Adolescence appears to be a turning point for sex differences in the microstructure of deep grey matter, according to a new brain-imaging study led by Christopher G. Filippi of The Hospital for Sick Children and Richard Watts of the University of Canterbury. Using Restriction Spectrum Imaging, the researchers analysed data from 1,643 individuals aged 9 to 35 drawn from two large public databases, and found that sex differences in the tissue structure of emotion, memory and reward centres emerge during puberty, rather than in early childhood.
Deep grey matter comprises several brain structures beneath the outer layer of tissue, including the amygdala, which handles emotions, and the hippocampus, central to memory. The nucleus accumbens, thalamus, and the caudate and putamen also play roles in reward processing, sensory relay and movement. Although these regions are intimately linked to conditions that show sex differences in diagnosis rates, the biological roots of those differences have remained elusive.
Animal research has long pointed to puberty as a period of substantial neural change driven by sex hormones. In humans, earlier work suggested pubertal development relates to the size of deep brain regions, independent of age. The new study builds on that foundation by asking when microscopic tissue differences between the sexes first arise during development.
The team analysed brain scans from 1,643 people aged 9 to 35, combining data from the Human Connectome Project Development dataset for children and teens with the Young Adult dataset for older participants. In their analyses, they statistically controlled for age and body mass index, and accounted for family relationships within the many twins in the adult sample.
To assess tissue microstructure, the researchers used Restriction Spectrum Imaging, which detects how water diffuses through brain tissue. Restricted diffusion reflects cell packing and membrane barriers, while the hindered diffusion signal arises from the extracellular spaces between cells.
Among the youngest group, aged 9 to 12, there were no statistically significant differences between boys and girls in these diffusion measures. Across the deep grey matter structures, tissue microstructure in childhood appeared similar between the sexes.
Differences began to emerge between ages 13 and 16, first in the hippocampus, amygdala and thalamus. Restricted diffusion rose with age in both sexes, but from puberty onward it tended to be higher in males—an indication the researchers say could reflect more densely packed cells or larger cell bodies. By ages 17 to 21, sex explained roughly 13 to 33 per cent of the variation in restricted diffusion across the six regions, and in adults the figure rose to 32 to 37 per cent in the hippocampus and amygdala.
In contrast, females showed a larger hindered component, meaning a greater share of the water signal came from the spaces between cells. Since both measures are expressed as shares of the total signal, the two patterns largely mirror each other.
The study also found regional nuances. In the nucleus accumbens, caudate and putamen, females displayed a slowdown in development in their late teens before accelerating again in the early 20s, while males followed a steadier trajectory. By the mid-20s, the sex differences in the hippocampus and amygdala had levelled off, though the measures themselves continued to change in both groups. In the thalamus, the divergence between the sexes continued to widen up to age 35.
“Sex differences in the brain have so far been primarily investigated at the macro-structural level (i.e., differences in overall brain size or differences in the size of specific regions),” explained Yumnah Khan, a doctoral scientist at the Autism Research Centre at the University of Cambridge who was not involved in the research. “What is interesting about this work is it provides evidence for differences in brain microstructure (i.e., what is happening at a finer level within brain tissue), which gives us a more detailed picture of sex differences in the brain.”
Balducci, a senior researcher at the University of Turku who was not involved in the study, said the findings build on earlier work. “Earlier studies had identified sex differences in subcortical brain structure and, more recently, in diffusion-derived microstructure in young adults,” he told PsyPost. “This study extends that work by showing that the differences emerge during adolescence rather than simply being present in adulthood.”
The timing of these changes aligns with the pubertal transition, prompting the authors to speculate about the biological drivers. Animal work indicates that sex hormones can influence synapses and dendrites, but the authors caution that this study cannot establish causality in humans. They suggest that combining human imaging with animal research provides a fuller picture of the mechanisms behind sex differences in brain development.
“Animal studies can provide much stronger evidence about causality and specific mechanisms, while human studies are generally observational and correlational,” Khan noted. “However, when both of these lines of research are considered and interpreted together, we get a better picture of the mechanisms underlying sex differences in brain development.”
Balducci stressed the descriptive strength of the study while urging caution about drawing broader conclusions. “The main descriptive finding appears robust because the study uses a large combined sample, finds a consistent developmental pattern across several brain regions, and applies correction for multiple comparisons,” he said. “However, the reasons for these differences remain uncertain because the data are cross-sectional, and in this setting it is difficult to distinguish age-related changes from pubertal development or establish a hormonal cause.”
Both researchers also cautioned against over-interpreting the findings. Khan warned that the biggest risk is viewing the results as showing fundamental differences between male and female brains, noting that differences tend to be small with substantial overlap. “In reality, sex differences in the brain tend to be relatively small, with a lot of overlap between the sexes.”
She added that the timing of any differences is sensitive to the ages included in a study, emphasising that adolescence should not be presumed as the starting point for all sex-related brain changes. “Although the study found minimal differences early in childhood, sex-by-age effects can be highly sensitive to the particular age range that is being studied. As such, the findings do not necessarily mean that differences are entirely absent earlier in development.”
Balducci also cautioned that diffusion differences are not a direct readout of behaviour or mental health. “The findings do not show that differences in water diffusion directly cause differences in behavior or mental health,” he said. “Diffusion MRI provides an indirect measure of tissue microstructure.”
Looking ahead, the researchers advocate longitudinal work that follows the same individuals before, during and after puberty, with measurements of pubertal hormones and developmental stages. “The most important next step would be a longitudinal study that repeatedly scans the same individuals before, during, and after puberty, while also measuring pubertal hormones and developmental phases,” Balducci told PsyPost. “That would help determine whether the observed diffusion changes track individual pubertal development and whether they are related to later mental-health outcomes.”
Khan likewise highlighted the value of extending this approach to earlier development, including the prenatal period, where sex differences in the brain may begin to take shape. “It would be very interesting to see this approach being used in earlier stages of development, such as the prenatal period,” she said. “The prenatal period is another critical window for the emergence of sex differences in the brain. Moreover, male and female fetuses are exposed to different levels of sex hormones, which may contribute to differences in early brain development.”
The authors describe the sex-specific trajectories they observed as a potential reference point for future studies exploring how deviations from typical development relate to mental health outcomes. The study, titled Sex Differences in Human Deep Grey Matter Microstructure Emerge During Adolescence, is credited to Christopher G. Filippi, Jay N. Giedd, Aziz M. Uluğ, and Richard Watts.
