How do sex chromosomes influence brain development?

Sex chromosomes (XX in females, XY in males) carry thousands of genes expressed differently in brain tissue; they interact with sex steroids to modulate gene expression and epigenetic programming, contributing to sexually differentiated neural phenotypes.

What role do prenatal sex hormones play in sexual brain differentiation?

Prenatal sex steroids shape brain circuits: fetal testosterone (from around week eight and high in early male infancy) masculinizes circuits, while early estrogen exposure in girls promotes female circuit development, producing epigenetic effects that influence later differences.

Which brain structures show sex-linked differences?

Sex-linked differences include volumetric and white-matter variations: boys often show more gray matter in the posterior lateral orbitofrontal cortex and larger amygdala and hypothalamus, while girls tend to have larger right temporo-parietal junction and related temporal regions.

How do sex differences affect neurodevelopmental and psychiatric disorders?

Neurodevelopmental conditions such as ASD and ADHD are diagnosed more frequently in boys (about fourfold), whereas disorders like anorexia, depression, and anxiety are more common in girls; differing biological factors and maturation timing partly explain these patterns.

What does research say about brain dimorphism versus polymorphism?

Large-scale research indicates brains are predominantly mosaics rather than strictly male or female: only about 0.8% of analyzed brains matched entirely male or female patterns, supporting a polymorphic model of sexual brain characteristics.

When do male and female brains mature differently?

Female brain maturation typically occurs about one to three years earlier than male maturation, especially in frontal, parietal, and temporal lobes, which influences developmental timing and behavioral trajectories.

The Sexual Brain: Neuroscience and Sex Differences

Thanks to biology, the physical appearance of men and women is different.

Recent research in neuroscience has revealed that there are also differences in brain structures between men and women.

Genetically we are different

Women have the XX chromosome and men the XY chromosome, but this has not always been the case. From the moment of fertilization until approximately 8 weeks of life, all circuits are female, in both male and female embryos.

The male embryo

Starting from the eighth week of pregnancy, its testes begin to produce testosterone, which is released to the brain and transforms those female circuits into male circuits.

At birth, the male brain during the first 6 months of life releases surprising amounts of testosterone.

The female embryo

By contrast, the girls up to two years of age also produce high amounts of estrogen in the ovaries that are released to the brain.

Because of this, some brain functions are substantially different from childhood.

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Key biological factors of the sexual brain

Sex hormones

Steroid hormones influence gene expression and other cellular processes. Thus, they are crucial in terms of the epigenetic effect that occurs in the brain when it is in full development.

Moreover, such early interaction with sex steroids from the embryonic period vitally influences the expression of later sexual differences in the nervous system.

Sex chromosomes

The sex chromosomes are another key biological factor of the sexual brain. They are XX in women and XY in men. However, both the X gene and the Y gene contain around 6,500 genes that are expressed differently in the brain of men and women, in at least one tissue.

Research highlights the interrelationship between sex hormones and sex chromosomes, acting together to give the phenotype of a sexually diverse brain.

Neuroscience and sex-linked differences

The neuroscience has studied the difference that exists in the brain volume in children, as well as in adolescents and adults.

The brain’s white matter shows specific differences in each cerebral lobe in the brains of men and women.

Recalling the fetal testosterone we mentioned earlier, it can also predict those volumetric differences observed in male and female brains.

The posterior lateral orbitofrontal cortex has more gray matter in boys than in girls.

The right temporo-parietal junction–posterior superior temporal sulcus, and the temporal plane/parietal operculum are larger in girls than in boys, resulting in an inverse effect of fetal testosterone.

The amygdala and hypothalamus are also sexually dimorphic, being larger in boys than in girls.

Neuroeducation and the sexual brain

In the field of neuroeducation, significant differences between the male and female brain have also been demonstrated.

Educational disorders such as autism spectrum disorder (ASD), the attention-deficit hyperactivity disorder (ADHD), Tourette syndrome and language impairments, among others, have a higher prevalence among boys. In fact, diagnosed cases of ASD and ADHD are four times those diagnosed in girls.

On the other hand, disorders such as anorexia, depression, or anxiety, among others, are more frequent in girls.

To understand this differentiation it is essential to understand the concept of temporality, since male and female brains mature at different speeds. Maturation is earlier in the female brain, occurring 1 to 3 years before the male brain, specifically in the areas of the frontal, parietal and temporal lobes.

Sexual brain… polymorphic?

A recent study, carried out by D.Johel , proposes the hypothesis that there is no dichotomy, male and female brain, there is no dimorphism but rather a polymorphism.

After analyzing 1400 brains, only 0.8% were completely female brain or male brain. The vast majority, on the contrary, were a rich mixture of diversity, which extrapolated to a single human being means that their brain will have a “mixture” of female and male areas, regardless of their sex.

In short, we must continue studying and delving deeper into this field that generates so much controversy and debate, in order to shed more light on what truly is the crux of the matter; the complexity and beauty of the human brain.

References:

Joel, Daphna. y Vikhanski, Luba. Mosaico de Género. Editorial Kairos, Barcelona, 2020.
Larry Cahill, «Dimorfismo sexual cerebral» en Investigación y ciencia, julio de 2005, n.º 346.