Structural differences between male and female brains are apparent from the very earliest days of life, according to a new study. Researchers found that even in newborn babies, female brains tend to have more gray matter, while male brains tend to have more white matter, along with an overall larger brain volume.
“We found it very interesting that several of the sex differences previously observed in older children and adults were already present at birth,” first author Yumnah Khan, a PhD student at the University of Cambridge Autism Research Centre, told IFLScience. “This emphasizes 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, including 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,” Khan explained.
“It has been incredibly challenging to collect MRI data from newborns and tailor MRI analysis techniques to the newborn brain. One reason is that to get a clear and accurate MRI scan, the subject needs to remain very still—something newborns famously struggle with.”
Because these infants were scanned so soon after birth, researchers were able to identify which sex differences were already present at birth. This helped them better understand how much prenatal factors may influence brain development, at a stage when postnatal environmental factors such as gender socialization are still minimal.
After applying exclusion criteria, the final sample included 514 babies: 236 birth-assigned females and 278 birth-assigned males. Of these, 56.8 percent were scanned within the first week of life.
“This is the largest such study to date,” said study supervisor Alex Tsompanidis.
The researchers found that total brain volume was, on average, larger in male babies, even after accounting for birth weight. Previous studies had consistently reported that male brains tend to be larger, and this study confirms that the difference is already present at birth.
After controlling for total brain volume, females were found to have more gray matter on average. Gray matter mainly consists of neuron cell bodies, including the nucleus and essential structures needed for cell function. Most gray matter forms the brain’s outer layer, known as the cerebral cortex, though some is located deeper inside the brain.
In contrast, males were found to have more white matter on average. White matter is made up of long nerve fibers called axons, which act like communication cables, allowing distant parts of the brain to connect and share information.
“The brains of males and females are more similar than they are different,” Khan said.
There were also differences in specific brain regions, but the overall message was that many of the structural differences seen in older children and adults are already present from birth.
However, researchers caution against overinterpreting these findings.
Questions about differences between male and female brains often lead to assumptions about thinking, behavior, or intelligence. But Khan emphasized that this study should not be understood that way.
“The brain is not ‘sexually dimorphic’ the way reproductive organs are,” she explained. “The brains of males and females are more similar than they are different, and it is important not to overstate or exaggerate these differences.”
She also noted that these findings reflect group averages and may not apply to every individual.
Understanding how brain structure relates to behavior is far more complex than simply linking size or tissue type to intelligence or personality.
“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 behavioral outcomes,” Khan said.
Some earlier research suggests that female brains may have more gray matter to balance their smaller overall size, while male brains may require more white matter to connect the different regions of their larger brains.
Still, Khan stressed that future studies must test these ideas directly.
This research may also help scientists better understand neurodevelopmental differences, including why conditions such as autism are diagnosed more often in males than in females.
The search for biological differences between male and female brains has often been controversial, especially when it intersects with social assumptions, stereotypes, or outdated beliefs.
The reality is far more complex and nuanced—and for many scientists, that complexity makes the subject even more fascinating.
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