Sex-based brain differences present at birth and stable in early infancy: study

A new study published in Biology of Sex Differences has revealed that structural differences between male and female brains are present from birth and remain relatively unchanged during the first month of life, highlighting the significant role of prenatal biological factors in shaping early brain development.

The research, conducted by scientists from the University of Cambridge as part of the Developing Human Connectome Project, analyzed brain scans of 514 full-term, healthy newborns — 278 boys and 236 girls — all within the first 28 days of life.

Using magnetic resonance imaging (MRI), researchers found that male infants had larger overall brain volumes, a pattern that aligns with findings in older children and adults.

However, after adjusting for total brain size, female infants were found to have more grey matter, the part of the brain responsible for information processing, while male infants had more white matter, which facilitates communication between brain regions.

Lead author Yumnah Khan, a PhD student at the University of Cambridge, said the findings challenge long-held assumptions that such sex-based differences in the brain are largely the result of social or environmental influences.

“Several on-average sex differences in the brain are already present from birth, indicating that prenatal factors play an important role in initiating sex differences in the brain,” Khan told PsyPost.

Exploring structural brain differences

The MRI scans revealed specific brain regions where the sexes differed. Female infants had relatively greater volumes in the corpus callosum, the bridge between the brain’s hemispheres, and the parahippocampal gyrus, linked to memory.

Meanwhile, male infants showed larger volumes in the medial and inferior temporal gyri, areas associated with visual and auditory processing.

Notably, these differences remained stable throughout the first month of life, suggesting they were established before birth rather than shaped by early postnatal experiences.

“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,” Khan said. “This emphasises that these differences are present from the very beginning of life and likely emerge prenatally.”

Implications for neurodevelopmental research
The findings have important implications for understanding why certain neurological and psychiatric conditions — such as autism, ADHD, and depression — occur more frequently or present differently in males and females. Researchers say these conditions may be linked to early structural differences in the brain, offering a potential pathway for earlier identification and targeted interventions.

Khan emphasised that the interest in sex differences is not just academic, but also practically significant.

“A better understanding of sex differences, their underlying causes, and the timeline of their emergence can explain why certain disorders affect males and females differently. This may also help tailor diagnostic and support strategies to improve health outcomes,” she said.

Caution against overgeneralisation

Despite the findings, the researchers were careful to warn against overinterpreting the results. The differences observed are average differences across large groups and do not suggest that male and female brains are fundamentally or universally different in function.

“It is important not to overstate or exaggerate the differences,” Khan explained. “The brain is not ‘sexually dimorphic’ like reproductive organs. The brains of males and females are more similar than they are different.”

The study did not investigate whether the observed structural differences translate into behavioral or cognitive differences, nor did it explore the precise causes — whether genetic, hormonal, or environmental — of these early brain differences.

“There is still much more to uncover,” Khan said. “We now need to determine whether these structural differences are linked to behavior, cognition, or future developmental outcomes. Understanding the origins and implications of these differences is the next critical step.”

This research marks one of the most detailed investigations into sex differences in the neonatal brain, offering a foundational understanding of how male and female brains begin to diverge—if only slightly—from the very first days of life. It also adds to the growing body of evidence that biological sex plays a role in brain development from the earliest stages, long before social and cultural influences take hold.