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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.