Quilted Science

Patchwork thoughts on psychology, neuroscience, and human behavior.

Male Brain, Female Brain

Sex differences in brain connectivity show up after childhood. Why?

A study recently published in PNAS examined biological sex differences in brain structure. I find studies such as this one frustrating and easy to misinterpret.

Males and females are obviously different. But are male and female brains different? If so, how?

According to this study, the answer seems to be yes. Female and male brains are structurally different. The study looked at the connectome in children, adolescents, and young adults.

“Connectome” is a new buzzword in neuroscience. It is modeled after the word “genome” and means a detailed (ideally complete) map of the brain’s neural connections. Human structural connectomes are generated with a magnetic resonance method called diffusion tensor imaging (DTI). A non-invasive technique, DTI maps how different brain regions are connected to one another by measuring water flowing through the brain’s white matter. White matter is made up of axons, which are the “wires” that connect neurons to one another.

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The University of Pennsylvania researchers asked how biological sex affected the brain’s white matter. They collected DTI data from 949 volunteers who ranged in age from 8 to 22 years. 

The authors report male and female brains are indeed wired differently and summarized their findings as showing that “male brains are optimized for intrahemispheric and female brains for interhemispheric communication.” In other words, male brains have stronger connections within a single cerebral hemisphere while female brains have stronger connections between the two hemispheres. Many popular press reports have pondered what these structural differences mean for how men and women function in this country and also for connectivity disorders such as autism.

But the authors went even further, suggesting, “male brains are structured to facilitate connectivity between perception and coordinated action, whereas female brains are designed to facilitate communication between analytical and intuitive processing modes.”

According to the authors, female brains are designed differently than male brains.

As troubling as the verb choice is, thankfully the data does not support the claim.

When the authors divided the male and female participants into groups based on age, they found no sex differences until adolescence (Figure 2B). Connectivity patterns in children were indistinguishable between males and females! 

This is an important point and is not discussed thoroughly.

Experience defines the brain. Your own experiences create new connections, replace outdated connections and strengthen existing ones. As Francis Crick, one of the scientists responsible for figuring out the structure of DNA, wrote in his book The Astonishing Hypothesis, “You, your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules.”

We are our neural connections, defined by our experiences.

I think the important question is why female and male brains wire differently after childhood

That is a hard question to tackle, but worthwhile questions usually are. I know the answer will have many factors, but I think experience will be the driving one.

One way to start untangling the role of experience would be to look at cross-cultural connectomes. Specifically, I want to know about connectomes from cultures, such as Asian-Americans, where there is minimal difference in academic performance between the sexes. I think those cultures will also show minimal differences between male and female brains. (Asian-Americans made up only 1% of the participant group in the PNAS study.)

Growing up, I always enjoyed math and science (in addition to writing, of course). It was not until my high school AP Chemistry class when I first thought I might be different. There were 11 students in that class; ten were male. Sometimes I consider myself lucky to have escaped the American norm of females not being good at math and science. 

That mentality is changing. Studies show that the females who pursue a math or science education are just as skilled as their male counterparts. And companies are successfully marketing toys teaching spatial reasoning and engineering concepts towards girls. I hope scientists studying biological sex differences start asking the questions that really matter.

Kimberlee D’Ardenne, Ph.D. is a neuroscientist by training, science writer by choice.

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