We've all seen it play out: Even in families that give their girls tractors and encourage their boys to cuddle dolls, more often the girls will choose a pink pony over a fire engine, and the boys will take Thomas the Tank Engine over Tinker Bell any old day.
Why is that? Some of this behavior is learned, no question about it. But the gulf between boys and girls goes deeper than upbringing, says Sheri Berenbaum, a professor of psychology and pediatrics at Pennsylvania State University.
Scientists suspect that even before birth, boys' and girls' brains are developing differently, shaping them into distinct little creatures.
Is there such a thing as a boy brain and a girl brain?
Yes. We know there are physical differences between a boy's brain and a girl's, both at birth and as children grow. But at least for now, exactly how those differences affect behavior, personality, and so on is a mystery.
For example, scientists say there probably is an area of the brain that propels many boys toward things that move and many girls toward nurturing, but it has yet to be identified.
How a boy's brain develops in the womb
Boys in the womb are little testosterone machines. In fact, says Margaret M. McCarthy, a professor of physiology at the University of Maryland who studies early brain development, male babies are born with as much testosterone as a 25-year-old man! After birth, testosterone plummets until a boy reaches puberty.
Among its many other jobs, testosterone shapes a male's developing brain. Animal studies show that it pares down the connections between brain cells (synapses) in some places and bulks them up in other places.
One study found that both male and female rats who were exposed to extra testosterone before birth performed better at maze tests shortly after birth. While scientists aren't ready to draw conclusions about humans based on this study, it is an indicator that testosterone may improve spatial reasoning.
Animal studies also show that in any male, some regions make connections typical of males, but some parts remain feminine. "There's really no such thing as a completely male brain," McCarthy says. "It's a mosaic of male and female."
How a girl's brain develops in the womb
Girls make some testosterone before they're born, too, but not nearly as much as boys, Berenbaum explains. And while girls do produce female hormones such as estrogen, these seem to have little impact on their developing brains.
In other words, girls have the brain that boys would have if theirs weren't reshaped by testosterone.
Comparing boys' and girls' brains as they grow
Once girls and boys are born, their brains continue to take different paths. MRI studies show that some areas grow faster in female brains while others grow faster in male brains. So, the brains of boys and girls who are the same age can be at different developmental stages. Eventually, though, they catch up with each other.
Size also varies. Male brains grow slightly larger than female brains, although the significance of this isn't clear.
Some research has shown that in girls, the region of the brain that helps control language and emotion – called the caudate – tends to be larger. (This part of the brain becomes especially active when someone looks at a photo of a sweetheart.)
Some studies also indicate that part of the larger corpus callosum, which connects the two sides of the brain, is larger in girls than in boys. Some scientists think this could mean that girls tend to use both the left and right sides to solve problems.
In studies on rats, males have been found to have a slightly larger amygdala, a region of the brain that controls deeper emotions, such as fear.
These seemingly small differences in brain structure don't necessarily mean boys will be better at certain things and girls at others. As Berenbaum explains, young brains are extremely plastic, and key regions grow or shrink depending on how they're used.
Do women tend to cry more easily than men because their brains are built to make them that way? Or are their brains shaped by their emotions? Or is it a little bit of both? We just don't know yet.
Adding to the mystery, individuals simply don't always conform to the stereotypes. There are plenty of "tomboys" who show little interest in dolls, and boys who are drawn to "girl" activities from an early age. These kids are well within the norm.
Do girls and boys think differently?
Scientists with the National Institutes of Health are piecing together the results from the MRIs of brains of 500 healthy young boys and girls to try to answer some key questions about the development of young brains. Already, they've made some interesting findings:
In most tests, boys and girls showed very similar abilities. They were equally competent at math, raising the possibility that any gap in math skills in later years is a product of culture, not biology.
Girls were somewhat better at memorizing and reciting lists of words, and they were slightly better at tasks that required finger dexterity and quick thinking.
Boys had the upper hand with spatial tasks, such as arranging blocks to form patterns.
The brain is only the beginning
Above all, the brain is flexible. Children build connections between brain cells, find fresh obsessions, and hone new skills as they read, listen, watch, and learn.
A girl who plays exclusively with dolls this month might move on to construction toys and blocks next month. Even if she never develops a fascination with toy cars, she may very well enjoy her bike and learn how to fix a chain (and later tune a carburetor). A boy may never pour imaginary tea for a doll, but he can learn how to take care of a pet (and later raise a baby of his own).
As Berenbaum says, "Biology is not destiny."