Over the past several decades, scientists focused their attention exclusively on one half of the population: They experimented with only male models in biomedical research because they were afraid that including female models would introduce unnecessary variability due to their hormonal cycles.
As researchers debunked this myth, many scientists called for the equal representation of sexes in biomedicine. Trace how the research community shifted toward more sex-inclusive work, and in doing so unearthed fundamental differences between how biological sex shapes brain function, drug responses, and other clinical outcomes.
Introducing Sex as a Biological Variable Policy to Counter Bias
Historically, biomedical scientists overlooked female subjects, assuming that their inclusion would unintentionally introduce variability. But in the 1990s, it became clear that female animals were not more variable than male ones, leading scientists to understand the treasure trove of information they missed out on. This led the National Institutes of Health to release the Sex as a Biological Variable policy in 2016, which mandated that sex be factored into research designs, analyses, and reporting in human and animal studies. Despite this mandate, scientists continue to question whether it truly alleviated sex bias in biomedical research, and efforts to answer this question remain ongoing.
How do Male and Female Brains Differ at the Molecular Level?
Catherine Woolley, a neuroendocrinologist at Northwestern University, studies sex differences at the level of molecular mechanisms in male and female brains. Specifically, Woolley and her team found that estradiol, a type of estrogen, influenced synaptic transmission in the hippocampus of both male and female brains. The team further found that estrogen receptors in both sexes acted differently, highlighting the need to include both male and female models to develop better-targeted drugs.
Studying the Female Brain
When scientists focused their experiments on male models, they did so even while studying conditions such as post-traumatic stress disorder and depression, which are more common in women. A limited understanding of the female brain prompted Rebecca Shansky, a neuroscientist at Northeastern University, to study both male and female animals. Through her work, she uncovered how stress and fear change brain structure and function in both sexes, highlighting the extent of information researchers miss out on by studying only one sex.
Anesthesia Recovery is Different in Female and Male Brains
The distribution of microglia (green) across the different cortical layers (orange) in the mouse visual cortex differs in female and male brains when recovering from ketamine anesthesia.
© Siegert group / ISTA
When researchers were studying the effects of ketamine, a general anesthetic, they discovered that female and male brains respond differently to it. Sandra Siegert, a neuroscientist at the Institute of Science and Technology Austria, and others in the field had also observed that ketamine affects the brain’s microglia, whose function is known to differ between the sexes. By tracing microglia in animals recovering from ketamine, Siegert and her team found that the drug triggered significantly more microglia-neuron connections in female brains compared to male ones. Digging deeper, they found that ketamine spiked a stress hormone only in female mice, which led to higher microglial activity.
Gene Expression Sets Female and Male Brains Apart
Doctors observe a clear bias in some brain conditions: Women are more prone to mood disorders, while men have a higher incidence of attention-deficit/hyperactivity disorder. To better understand whether sex-dependent genetic makeup influences these outcomes, Alex DeCasien, a neurogeneticist at the National Institute on Aging, and her colleagues analyzed gene expression in nearly 170 brain samples from 15 men and 15 women. They observed a link between sex and gene expression changes of more than 3,000 genes that could shape disease. While more work must be done to tease apart the effect of genetics and different socialization of genders, the results emphasize the importance of studying both female and male brains.
Brain Proteins Differ Based on Biological Sex
Further offering insights into why some brain conditions exhibit sex bias, Emory University neurologist Thomas Wingo and his team assessed the proteomes of more than 1,200 donated post-mortem brain samples. Their results indicated that more than 30 proteins had sex-biased expression. While Wingo acknowledged that environmental factors could also influence brain protein differences, he added that these results could help explain why certain conditions are more common in men and others in women. This could help develop better therapeutic interventions.
Why Women Take Longer to Resolve Pain
Women often report more long-lasting pain than men, an observation corroborated by evidence from animal studies. Geoffroy Laumet, a neuroimmunologist at Michigan State University, and his colleagues investigated inflammatory cytokines in mice. They observed that male mice showed increased activity of anti-inflammatory signaling compared to their female counterparts, likely driven by sex hormones. So, inflammation and pain resolved in male mice sooner than they did in female ones, insights that could someday help women receive more appropriate pain-relieving treatment.
