As a mother, I understand the powerful bond that can exist between a parent and child. I love my son, and I really would do anything for him. Society assumes that all women, with enough exposure to their babies, will feel the same sense of endless, selfless love. We’re thought to be biologically programmed this way. But the truth is far more complicated.

It’s hard for any woman to escape the expectation to be a mother. The maternal myth suffuses every human culture, from Catholicism’s Virgin Mary to Hinduism’s goddess mother. It’s considered the most natural state of womanhood, leaving the childless woman the object of pity. Let’s not even mention the woman who doesn’t want or like children at all.

At the weekend Thomasina Miers, chef and founder of the Wahaca chain of restaurants, suggested Theresa May had abandoned the fight against childhood obesity – including a ban on junk food adverts, and free primary school lunches – because she isn’t a mother. Miers’ implication was that motherhood somehow makes a person automatically care about not only her own children but everyone else’s as well; and that women who aren’t mothers don’t have the same caring sense towards future generations.

So powerful is this idea of maternal instinct – the endless well of selfless love – that it’s thought not to be exclusive to humans. Just look at how other animal mothers cling tightly to their newborns. London-based primatologist Dawn Starin, who has studied red colobus monkeys in Gambia, documented once how, even after an infant died, the primate mother would faithfully carry around its corpse, fruitlessly grooming and caring for its body for days. It is this kind of devotion that informs our image of archetypal motherhood.

But maternal instinct is not a switch that exists in every woman, ready to be flipped as soon as she smells a baby. Relationships between mothers and their children are frequently far more fraught than the myth leads us to believe. It shocks us that mothers can be selfish. It surprises us that women may not want kids at all. Yet fewer women are having children: 17% of women born in 1970 are childless, compared with 12% of those born in 1943.

There is scientific evidence to suggest that the maternal instinct may even be contingent on a woman’s circumstances. In her groundbreaking work, the influential American anthropologist Sarah Blaffer Hardy looked to other primate species for insights. For many monkeys and apes, a female can’t become pregnant until her last infant is independent, allowing her to cope with the physical burden of childcare.

Co-parenting is key in the few primate species that can have more than one baby at a time: tamarin monkeys naturally bear twins, and the father is extremely hands-on. But if one of the parents dies there’s a high chance of the babies being abandoned. In humans, too, a similar argument can be made that maternal instinct may sometimes depend on whether a mother has the support she needs. We’re not a species designed to cope alone. Indeed, we’re at our most social when it comes to parenting, often recruiting many people around us to help. It really does take a village to raise a child. Without any help, it can be desperately tough.

Fewer women are having children: 17% born in 1970 have remained childless, compared to 12% of those born in 1943

For some, the prospect of raising a child they never wanted, or feel they can’t raise, can be unbearable. A 2015 study titled Regretting Motherhood, by Israeli sociologist Orna Donath, lifted the lid on the deep regret some mothers feel after having children. I myself have had to face down questions from family, friends and even strangers who don’t understand my wish to have only one child. The thought of having more children terrifies me, and has nothing to do with the love I feel for my son.

The reasons why women feel the way they do about children is under-studied. At the extreme end, in cases of neglect or infanticide, it is often assumed that a person must be evil or psychotic. Nothing else could explain this level of cruelty towards a child. But statistics show that, tragically, mothers resort to killing their babies more often than we like to think, and many of them are not mentally ill. The homicide rate is greater for children aged under one year than any other age group. A 2013 academic study that analysed cases of filicide in England and Wales in the decade up to 2006 found that just under a quarter of female perpetrators were teenagers – girls often in situations that left them feeling they had no choice, who were desperate and scared.

Such uncomfortable facts reveal that motherhood is not always an against-all-odds epitome of selfless caring. Sometimes it can involve emotional calculation, weighing the needs of both parent and child. We all assume that a mother always wants the best for her child, above her own needs. What we seem to deliberately ignore is that a child’s welfare can also depend heavily on the mother’s own needs being met.

For the sake of both mothers and children, we need to begin detaching the myth of motherhood from the reality. It’s unfair of any society to expect women to be the best mothers they can be without economic or emotional support, just because they should love their children. Not all women are happy to be mothers. And when it comes to abortion, it is also wrong of pro-lifers to assume that keeping a baby will always work out fine because some maternal instinct will inevitably kick in.

Many mothers will know that birth doesn’t always signal a rush of immediate love. The maternal bond may build slowly over time. For a small few, it may never appear. And some never experience the urge to have children. We think of all these as unnatural exceptions, bucking the normal trend of how a woman is supposed to feel. But the scientific and historical evidence shows that none of it is strange at all.

The spectrum of human behaviour encompasses those who want children, those who don’t, and those who aren’t yet ready to have them. The most unnatural thing of all is forcing a woman into motherhood in the anticipation that she will biologically fall into line when a baby arrives.

• Angela Saini’s new book, Inferior: How Science Got Women Wrong and the New Research That’s Rewriting the Story, will be published by Fourth Estate on 15th June.