Quotes of the Day

Pardon the sexism, but a question: Why are girls so girly?

For the better part of the past half-century, feminists, their opponents and armies of academics have debated the differences between men and women. Only in the past few years have scientists been able to use imaging technology to look inside men's and women's heads to investigate whether those stereotypical gender differences have roots in the brain. No concrete results have emerged from these studies yet, but now a new functional magnetic resonance imaging (fMRI) study of children offers at least one explanation for some common tween social behaviors: girls are hardwired to care about one-on-one relationships with their BFFs (best friends forever), while the brains of boys are more attuned to group dynamics and competition with other boys.

The study, conducted by researchers at the National Institute of Mental Health (NIMH) and Georgia State University, begins with a premise that every parent of a tween knows: as kids emerge into puberty, their focus changes dramatically. They care less about their families and more about their peers.

So what's actually going on inside these young brains? Scientists asked 34 healthy kids, ages 8 to 17, to look at pictures of 40 other boys and girls and judge how much they would like to interact with them online. The kids were asked to rate those in the photos on a scale from 0 ("not interested at all") to 100 ("very interested"). The NIMH scientists told the kids that their ratings would be revealed to the boys and girls in the pictures, and the scientists said they would arrange online chats between the kids and those they liked. The chats were supposed to occur two weeks later.

On the appointed day, the study volunteers were once again brought into the NIMH lab. This time, researchers monitored the kids' brain activity using fMRI while showing them the same pictures. The participants were asked to guess which of the kids in the pictures (the same kids they had rated — and who, they believed, knew those ratings) would like to interact with them.

It was all an elaborate ruse: the kids in the photos were actors, and there were no chats arranged. The purpose of the deception was to look inside participants' heads when they were highly engaged in a potential social interaction. Partly because the study design was so complex — it's difficult to study actual social interactions on fMRI — no experiment like this had ever been conducted before.

The results suggest that as girls progress from early puberty to late adolescence, certain regions of their brains become more active when they face a potential social interaction. Specifically, when an older girl anticipates meeting someone new — someone she believes will be interested in her — her nucleus accumbens (which is associated with reward and motivation), hypothalamus (associated with hormone secretion), hippocampus (associated with social learning) and insula (associated with subjective feelings) all become more active. By contrast, boys in the same situation show no such increase in activity in these areas. In fact, the activity in their insula actually declines.

Boys, it seems, aren't as interested in one-on-one interactions as girls are. Previous research has shown that male adolescents instead become more focused on competition within larger groups (like between sports teams). Perhaps it's evidence that evolution has programmed boys to compete within large groups, so they can learn to eliminate rivals for women — and that girls have been programmed to judge, one-on-one, who would be the most protective father for offspring.

The authors of the study are reluctant to draw such broad conclusions about the gender disparities. "There are many different possible explanations," says NIMH neuroscientist Daniel Pine, who suggests a much more ordinary reason for the girls' more emotional response. "It might be possible that the girls are trying to remember what they wrote earlier [about the kids in the photos]," he says. "You can imagine a scenario where they say, 'Oh, did I write something bad about that girl?' Boys are simply doing that less." In other words, it may be that boys are cads because they're not wired to be any other way.

• John Cloud
• It's no surprise that tween girls like one-on-one relationships and boys are more interested in groups. A new study suggests those preferences are wired in the brain