Sex Differences In The Human Brain Observed From Birth In Largest Study Of Its Kind

“We found it very interesting that several of the sex differences that were previously observed in older children and adults were already present at birth,” first author Yumnah Khan, a PhD student at the University of Cambridge’s Autism Research Centre, told IFLScience. “This emphasises that these differences are present from the very beginning of life and likely emerge prenatally.”

The study used data from the Developing Human Connectome Project, comprising nearly 1,000 MRI scans taken from newborn babies, most within the first few days of life.

Little is known about the newborn brain, leaving many questions, such as this one, unanswered.

Yumnah Khan

“It has been incredibly challenging to previously collect MRI data from newborns and tailor MRI analyses technique to the newborn brain,” Khan explained. One reason for that is to get a clear and accurate MRI scan you need your subject to keep very still, something newborns famously struggle with.

“As a result, little is known about the newborn brain, leaving many questions, such as this one, unanswered. Since these infants were scanned so soon after birth, we were able to identify what sex differences are already present at birth, allowing us to understand to what extent prenatal factors impact sex differences in the brain at a stage where postnatal environmental factors such as gender socialization are relatively minimal.”

The final sample for this study, after a number of exclusion criteria were applied, was 514 babies, of whom 236 were birth-assigned females and 278 were birth-assigned males. Of these, 56.8 percent were scanned within the first week of their lives. “This is the largest such study to date,” commented study supervisor Dr Alex Tsompanidis in a statement.

Total brain volume was found to be higher on average in the male babies, even when birth weight was taken into consideration. It’s been consistently reported that male brains are larger, but this study has confirmed that this difference is present all the way through life.

After controlling for overall brain volume, females were found on average to have more gray matter. This brain tissue mostly consists of the cell bodies of neurons, including the cell nuclei and all the organelles they need to function. Most gray matter forms the outer layer of the brain, the cerebral cortex, but there are pockets deeper inside as well.

By contrast, males had more white matter on average. This tissue is made up of the long axons that project from neurons, a little bit like electrical cables. It’s through axons that cells in distant regions of the brain can form connections.

The brains of males and females are more similar than they are different.

Yumnah Khan

There were some significant differences in specific brain regions too. Overall, though, the message is that differences that have typically been observed in the brains of older children or adults have now been shown to be present from birth.

But what do these findings actually mean?

Questions about the possible differences between male and female brains have persisted through the ages. Investigations into whether sex affects how the brain looks inevitably lead to theories about how this might affect function – do men and women think differently as a consequence of how our brains are structured?

But Khan explained to IFLScience that this is not really the right way to think about this study. “The brain is not ‘sexually dimorphic’ the way that the reproductive organs are. The brains of males and females are more similar than they are different, and it is important not to overstate or exaggerate the differences.”

“Any sex differences that we have observed here are simply in group averages, and may not apply to each individual male or female.”

What’s more, figuring out how a complex organ like the brain works is not simply a question of linking a structural observation to a functional outcome: “It will be important for future studies to directly test whether there are any associations between sex differences in brain structure at birth and cognitive and behavioural outcomes to fully understand the functional implication of these findings,” Khan said.

There have been some hints from prior studies. Khan explained that female brains may have more gray matter to compensate for their smaller overall volume, while male brains may need more white matter to connect up all the parts of their larger brains.

“But again,” Khan concluded, “it will be very important for future research to directly test these links.”

Some of that research may focus on neurodiversity, helping scientists to understand why, for example, autism is diagnosed more often in males than in females – and that’s not the only area where we see a sex-based disparity in diagnosis.

The quest to identify fundamental differences between male and female brains has been fraught. Arguments have been waged over viewpoints some have decried as pseudoscience – sometimes with a sprinkling of misogyny thrown in there too.

The true picture is much more complicated and nuanced than many would like to think – but in our humble opinion, that usually makes things all the more interesting.

The study is published in the journal Biology of Sex Differences.