Several years ago, a high-school friend of mine called to ask me if she could borrow some balls. She was in town to visit her parents and wanted some toys for her 2-year-old son. That was easy. My daughter had a whole basket of balls that she never played with. I felt kind of guilty giving these to my friend–a collection of boring, ignored toys. When my friend returned the toys at the end of the week, she thanked me profusely: “Jackson LOVED the balls. The first thing he said each morning was, ‘Ball!’”

I used to think that boys and girls were born alike (aside from those obvious anatomical differences). This is the kind of absolute belief only a childless person could have.

When my daughter and her peers were toddlers, I began to notice certain inescapable tendencies. The toddlers who were obsessed with balls, cars, trucks or trains tended to be boys. The toddlers who were obsessed with stuffed animals, dolls, dressing up, or playing house tended to be girls. Some interests, like dinosaurs, crossed gender lines.

These toddlers lived in families that provided them with all different kinds of toys–the boys had dolls and the girls had trains and balls. These children had mothers who worked, and fathers who cooked and changed diapers.

I suppose the revelation that boys and girls are different is not so shocking to many people. However, it did come as a surprise to me and some of my friends, who grew up reading Ms. magazine and thinking that all differences between the sexes were due to culture and upbringing.

I have even developed my own cave-people theories (my apologies to paleo-anthropologists). Boys like things they can throw and vehicles that cover a lot of ground because males have spent eons traveling around searching for animals and then throwing rocks and spears at them. (Imagine how much the cavemen would have enjoyed having a Ford Explorer to patrol their hunting grounds.) Girls like inventing relationships between stuffed animals and playing house because females have spent eons back at the cave raising the children, and gathering berries, talking all the while.

Why in the world wouldn’t boys and girls, and men and women, be different from one another? Why wouldn’t those with bigger bodies and stronger arms go in search of the meat? Why wouldn’t those with mammary glands stay with the children?

I spent the first several years of my daughter’s life being amazed at this discovery of mine, and noticing it in all its forms. However, now my daughter is in first grade. She is growing into a more complex person every day, no longer reducible to one or two gender-appropriate obsessions.

When she was 2 and 3 years old, she wore only dresses, played with her stuffed animals daily, and became enamored of ballet, with its princess-like costumes. She wanted to be a ballerina when she grew up. Today, she ignores her closet-full of dresses because she has discovered that shorts and pants are much more practical on the playground. She still loves to dance. However, princess costumes aren’t so important to her anymore, and this has freed her to become interested in other types of dancing. She wants to be a dentist when she grows up.

But as my daughter and her friends grow more complex, ready to explore a wider variety of interests, our culture hems them in. For example, I know a family where the two girls love Barbies. Relatives and friends shower these girls with Barbies. They have dozens. Yet in another family I know, a little boy likes Barbies. Every Christmas, he asks for one and his parents have a big argument about whether or not to get it for him. (So far, they always have.) But he hides his Barbies from his friends.

Almost everything you can buy for children is divided into boy and girl categories–lunch boxes, backpacks and computer games–forcing children to choose and announce their allegiances. The aisles in Toys ‘R’ Us aren’t labeled “boys” and “girls,” but they might as well be. The girl aisles are easy to recognize–think pink. A friend of mine, who is philosophically opposed to makeup, nail polish and gender stereotypes, was dismayed when her girls referred to the Lego aisle at Toys ‘R’ Us as a boy aisle.

Believe me, I have not been able to come up with a cave-people theory about why girls like pink. It seems pretty obvious that girls like pink because the toys and clothes we give them are pink.

We no longer need to dispatch hunting parties to the woods to bring back meat. We no longer need to gather our own berries. Even though I believe it’s often true to say, “Little boys like balls and little girls like dolls,” people who are out of preschool are much more complicated than that. Now that we’ve left caves behind, we are free to choose different roles for ourselves. Family life is richer when fathers work jobs that allow them to spend a lot of time with their children. Mothers who want to work outside the home are happier when they are able to do so.

I want my daughter to be able to dance and practice dentistry, and I want our culture to give her the space to do so. That’s why I still keep that basket of balls around. To give her room to grow.