Throughout her life, Sarah Thornton hadn’t given much thought to her breasts. They were there, of course, and they’d fed two children. But they had also attracted unwanted attention, and latterly they’d become a source of concern – with a history of breast cancer in her family, and after years of vigilance and tests, in 2018 Thornton was about to undergo a preventive double mastectomy. Preparing for the operation, she realised she still hadn’t given them much consideration, nor what it would be like to have “new” breasts in the form of implants. When they turned out to be bigger than expected, she was shocked, “but in the end,” she says, “it wasn’t the aesthetic form as much as the feeling. It was like losing sentience. And it put me on a quest to understand these things that I’d never thought too much about. These things I’d kind of dismissed as dumb boobs.”

Thornton’s new book, Tits Up: What Our Beliefs About Breasts Reveal About Life, Love, Sex and Society, is a deep dive into the bosom of our fixation with boobs. Writing the book, she says, has transformed how she views her own breasts. “I really did go from dismissing them as a kind of shallow accessory, to thinking of them as a really important body part – one we wouldn’t have a human species without,” she says. “Our top halves have been invaded by male supremacy and I did not realise how deeply patriarchal even my own view of breasts was. I was dismissing them as dumb boobs, partly because they’re positioned primarily in culture as erotic playthings and I didn’t want to just be an erotic plaything.”

She doesn’t want to be a killjoy, she says. “Breasts are not evolutionarily, or universally, erotic. But the sexualisation of breasts causes many women a lot of stress, anxiety and dissatisfaction. That is a real shame, if not a serious political problem, and I think elevating the esteem of this body part that’s so emblematic of womanhood is important.”

I’m speaking to Thornton at home in San Francisco. She grew up in Canada, with a British mother, then spent 26 years in the UK, where she was an arts journalist, academic and author of books including Seven Days in the Art World. She has lived in the US for 12 years, now on the west coast with her wife, the gallerist Jessica Silverman. They often use the word “titties” (“I have a lot of affection for the word,” she says) with their 18-month-old daughter. “I was really struck by the fact that in Chinese one of the dominant slang words is the equivalent of ‘milkies’. That’s just not true in the US, or Anglo-Saxon culture. The only equivalent word would be ‘jugs’, as something that suggests nutritional function, which is the evolutionary raison d’etre of our tits.”

Thornton likes “tits”, and the “tits up” of her book title is American showbiz slang for good luck, a much more positive association than the British version meaning hapless or disastrous. “Tits is the No 1 word used on the internet for breasts,” she says, “and it seemed to me that if women were going to reclaim these words, then we needed to branch out. In the US, men use a much broader vocabulary to describe breasts than women do, and that struck me as a red flag. How come teenage boys can use 10 words and teenage girls use one? It’s like, who thinks they own them?”

And so Thornton is keen to reclaim words such as tits. She also appreciates the word “rack”, something, she adds, which gives the impression of “all the cultural baggage” hanging off it. “A liberated rack has no particular appearance, it is what it is and it just works for its owner. A liberated rack isn’t ashamed, it does what it wants to do. So if you want to free the nipple, you go there; if you want to bundle up in a triple-thick sports bra, do that too. There’s not a singular liberated rack, there’s lots of ways to do it. I know this pluralism sometimes feels over liberal, but when it comes to women’s bodies, it’s hard to be liberal enough.”

For many girls, the development of their breasts is often the first time they become uncomfortably aware of heterosexual male attention. When Thornton was 15, her breasts were groped by an older male colleague at a golf club restaurant where she was working. Not long afterwards, at a sleepover, she was assaulted by the much older boyfriend of her friend’s sister in the middle of the night. Her breasts, she writes, “had become defeated fools – boobs in the literal sense – that needed to be buried in oversized sweaters.” Looking back, she says, “it was a significant event in my body’s history. I’m sure that fed into me being the kind of person who was not someone to flaunt my cleavage. I have such deep respect and love for women who love their cleavage, I just wasn’t good at that. I felt so awkward and vulnerable.”

When Thornton came to breastfeed her two older children, now in their 20s, her breasts took on a different meaning, but it wasn’t a particularly positive experience. “I really wish I had loved breastfeeding more than I did. I didn’t love it and it’s partly because my breasts were such a source of conflict for me.”

Thornton’s research took her from strip clubs to cosmetic surgeons’ clinics to donor milk banks. “The whole book is really told through women’s eyes,” she says. For one transgender woman she interviews, breast surgery was “an essential part of her validity as a woman”. The women who donated to milk banks were not exploited subordinate wet nurses but “allomothers” in the millennia-long precapitalist tradition of communal child rearing. In the strip and lapdance clubs – Thornton is in the “sex work is work” camp, which may jar with many feminists – she comes to the conclusion that, for the women who work there, breasts are not so much sex objects “as much as salaried assistants”. One dancer suggested that having men confronted with her breasts felt more humanising – they were also forced to look at her face – than when it was her bottom being objectified.

An artist, Clarity Haynes, who does a portrait of Thornton’s breasts, used to be a stripper. “She said it was fine if she was getting paid for it,” says Thornton, “but she would get so irate if she was just walking down the street and guys decided to ogle her.”

Given our breast-obsessed culture, it’s thrilling to realise that it was only relatively recently that breasts took on quite so much sexual importance. Thornton traces their sexualisation in the west to 15th-century France. “You need breasts to be disconnected from their primary use in order for them to be fully eroticised, and the first real cultural evidence of that is in French Renaissance painting, with portraits commissioned by French kings of their mistresses who had these pristine breasts – you even have the wet nurse in the background with her heavy, saggy milk-filled ‘jugs’ as a contrast to the perky unused breasts of the mistress.”

Thornton argues that there is a strong link between the sexualisation of breasts and higher rates of formula feeding. As formula became more widespread and affordable – in the US, breastfeeding hit its lowest rates in the 1970s – breast fetishism exploded. She believes in the right of women to choose to formula feed – “it would be inconsistent and preachy to tell another woman what to do with her boobs” – but she points out that she has always had a problem around the word “choice”, “because choices are not equal, we don’t make our choices on a level playing field.”

In a culture where breastfeeding in public can be uncomfortable for many women, support to get it established is missing and lack of employment rights can hinder it. Then there are the women who can’t, or won’t breastfeed for any number of reasons. “One of my interviewees was the victim of sexual abuse and she decided prior to giving birth that she would not breastfeed.”

The woman, Elysia, turned out to be such an abundant producer of milk that she ended up donating 80 US gallons to a milk bank that feeds premature babies. Her son also thrived on her milk. “But she never breastfed. She pumped and delivered raw milk to him, fresh, from ‘jug’ to jug. What’s really beautiful about that story is that she totally changed her relationship with her breasts through that experience. These things that she felt excruciating pain and shame around became something she had love for because they nourished her boy so well.”

Breast fashions change, and in the latter half of the 20th century, large breasts were desirable, and women’s feelings about their breasts “started to be influenced by implant shapes – the round Pamela Anderson shape and size”. Even if that is no longer fashionable, “breast surgery is not going away,” says Thornton. “The lift is on the rise.”

Although she met some male surgeons, “one of whom I call Dr More, because it was always more, more and more”, in her book Thornton chose to focus on female surgeons, who tended to have a far more natural and subtle approach. Thornton sat in on one operation on a woman in her 40s who was having her large implants removed and her breasts lifted. In the US, the number of implants peaked in 2007, writes Thornton. Dr Carolyn Chang, whose operation Thornton watches, tells her that “implants, or at least large ones, are becoming less fashionable. Women want athletic bodies.”

When Thornton ventured into the world of bra design, she found a dominance of foam cups that created a smooth, round appearance and hid the wearer’s nipples – a word that is rarely uttered in the industry, which instead prefers to talk about a breast’s “apex” (one notable exception being the Skims “nipple push-up bra”, which features a moulded nipple shape, launched recently with much hype).

This brought Thornton to the Free the Nipple movement, which began in 2012 to highlight the sexualisation of female nipples and give women the same shirt-free rights as men. In the beginning, Thornton says, she had doubts about it. “I was like, is it really important? After doing the research, I’ve come to the conclusion that it’s a really fundamental problem.” She writes that she believes “hiding this fundamental mammalian marker is integral to women’s inequality and disempowerment.” Thornton smiles. “Do you think that’s an overstatement? What I would say is men’s and women’s chests are not treated equally in our society and aren’t associated with the same thing.”

It comes to a point with nipples in particular. Men’s nipples are visible everywhere, but also unnoticed. “I didn’t even notice them until I started working on this book and then I just saw men’s nipples everywhere. A white shirt is a recipe for a male nipple.” Women tend not to be comfortable showing theirs, “and it’s partly because there’s this notion that our breasts are primarily sex objects, they don’t belong to us and if we take our top off, we’re going: ‘Come fuck me.’ I genuinely believe that the dismissal of our breasts for the complex things they are is a serious problem for women.”

She hopes that her book will go some way to help elevate the status of breasts, and that women may feel “less critical, more accepting”. Will sagging, ageing breasts ever not be considered, at best, a joke in our culture? Thornton sighs. “Ageing is not generally accepted. We live in a world that is so fast-changing that the meaning of wisdom has shifted.” She misses her “saggy boobs”, she says. “I wish I could give the affection to them that I now feel.” Before her double mastectomy and implants, she took her breasts on what she calls their “final outing” – to the pool at a fancy hotel and, as she swam, thanked them, and apologised for not appreciating them enough over the years.

She is silent for a while and looks suddenly emotional, then says she recently switched gyms; her new one has a lot of older women. “I see a lot of saggy boobs in the dressing room, and I actually feel love for them, genuine affection.” It took her a few years to accept her new breasts. “I’m very grateful that I dodged the bullet of breast cancer and that the experience led me to a place where I learned a lot.” She remembers one of the women she interviewed, a voluptuous burlesque dancer named Dirty Martini. “She said breasts are a gateway to body positivity and I actually think that’s true for a lot of women. They’re front and centre, part of us.”

• Tits Up by Sarah Thornton is published by Bluebird (£20). To support the Guardian and Observer order your copy at guardianbookshop.com.

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