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Boys Like Trucks and Girls Like Dolls, but Is It Genetic?

The science of kids' gender roles

By Heather Turgeon |


Earlier this month, my son turned two. We didn’t rent a bouncy house or congregate at a kid’s gymnasium; instead we went on a tour of the city recycling center. Forklifts, dump trucks, men in work clothes yelling and hauling crates of bottles and cans onto massive conveyor belts – it was toddler-boy heaven.

Now, I like to think of myself as having taken a gender-neutral stance toward raising my son. Early on, we bought him a stuffed baby doll to encourage empathy and caretaking (he now sings to and sleeps with it every night), and I’m quite proud of his enthusiastic ballet moves. But around the time he turned one, it was like a gene for wheels and motors turned on, as if an obsession with vehicles and machinery was coded into the DNA on his Y chromosome.

And my son isn’t the only one. The more parents I talk to, the more stories I hear of boys’ fire truck fascinations or girls spending hours in ballet slippers enacting the entirety of the Nutcracker. When it comes to our children’s love for stereotypically boy and girl things, clearly something biological is at work.

Are Hormones at Play?

When you see a three-year-old boy pretend machine-gunning his siblings or, as I saw recently at my friend’s house, a five-year-old on a mission to kill ants with a hockey stick in the driveway, a popular conclusion to draw is that they must have higher levels of circulating testosterone.

Not true. According to the well-known author and neuroscientist Lise Eliot, boys and girls have equal hormone levels until puberty. The exception to this is that, in the womb, boy fetuses have a testosterone surge between six weeks from conception and the end of the second trimester, and for a few months after birth, boys and girls have a “mini-puberty,” in which estrogen and testosterone rise. But after six months, it levels off, and doesn’t spike again until the pre-teen years. Interestingly, boy and girl play is similar during these hormonal infant months; it’s actually not until well after the chemicals settle down that the gender play kicks up.

Many scientists, including Eliot, do believe, however, that those early prenatal and infant hormone boosts are enough to wire up little boy and girl brains with slight differences. Early humans obviously didn’t have Tonka trucks, but it might be that the boy brain has a slightly stronger proclivity for high-energy objects that move.

One population that has helped doctors sort out this question is girls with a condition known as congenital adrenal hyperplasia (CAH). Starting in the womb, these babies produce high levels of androgens (male hormones). And even though hormone levels are corrected with supplements after birth, as kids, the girls with CAH tend to be more aggressive and fond of traditionally boy activities. There is no question that, once born, social influences further divide kids into pink and blue camps (studies reveal that parents unconsciously promote stereotypical gender play), but science has shown convincingly that prenatal hormones do play a role.

Gender Identity

But even though my son sometimes seems as male as a junior Chuck Norris, the other day he lifted his shirt and told me he had a baby in his tummy. He declares in the bathtub that he and dad have penises, but every once in awhile I get included in that club as well. That’s because a toddler’s gender identity (the fixed sense we have of our own male or femaleness) is a work in progress – it takes years to establish fully.

By two and a half, most toddlers can correctly tell you which sex they are. And it’s at this point that the explosion of pink princesses and cement mixers hits, even in the most open-minded, gender-equal households. A few of my girlfriends have been thrown by this phenomenon, not considering themselves the “girlie” type and wondering where the insistence on dresses came from. It’s an indication, however, of how little kids see the world in black and white. While they’re still working out the complexities of life, they tend to sort and categorize, making them even more gender-conscious than we are as adults.

So preschoolers know they are boys or girls, but they don’t yet grasp that gender is a constant – a stable trait that will be part of them for life. At this age a boy might still have the belief that he could grow up to be a mommy or that girls have penises when they get older. It’s not until they are six that most children fully own their gender and can wrap their heads around its permanence.

Avoiding Our Own Stereotypes

It’s easy to let the trucks and imaginary guns give you tunnel vision as a parent. When the gender-stereotyped behaviors sprout up, we bring in our own host of preconceived ideas of what it means to be male or female. When you see your son violently colliding trains together, it’s important to remember that he is also a sensitive, emotional, empathetic little person too. He needs just as much for you to quietly read a book, rub his back and ask him how he’s feeling. And when your daughter insists on only pink and tutus, just remember to look out for her competitive, aggressive side as well. She may be destined for math or engineering greatness – it’s just hard to see past the princess costume.

Personally, I’ve embraced my son’s passion for all things on wheels. In fact, I find myself getting excited when I spot one of his favorites. I’ve actually heard myself shout, in genuine enthusiasm, “Look! 18-wheeler!” when we see one pass by. I just try to stay open to the other side of his little personality too. So when we get home, who knows, we may just set the table and have a tea party with our stuffed baby dolls.


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About Heather Turgeon


Heather Turgeon

Heather Turgeon is currently writing the book The Happy Sleeper (Penguin, 2014). She's a therapist-turned-writer who authors the Science of Kids column for Babble. A northeasterner at heart, Heather lives in Los Angeles with her husband and two little ones. Read bio and latest posts → Read Heather's latest posts →

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15 thoughts on “Boys Like Trucks and Girls Like Dolls, but Is It Genetic?

  1. Lamprey says:

    Boys are boys and girls are girls. Simple.

  2. lookaroundyou says:

    Girls who want to be ballerinas or princesses when their parents want them to be engineers have simply learned independently (through pervasive marketing) that these archetypes are the most potent representation of femininity in our culture.

  3. TravelMama says:

    This is so relative, my daugther only likes cars and trucks. She says she is a race car herself, and when she goes to her ballet classes she keeps telling me she is a race car with slippers and tutu.

  4. ImprovGal says:

    I agree with TravelMama; this is definitely relative. My 4-year-old sons like to play with their toy trains, but they also delight in watching “Barbie in a Mermaid Tale” with their 6-year-old sister and pretending to be any number of characters, whether male or female, human or animal…it just depends on their mood. To my father-in-law’s horror, they also adore playing dress up in my daughter’s outgrown dresses and leotards. I am enjoying this time when they don’t feel any need or outside pressure to be embarrassed about pursuing their interests.

  5. Stoich91 says:

    Nothing wrong with treating a boy like a boy and a girl like a girl. You can’t *impose* gender on kids’, as is the popular belief, today. “Boys will be Boys”, as the saying goes, and even though some little tykes get in tune with girly things, and vice versa, there is really no reason to worry about “sterotyping” a baby. If we’re more concentrated on their overall health and wellbeing, their proper gender roles will fall into place, without any prodding or paranoia on the parents’ part. I find it interesting that in utero hormones play a role in gender-typing, though. Cool science! :)

  6. jmagnus says:

    My garbage-truck-obsessed son, age 2, also likes to “nurse” his baby doll and tell me that he has a baby in his tummy (I’m pregnant). But I know what you mean about the contagion of truck excitement; lately I’ve been known to excitedly point out tow trucks and fire engines…even when I’m out with adult friends and my son is not with me!

  7. Gigi says:

    I’m a full-time nanny to a 2.5 year old girl who could not care less about anything princessy or stereotypically girly- she’s obsessed with big trucks, trains and dinosaurs; she has a doll stroller with which she uses mainly to race toys around the house.

    Add me to the list of those who find themselves shouting “Look, a big truck!” in mixed company!

  8. anonymous says:

    To the comments saying “boys are boys and girls are girls” – I scoff. Boys are “boys” because of what they see, how they are socialized and what their parents pass off as something boys do or dont do. same for girls

  9. Lamprey says:

    anonymous – decades of research would disagree with your opinion that gender identity is solely a result of what they see or how they are socialized. Ever hear of DNA?

  10. Anonymous says:

    When my cousin was a Toddler (now he is 18yrs old) he only wanted to play with dolls and his favorite color was pink till he was 6 yrs old, never had an interest in “BOY” play or trucks etc. His mother was very uncomfortable with him playing with dollies, so when he would come to our house we gave him all the dolls and Barbies he wanted..Like a secret from his parents! We would joke that he is Gay..or come to that realization as an adult. Sure enough he is 18yrs old Gay and proud of it…and we are proud of him also..

  11. comeon says:

    Oh good, you helped make your cousin gay! Barbies are for girls. Trucks are for boys. Don’t confuse them.

  12. zim says:

    “Boys are boys and girls are girls. Simple.”

    Yes, what a mistake it was to force women into men’s jobs! Articles like these and comments like yours show that the work of feminism is nowhere near completed. As long as many people think in stereotypes as you do and have nazi-like views of everything being simplistically rooted in genetics, sexist discrimination against women and girls will continue to be rampant.

  13. zim says:

    “anonymous – decades of research would disagree with your opinion that gender identity is solely a result of what they see or how they are socialized. Ever hear of DNA?”
    Many of the same researchers say that the problems of black Africa and the black inner city are rooted in DNA. I could say that men are genetically evil due to Y chromosome activated testosterone poisoning (and indeed some of the nuttiest victim-feminists do) but that wouldn’t make it true.

  14. Ty says:

    Just look at the psychologist experiment with a boy when he was incorrectly circumcised. The psychologist decided to run a social experiment on gender identity on this young boy so they changed his Gentiles to look more female. They decided to raise him as a girl. The boy fought this not wanting to play with doll and often shared toys with his brother. They tried to force him into the stereotypical female gender, and he nothing but fought it. When he found out he was angry and ultimately committed suicide to prove a point.

  15. KingreX32 says:

    I really dont see any problem with buying your Son Trucks or your daughter dolls. Especially if they are already into them. Its not steriotypical.

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