Everyone knows that breast is best. Not a day goes by without more evidence. "Breastfeeding for 20 months decreases risk of ovarian cancer by 50%." "Breastfeeding burns up to 500 extra calories a day." "Breastfeeding reduces infant mortality by 15%." These are just three stories on Twitter from the last couple of days from, respectively, a sample of 1,000 Chinese women by Curtin University in Australia, a New York state health department campaign and the health education and literacy programme in Pakistan.

But is breastfeeding far and away the best thing? Or have we done women a disservice by overstating its benefits? This week one of the leading figures in a backlash against breastfeeding will argue in an open lecture at the University of Kent, Canterbury, that "the scientific evidence for breastfeeding's health benefits is weak".

Joan B Wolf, professor of gender studies at Texas A&M University, contends that breastfeeding has been oversold because of three factors. First, we have a cultural obsession with eliminating risk, she says. Second, much of the research into breastfeeding is methodologically flawed. Third, we're in thrall to "an ideology of total motherhood that stipulates that a mother can and should eliminate any risk to her children, regardless of how small or likely the risk or what the cost is to her own wellbeing in the process".

Without disputing the fact that breastfeeding does have obvious benefits, increasingly academics are starting to speak out against the limitations of the scientific evidence. In her book Is Breast Best? Taking on the Breastfeeding Experts and the New High Stakes of Motherhood, Wolf argues that the science behind many breastfeeding studies is problematic. "In the science we trust most, we do randomised controlled trials. But you can't do that with breastfeeding. The groups are self-selecting."

"Women who choose to go through the labour of breastfeeding have made a commitment to go the extra mile for the sake of their baby's health," says Wolf. "They are likely to be doing all kinds of other things too. Their homes are clean. They wash their hands. They will be reading more, talking more, serving more fruit and vegetables …

"When you look at all of those things and hold them up to the very small differences that researchers find, it could very well all be down to these environmental factors."

Tom Jaksic, an expert on neonatal nutrition at Harvard Medical School in the US, has expressed concern that most of the studies are "population-based", meaning that it is impossible to separate the effect of breastfeeding from the combined impact of caring parents and a positive environment. Dr Nancy Butte, professor of paediatrics at Baylor College of Medicine in Texas, and an authority on childhood obesity, has said that it's "hard to distinguish between a well-cared-for bottle-fed infant and one who's breastfed".

We are in danger of ignoring the drawbacks of breastfeeding, says Wolf, such as the potential loss to women's earnings. And we are in danger of "holding mothers accountable for all sorts of things they don't have control over". She says: "You end up having this very strict set of rules about what mothers can and can't do. Once you begin to say 'better safe than sorry' – which is what people say about breastfeeding – there is no end to that kind of logic. One of the greatest lies promoted by breastfeeding advocates is that breastfeeding is free. But it's not free if you count mother's labour. For many you could say it has an extraordinary cost and is probably not worth the effort of continuing to do it."

Wolf's message has not been received warmly by the breastfeeding lobby. "Many people were and remain very angry with me." She has been called "a gender-confused cow", likened to a "Holocaust denier" and accused of hating mothers and children. She admits the response to her book has been "mostly negative". One blogger writes: "It bespeaks great scientific arrogance (of the kind that Wolf supposedly decries!) to maintain that an artificial formula could offer absolutely every crucial natural constituent of mother's milk." But others have applauded her for "bringing sunlight to this topic" and opposing the "breastfeeding mafia".

The arguments in the US are hotting up since a "baby-friendly hospital initiative" was launched in New York last year with the backing of mayor Michael Bloomberg. As part of the Latch On, NYC campaign, formula is to be kept under lock and key on maternity wards. If a parent requests it, a nurse must deliver a lecture on breastfeeding and then document a medical reason for giving out the formula. The Huffington Post reported that mothers would have to "sign the formula out like medication". "Baby formula is not Budweiser," protested the Daily News.

In the US the debate is highly politicised, largely because of the absence of maternity leave. Those who caution against placing breastfeeding front and centre of health campaigns point out that working mothers (who have no choice but to return to work in the early weeks or months if they want to keep their jobs) are sentenced to spending time at work expressing breast milk to maintain their supply. This is such a part of American working life that in 2010 the authorities took the decision to make breast pumps and supplies tax deductible.

In Britain, meanwhile, the strange thing about breastfeeding is that all the promotions and campaigns exist alongside a climate of supposed guilt and non-compliance. Anyone who has breastfed a baby past the age of six months, let alone past one year, knows that it's quite hard to find other mothers who are still breastfeeding partially or solely by that age. If you breastfeed past a certain number of months, you will get funny looks in public and questions about why the baby hasn't grown out of it. All the official information suggests that a six-month-old should be breastfed. And yet the statistics tell a different story: at two weeks old more than half of British babies have had formula.

Last year Unicef UK's report Preventing Disease and Saving Resources: The Potential Contribution of Increasing Breastfeeding Rates in the UK showed that breastfeeding rates at birth have increased from 62% in 1990 to 81% in 2011. But by six weeks more than half of parents have introduced formula. From Unicef UK: "Only 45% of women reported that they were breastfeeding exclusively [ie no formula] at one week after birth. Fewer than 1% were still doing so at six months." The report concludes: "The great majority of babies in the UK are fed with formula in full or in part at some time during the first six months of life. And by five months of age, 75% of babies in the UK receive no breast milk at all."

This is contrary to the cultural messages we receive about breastfeeding: that we should be doing it and we should feel bad if we don't. In fact, only a minority of mothers are doing as they're told, and, despite all the fuss about breastfeeding, there is not much research into this. One 2007 study in the Journal of the Sociology of Health and Illness showed that only 32% of formula feeders feel a sense of failure about not breastfeeding. A total of 23% "worry what the midwife/health visitor might say". And even fewer worry about the effects on the baby's health. Instead, 88% are just "relieved their baby is being fed". There's doublethink going on here, a disconnection between what people are supposed to say and what they're actually doing. And yet the message persists that breast is the only way forward.

Christina Hardyment, author of a history of childcare, Dream Babies, says: "The history of breastfeeding fashion tends to be a middle-class thing. Most people just do whatever they can do and what they can afford. The more women go out to work, the more acceptable it is to not breastfeed."

But there is also a sense that there is a historical cycle at work here, she says. "At the beginning of the 20th century they were obsessed with formula and thought breastfeeding was unscientific. There was a big reaction in the 1920s led by Truby King. He said that babies needed their mother's milk and this sparked a terrific fashion for breastfeeding. It was about feeding by the clock, though, not on demand. People became pro-breastfeeding to the point that mothers began to be desperate. Then in the 1960s the Dr Spock "just relax" business came in and people started to say, 'If you can't breastfeed, then formula is just as good'."

As a historian she argues that the scientific research proves "there is no doubt mother's milk is made for babies". However, as a mother of four and grandmother of eight, she says: "I would not exaggerate the hugeness of the research into the benefits of breastfeeding. It's lovely if you can do it. But formula is the next best thing. I don't think it makes much difference either way if you provide lots of love and cuddles." Some things science can't disprove.