We know that partly because it happens with monkeys, too, and their societies have nothing to say on the matter. Experiments with adolescent rhesus monkeys have found that, when offered a choice, the girls go for dolls and the boys for trucks.
Further experiments suggest that all this has something to do with exposure to hormones in the womb or shortly after birth. One study found that the more testosterone three to four month old (human) boys had swimming around in their systems, the more time they spent looking at boy toys like trucks and balls.
So the choice seems to be hardwired, but many parents would agree that the wiring sometimes goes haywire. From my experience I'd broadly accept the notion that boys inherently like trucks and girls like dolls, but I'd also add that kids rarely conform perfectly to such black and white categorisation.
To put it simply, little girls might generally prefer playing with dolls, but sometimes they like to take the dolls to doll hospital and chop off their arms. Where that fits into the science goodness only knows.
With my kids, the confusion arises on both sides. Luca, my six-year-old, is in many ways a typical boy child, and the scientists would consider him the perfect embodiment of their conclusions - most of the time.
In a nutshell, he likes guns, superheroes and vehicles. My wife and I spend large chunks of most days staring blankly into space as he explains in great detail the imaginary weapons systems of whatever toy he happens to have at hand.
Pretty much anything he picks up, from a twig to an old hinge and bracket, will soon be assigned destructive powers. At six, his party trick is mimicking - brilliantly - the noise of a machine gun.
And then sometimes the neurons misfire, and he forgets the message. Sometimes he'll come across, say, one of his sister's miniature dollhouse dolls, and carry it round with him all day, keeping it in his pocket when he goes out, and tucking it into bed next to him at night.
You'd almost say he was being nurturing, which is not at all what evolutionary psychology says he should be.
A couple of years ago he went through a Toy Story phase, as many kids do. He had a mini sheriff Woody and a mini spaceman Buzz, but after he'd seen the second film these were soon replaced in his affections by a mini cowgirl Jessie.
Now she may be brave and kinda sassy, but Jessie is undeniably a girl. Even today, if he plays with any of them it's invariably Jessie, who he treats as a friend rather than a fighter.
More recently, after turning six, Luca hit the Star Wars phase, drawn in by the promise of alien worlds and high tech intergalactic warfare. For Christmas he wants Star Wars figures, and on top of the list is, um, Princess Leia.
At two and a half, Poppy is showing signs of similar gender confusion. As a baby, she took to soft toys in a way her brother never did, and still carries her cuddly cat everywhere she goes.
Without any encouragement from us she has started putting make-up on her dolls, which she thinks makes them look 'gorgeous' but in fact makes them look like Bride of Chucky.
In other words, she's a girly girl, right up until she catches sight of one of her brother's Lego Heroes. For the uninitiated, Lego Heroes are evil looking killer robot things, sporting a selection of deadly looking weapons and protected by spikes and plasma shields.
They are undeniably built for mayhem rather than make-up.
And, when the testosterone rises, Poppy recognises this. She will happily try and use one Lego Hero to murder another. Mimicking her brother, she makes the sounds of battle and then chucks them across the room, laughing demonically when they split apart on impact.
It's about as far from nurturing as it's possible to be.
Like all sensible parents, when both kids are engrossed in a game like this my wife and I sneak out of the room and enjoy a peaceful cup of tea.
Then sometimes we'll wake up from our daydreams and realise all is unusually quiet. We'll sneak up to the kids' rooms and peek in. And there's Luca, chatting to Jessie about her elegant hat. And there's Poppy, in another scene straight from Toy Story, haranguing Lego Hero Jawblade for spilling his tea.
Of course, none of this proves or disproves the science, but it does add a certain nuance to the findings. I'd say from my entirely unscientific experience that boys would rather play with trucks and girls with dolls - most of the time.
But in each of our kids there's just a little chunk of the other trying to get out. Sometimes it creeps into the open and they play completely against type for an hour or two, until the dominant part of their personalities reassert themselves.
I quite like it. A boy with a feminine side and a girl who can play tough when she needs to seems to strike a decent balance. The biggest downside at the moment is that well meaning relatives turn up with surprise toys at their peril. On one occasion, with Poppy in full-on fighting mode, a lovely doll was eyed suspiciously, patted once, and chucked out of the window.
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