ended with symmetrical houses and white wedding dresses and Laura Ashley sofas. I had spiked the magazine pile with my partner’s railway-modelling magazines and glossy Sunday supplements in the hope of inspiring something different – a new job, an interesting hobby, some travel, perhaps? – but to little avail. “What else would you be doing in the future?” I asked Cathy*, looking at the scenes of domesticity she aspired to. “You’ve been writing some beautiful poetry about your experiences,” I told her. “I could help you get them published as part of a campaign for prison reform.”

Cathy was about my age (29) but since leaving the care system had only known the control of either a man or the state. She, like many others, was a shoplifter and was frequently in and out of prison for theft and drug possession. “It will be finding ‘the one’ that will get me out of my mess,” she said. “He will look after me and keep people away who come round trying to sell me gear [heroin] again.”

Cathy’s was an oft-told story. She had been prevented from seeing her children by social services because she couldn’t stop seeing an abusive partner. He kept coming round and, against her best judgment, she opened the door.

What I wanted to say was that she didn’t need a man to straighten her life out for her, that she had “everything she needed inside of her” (life advice that works best when Instagrammed over a picture of a thin white girl walking into a sunset).

In time I came to realise that she was probably right. Ambition and independence are a good deal further up the hierarchy of need than security. It’s pretty realistic to assume that the quickest way to ward off a coercive and abusive man is to find another man who is kinder and stronger to stand in the way.

Prison changed my attitude to sex work, too. The jail had just appointed its first female governor and she was keen to put feminism on the agenda for International Women’s Day that year. My projects (I had previously hosted a lecture by a non-binary Mars astronaut, run a workshop on gender-neutral pronouns and started a choir that sang protest songs) that would have been met with eye-rolls before her arrival were now enthusiastically scheduled: a showing of the film Made in Dagenham followed by a debate on the gender pay gap. The learning and skills department was running a concurrent visit from a local museum, featuring artefacts from the suffragette movement.

I had engaged with ideological debates on the topic of sex work before, of course I had. I knew that to question whether sex work is really like any other work would make you a dreaded “swerf” (sex worker exclusionary radical feminist) and – like the patriarchy – is an attempt to control what women do with their bodies. People don’t need rescuing, the theory goes, they need rights and unions.

I had assumed that sex work was well paid. Most women could only keep 10%

Phoebe was the first to speak after I had introduced the topic of equal pay and sexism in the workplace. She was, as many of the women in prison were, “on the game”. She worked as an escort, managing her business online, so she could charge her own prices and didn’t need to rely on a pimp. She was fairly new to the jail, and had been transferred from a big city prison, ruffling the other inmates with her manicured nails and diamante sliders. “I don’t think women get it worse at all,” she said. “We get doors opened, bought presents, we got everything we need to get what we want right here.” She ran her hand down the length of her body and winked. “You can’t do that if you’re a man.”

She directed her comments pointedly to the woman next to her. Lesley was the kind of sex worker we more often see in prison, known as a “survival sex worker”, someone on the game because they have no other choice. Lesley sold £6 blowjobs from street corners to fund her and her partner’s heroin addictions, the signs of which showed on her face, with hollowed cheeks and yellowed teeth. Before I worked in prisons, I had assumed that sex work must be quite well paid per hour – that even those with a pimp could surely enjoy a 60:40 split of the takings. This was wishful thinking. Although there are plenty of women like Phoebe, they are less likely to end up in jail, meeting people like me. Most of the women I met were getting more like 10%, or being paid in drugs and housing rather than cash. The almost universally male pimps often have a group of women in their employ and rake in the takings in return for protection. In other words, it’s a racket. Survival sex work has a worse gender pay gap than almost any other industry. “Does anyone disagree with Phoebe?” I asked. There was a silence, then Lesley piped up. “I don’t see anyone’s boyfriends having to go out on the game,” she said. Phoebe rolled her eyes. “No one’s making you, babe. And no one is setting your prices that low either.” I started to sense that there was some sort of argument from the wing going on here that I was not aware of, and the spat was working its way into our discussion in the guise of a price war. “You’re bringing everyone’s prices down with what you charge on the pavement, you slut.” By this point, they had both got up. “We do the same thing,” Lesley shouted. “Don’t you go thinking you’re better than me.” By this point, I had lost the room. Security had arrived. And the questions I had planned about unionisation and female solidarity were drowned out by more immediate concerns.

It was not the International Women’s Day I had planned. But as happened so often in my time in prison, the theories and beliefs I came in with sat uncomfortably next to the nuance of the reality. The majority of sex workers I met in prison, who arrived with bruises and track-marks, would rather have been doing anything else. They needed their rights protected, sure, but they also wanted a route out. The reality was not simple. It rarely is.

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I learned that my idealism had made me treat feminism like a club rather than a journey. My self-congratulatory, cross-legged-femcast feminism was not flexible enough to accommodate the volume of women’s conflicting experiences, thoughts and feelings in the prison.

The truth is that some women want to start a business, and others would like the safety and security provided by giving control to a trustworthy man, at least for now. Some sex workers want unions, others want an escape route. For feminism to be at all useful, it has to be uncomfortable. It has to include people whom it would be easier to leave out: women who say they’re not feminists; who think they need a man to save them, or who say they fancy Piers Morgan.

Working in prison messed up my ideas of what feminism should be. It didn’t invalidate the ideas I had learned and fought for, it just disrupted the clean lines, leaving me with unresolved tensions and fewer opinions. I went into prison thinking that I would be able to use feminism to help empower women, and to reform a system. Instead I met women who taught me about feminism, and saw myself change instead.

* Women’s names have been changed.

• Jailbirds: Lessons from a Women’s Prison by Mim Skinner is published by Seven Dials (RRP £16.99) To order a copy go to guardianbookshop.com or call 020 3176 3837. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.