Summary: The human male and female brains share far more similarities than popular media often suggests, writes Julia Gottwald.
Source: University of Cambridge
If you were a fruit fly and smelled male pheromones, your response would be direct and consistent: a female would initiate courtship, while a male would respond with aggression. In fruit flies, specific pheromones activate different clusters of neurons depending on sex. Researchers even identified a single master gene, fruitless, that controls the male courtship ritual — disabling it prevents males from mating, while activating it in females can induce male-typical courtship behaviours.
Humans are not fruit flies. Studying sex differences in the human brain is far more complex, controversial, and emotionally charged than in other species, and media coverage often oversimplifies or misrepresents scientific findings. Sensational headlines can promote stereotypes and misunderstandings; for example, studies on worms or flies are sometimes exaggerated to explain human behaviour. The reality is subtler: male and female human brains have many common features, and the differences that do exist are often smaller and more variable than many reports imply.
The Royal Society recently dedicated a special issue to sex differences in the brain, including an opinion piece arguing that human brains do not split neatly into two distinct categories. That assessment draws on a large study of roughly 1,400 human brains that analyzed regional volumes and connectivity patterns to identify areas that differed most between sexes. For each region, the study labeled the extremes of the distribution as more typical of one sex or the other. If brains came in two distinct sex-specific types, individuals would consistently show all male-typical or all female-typical features. Instead, the researchers found that such consistently sex-typed brains are rare. Most people exhibit a mosaic of features: some regions are more common in males, some in females, and many are typical in both sexes.
Biology alone cannot explain this patchwork. Environmental and life-course factors strongly shape neuroanatomy: prenatal stress, early social relationships, exercise, nutrition, and education all influence brain development. Experiences across the lifespan continue to modify neural structure and connectivity. Gender — the set of social expectations, roles, and norms associated with being male or female — further interacts with biology to influence behaviour and brain organization. In short, genes and environment together shape brain structure.
Average neuroanatomical differences between men and women do appear in some studies, but structural distinctions do not automatically translate to clear behavioural differences. Complex behaviours such as mating, spatial navigation, or writing an essay are governed by large interacting neural networks and are modulated by internal states like stress, hunger, and fatigue. A modest anatomical difference in one region rarely dictates a single outcome; behaviour emerges from many interacting pathways.
Some researchers propose that brain differences between sexes may act to reduce behavioural divergence rather than amplify it. Geert de Vries, director of the Neuroscience Institute at Georgia State University, has suggested that male and female brains might develop distinct neural architectures to compensate for substantial physiological and hormonal differences, promoting similar behavioural outcomes across sexes. This compensatory idea is plausible but not yet confirmed for many structural findings.
Compensatory mechanisms are familiar in biology. For example, female mammals carry two X chromosomes while males have one. To avoid a double dose of X-linked gene expression in females, one X chromosome is largely silenced through X-inactivation. Analogous balancing processes could conceivably operate at the level of brain networks, producing different anatomical routes that support similar functions in men and women.
Why then study sex differences in the brain? One practical reason is that much biomedical and neuroscience research historically over-relied on male subjects. Reviews show many more all-male than all-female animal studies, and until recent policy changes, female hormonal cycles were often treated as confounds to be avoided. But male-only results do not always generalize to females. Pharmacokinetics and pharmacodynamics can differ by sex: some drugs are absorbed, distributed, metabolized, or excreted differently in men and women. Sex also matters for disease prevalence and presentation: autoimmune conditions such as multiple sclerosis, as well as depression and eating disorders, are more common in women, while autism spectrum conditions and some substance use disorders are more common in males. Ignoring sex as a biological variable risks missing important differences in risk, diagnosis, and treatment.
Recognizing this, policy changes have required the inclusion of women in clinical trials and, more recently, mandated consideration of sex in animal research funded by major agencies. Journals increasingly require authors to report the sex composition of their samples. These steps are necessary to build a more complete and accurate picture of how sex and gender shape brain development and health.
Findings about sex and the brain require careful analysis and nuanced communication. Men and women may differ in subtle, statistically detectable ways, but their similarities are generally greater than their differences. Small anatomical variations rarely produce wholesale changes in behaviour — human brains develop and operate within complex biological and social contexts. Unlike fruit flies, people live lives shaped by both genes and experience.
Julia Gottwald is a third-year PhD student in the Department of Psychiatry.
Source: Julia Gottwald — University of Cambridge
Image:
Joel, D., & Fausto-Sterling, A. (2016). Beyond sex differences: new approaches for thinking about variation in brain structure and function. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1688).
Joel, D., et al. (2015). Sex beyond the genitalia: The human brain mosaic. Proceedings of the National Academy of Sciences of the United States of America, 112, 15468–15473.
De Vries, G. J., & Boyle, P. A. (1998). Double duty for sex differences in the brain. Behavioural Brain Research, 92, 205–213.
Beery, A. K., & Zucker, I. (2011). Sex bias in neuroscience and biomedical research. Neuroscience and Biobehavioral Reviews, 35, 565–572.