Enid Blyton's gender stereotypes infuriated Cordelia Fine as a child. Now, as a psychologist and a mother, she can't accept that boys' and girls' preferences, aspirations – and even their brains – are as different as we assume

From an early age, I was incapable of reading Enid Blyton books (which I adored) without offering up a scathing feminist critique to anyone within earshot: "Oh, yes. Of course the boys go first! In case it's dangerous." I vividly remember coming across a sentence that so outraged me – a boy telling his companion that she couldn't take part in some adventure because she was a girl – that I stopped reading and spat on the offending lines.

Even today when reading to my own children it's hard not to want to edit Blyton. When I do, my eldest, even with his eyes closed, knows it immediately: "Mum, are you swapping the characters around again?" he'll ask the instant I put a girl behind the controls of the toy plane that will fly everyone to safety.

But how is it that even before he went to school my son was already so well versed in the different ways girls and boys are expected to behave? And how do I, as someone who once proudly spat on an Enid Blyton book, feel about how well these easy cliches thrive?

It wasn't until I became a parent that my feminist fire, my "inner spitter" if you will, was rekindled. At first, I was simply struck by how parents seemed to see children through the "lens of gender", as the psychologist Sandra Bem put it. Then, after the birth of my second child, I was astounded one day at playgroup. About a dozen young children were sitting eating and the playgroup leader's daughter, a boisterous five-year-old, started to lead them in a chorus of shouting and foot stomping. For some reason, only the other girls joined in; my two sons and the few other boys carried on eating quietly. "Aren't boys noisy!" one of the mothers exclaimed over the girlish uproar.

I was also surprised – especially given how politically correct we all supposedly are – by how quick parents were to chalk up their children's behaviour and traits to some deep gendered core. When among our group of friends the second crop of children came, a common question was, "Are they different?" Of course, the answer was always yes. But while the parent of two sons or two daughters would answer by talking lengthily about the unique, idiosyncratic personalities that made up the essence of Jack-ness or Sarah-ness, parents with one of each, I noticed, would often say instead, "Oh yes. Boys and girls are so different."

Different – I hear it all the time in conversations with other parents. And to a casual observer it can often seem that boys and girls play in a very different, easily defined way. But why does it happen and is it really as rigid as we think?

The casual sexism of the Edinburgh primary school my sister and I went to shocked my parents, too, at times. I can recall coming home one day with the news that we were being taught how to sew. "We" the girls, that is. The boys were doing woodwork. The next day, my father was carefully dressed in his best suit and sent to visit the head. The hastily purchased copy of the Daily Telegraph tucked under his arm was intended to make the point better than mere argument. My mother hoped the thought would strike that when even conservatively dressed male Telegraph readers find your school practices sexist and outdated, it's time to embrace progress.

It was only a partial success. The sexes were put together in a single "craft" class, but while the girls sewed pretty aprons, the boys were emasculated as little as possible by being given felt space rockets and stars to sew on to the black canvas of outer space. Undeterred, my mother [the writer Anne Fine] went on to write Bill's New Frock, a still popular children's book in which the different treatment and experiences of girls are made plain through the eyes of Bill, who unaccountably wakes up one morning as a girl. I loved that book, as my sons do now.

But gender wasn't a topic I gave too much thought to during my teenage and early adult years. When my own children came along, I became a voracious reader of parenting books, and when they were about two and four I came across a book claiming that differences between male and female brains have important implications for education and parenting. Curious, I looked up the studies used as evidence, and was shocked to discover how badly neuroscientific data was being misrepresented. I looked at some of the other popular books newly on the scene, also proclaiming important neurological differences between the sexes. Same thing. Yet people, educators – my son's kindergarten teacher! – appeared to be taking these pseudoscientific claims seriously.

Next I read study after study from social psychology, which built up a picture of a surprisingly fluid mind in constant interaction with the environment around it. I found out that when gender is in the background, the thinking and behaviour of the sexes becomes remarkably similar. But when the context makes gender salient – as social psychologists do in the lab and the real world does constantly – stereotypes and social expectations start to influence our self-perception, our interests and even our cognitive and social abilities.

Then there was the research about children. Their behaviour in many parents' minds is all the proof one could need of hardwired sex differences. But a closer look at the social world into which children are born reveals an environment in which gender is emphasised above all social categories, from birth. How should children ignore gender, not be influenced by the assumptions and expectations it brings, when they continually watch it, hear it, see it; are clothed in it, sleep in it, eat off it? Little wonder that children become "gender detectives" eager for their behaviour to fall on the right side of the all important social divide.

I'm pleased to say that the sheer extensiveness of the scientific terrain I covered enabled me to be tiresome in all sorts of different ways. Among friends, a well-timed sentence beginning with "Interestingly …" became my favourite way to spoil a perfectly pleasant conversation. "Interestingly, in humans there's no clear causal relationship between testosterone and aggressive behaviour," I would say casually to a parent describing a group of boys' behaviour as "testosterone-fuelled". A dear friend was gently rebuked with the same word when she mentioned having to stock up the present cupboard with more "girl toys". I couldn't help myself.

"Interestingly," I remarked, "a recent laboratory study of children's play behaviour found that girls spent twice as long playing with 'boy toys' as they did with 'girl toys'."

But it's at the local toy shop that my feelings really come to a head. The first time I shopped there I readily accepted the offer to have the present I'd just bought for my nephew gift-wrapped, not realising the agonies the shop assistant's next question would put me in.

"Is it for a boy or a girl?"

I realised that if I admitted that the gift was for a boy, out would come the ubiquitous cars, space rockets, tools or sports paraphernalia – reinforcing those as "for boys".

I hesitated so long before answering that, by the time I finally did, two other assistants were also waiting curiously for my response. "I'm not going to tell you," I said, a small rebellion that turned my face bright red.

I know it's just wrapping paper. But it's also a manifestation of something pervasive and powerful. When I discovered research showing that preschoolers are beginning to grasp not just the concrete correlates of gender, but also the metaphorical cues – that what is soft or curved is female, and what is hard or angular is male – I know that children are getting the message conveyed to them (however inadvertently) from the way their clothes and bedding, toys and crockery, greeting cards, and, yes, even wrapping paper, comes gender-labelled from birth. And when those gender labels lead five-year-old children to the conclusion that a black, spiky My Little Pony has to be for boys, while a lavender satin gun and holster set must be for girls, it becomes clear that these gender cues pack a psychological punch.

Next time I visited the toy shop I was ready for "that question".

"It's for a girl," I said. "But I really don't see why she shouldn't have that space rocket wrapping paper. After all, it's not as if girls can't grow up to become astronauts."

To my delight, a girl of about eight standing by the counter chimed in. "I like space stuff. And so does Charlotte. Mum, can we have her present wrapped in rocket paper too?"

"Well, of course," her mother answered. "Why ever not?"

Why ever not, indeed.

Delusions of Gender, The Real Science Behind Sex Differences by Cordelia Fine is published by Icon Books at £14.99. To order a copy for £10.99 with free UK p&p go to theguardian.com/bookshop or call 0330 333 6846