Gray matter and white matter, both essential components of the brain’s structure, play distinct roles in how the brain processes information and governs various functions. Gray matter is composed primarily of the cell bodies and dendrites of neurons. Dendrites are the branch-like structures that receive signals from other neurons, while the cell bodies house the nuclei and other organelles necessary for cell functions. This region is crucial for processing and interpreting data, including sensation, perception, learning, speech, and higher-order cognitive functions like thinking, reasoning, and decision-making.
On the other hand, white matter consists mainly of axons, the long, slender nerve fibers that connect different areas of the brain. Axons serve as highways for transmitting electrical signals between various regions of the brain and spinal cord, facilitating communication between neurons. These long fibers are covered in a fatty substance known as myelin, which helps to speed up the transmission of neural impulses, and the name “white matter” comes from the whitish appearance of this myelinated axonal coating.
Understanding the distribution and characteristics of gray and white matter across different individuals has been a subject of considerable scientific inquiry, particularly concerning how these aspects may differ between males and females. An intriguing aspect of recent research is how these differences manifest at birth, offering new insights into the early development of the brain. A major study led by Yumnah Khan, a Ph.D. student at the Autism Research Center at the University of Cambridge, examined these differences, bringing attention to the fact that even in infancy, male and female brains show distinct patterns of development.
The research team’s investigation provided evidence suggesting that sex differences in brain structure, specifically in the relative amounts of gray and white matter, were already evident in newborns. Historically, scientists had questioned whether such differences existed at birth, as this inquiry was often complicated by sample sizes that were not large enough to draw definitive conclusions. By analyzing a dataset from the Developing Human Connectome Project—a large-scale initiative that collects MRI brain scans of infants—they were able to explore these sex differences in greater depth. Importantly, the study included over 500 newborns, a sufficient sample size for detecting subtle brain differences between the sexes.
One of the key findings from this research was the distinction in the total brain volume between male and female infants. On average, male babies had larger brain volumes compared to females, even when differences in birth weight were considered. This larger brain size in males is consistent with broader trends seen in adult brain morphology, where males typically have slightly larger brains than females. However, this does not imply a direct correlation with cognitive function or intelligence, as the brain’s functional and structural components differ in complexity, which plays a critical role in determining an individual’s cognitive capabilities.
When these researchers adjusted for total brain size to examine the regional volumes of gray and white matter, distinct sex differences were found. Female infants, on average, had larger volumes of gray matter in areas linked to memory and emotional regulation, suggesting that these regions may develop differently across genders right from the onset of life. Gray matter plays a significant role in cognitive functions such as processing sensory input, memory storage, and emotional responses, and its larger volume in females could be a factor contributing to gendered differences in these abilities as the brain matures.
Meanwhile, male infants displayed a greater volume of white matter in regions related to sensory processing and motor control. These findings are in line with later developmental differences observed in childhood, where boys typically exhibit better performance in tasks related to spatial reasoning and motor coordination, whereas girls may show more aptitude for verbal and memory-related tasks. It is essential to recognize that such findings are general patterns and that within-group variation is substantial—there is considerable overlap between the sexes, meaning that not all males have more white matter or larger brain volumes, nor do all females have more gray matter in specific areas.
The implications of these findings are fascinating, as they suggest that sex differences in the brain, which become more apparent in adulthood, are already shaping up during the earliest stages of life. According to Khan and her colleagues, these differences in the brains of male and female infants may reflect biological sex distinctions that occur during prenatal brain development. The prenatal environment, influenced by genetic and hormonal factors, might significantly contribute to these observed variations in brain structure.
An additional concern that the study addressed was whether such differences might simply be an artifact of body size. Larger males might naturally have larger brains, which could explain some of the disparities between sexes. However, the researchers accounted for these variations by controlling for birth weight and other factors. They found that even after controlling for these variables, the sex differences in brain structure persisted. Therefore, they concluded that these differences were not merely due to overall body size but were specific to the brain’s development.
This work builds upon the premise that understanding brain differences is essential for appreciating the role biology plays in the development of cognitive functions. The research extends beyond just male and female differentiation and provides valuable information that can lead to deeper insights into neurodevelopmental conditions, especially those related to neurodiversity.
Dr. Alex Tsompanidis, the study supervisor, emphasized that future research aims to examine more closely the factors influencing these differences, particularly those during the prenatal period. This approach could involve exploring birth records and utilizing cellular models of developing brains to better understand the contribution of biological and hormonal factors. For instance, studying how testosterone and estrogen affect brain growth during pregnancy could further clarify the mechanism behind these early sex differences. By understanding the biological basis of brain development, it may become easier to unravel the mechanisms that underpin conditions such as autism, which shows a higher prevalence in males than females.
Despite the critical importance of these findings, the researchers stress that brain differences between males and females are average differences rather than defining traits of individuals. As Dr. Carrie Allison pointed out, these differences do not apply universally to every male or female and only emerge when comparing the sexes as large groups. As such, it is important to approach these differences without reinforcing harmful stereotypes or perceptions. Brain structure is only one facet of human complexity, and factors such as personality, emotional intelligence, and life experience also play pivotal roles in shaping an individual’s cognition and behavior.
Moreover, the findings encourage the understanding that the brain’s gender differences are simply a reflection of neurodiversity—a term that recognizes the diversity of human brains and cognitive patterns. The research could also pave the way for a better understanding of how the brain of children diagnosed with conditions like autism may differ from those without such diagnoses. Since autism is more commonly diagnosed in males, the observation of gray and white matter differences might offer valuable insights into how the brains of individuals on the autism spectrum diverge from the norm.
Reference: Yumnah T. Khan et al, Sex Differences in Human Brain Structure at Birth, Biology of Sex Differences (2024). DOI: 10.1186/s13293-024-00657-5