When I began studying the neural circuitry underlying spatial navigation in the fall of 2019, I expected that my experimental trajectory would be fairly straightforward: read up on what others had done before me, find the gaps, and try to fill them. But as I started my research, it became clear that there was one gap that was much bigger than I had anticipated. In seeking a research focus, a new question had appeared: Where are the women?

The past decade has seen gains in the number of female subjects included in biomedical research trials. According to a bibliometric analysis that came out last year, 49 percent of mammalian biological studies published in 2019 reported testing on both males and females, up from only 28 percent in 2009. That’s a hopeful statistic. But a third of all research studies that included both male and female subjects failed to report their results by sex. When it comes to quantifying sample size by sex, neuroscience is among the worst offenders.

A large proportion of neuroscience research is conducted in rodents, and most of those rodents are male. Despite the National Institutes of Health’s 2016 Sex as a Biological Variable Policy, which requires proposals to provide fulsome justification for the exclusion of females from experimental designs, the 2020 analysis shows that roughly 30 percent of neuroscience studies still use exclusively male subjects. In fact, while you might expect that the NIH’s policy would have narrowed the scientific sex and gender gap, in some fields the opposite is true. In neuropharmacology, for instance, the proportion of female subjects in research trials has decreased by 5 percent since 2009.

What is the source of this disparity? Despite a complex history of inequality in science, this problem can be boiled down to one tenacious misconception: females are the more variable sex. The root of this misconception lies in the dynamic estrogen fluctuations people with ovaries experience over the course of their four-week menstrual cycle. The analogous cycle in rodents lasts only four to five days, and is known as the estrous cycle. 

Despite a complex history of inequality in science, this problem can be boiled down to one tenacious misconception: females are the more variable sex.

The term “estrous” itself reflects the idea that the hormonal cycle is a source of inherent instability in the female sex. “Estrous” comes from the Latin oestrus, meaning “frenzy” or “madness.” Zoologists first used this term in 1900, and more than a century later the idea of “estrous frenzy” is still pervasive across scientific disciplines. In a 2021 survey of scientists’ opinions of the Sex as a Biological Variable Policy, a significant number of scientists cited heightened variability from the estrous cycle as a concern when considering adding female animals to studies that traditionally included all-male subjects.

Such concerns highlight the notion that the variability inherent in naturally cycling female animals makes it harder for experimental data to achieve statistical significance. Practices such as ovariectomization, or surgical removal of the ovaries, reflect widespread efforts to subvert the effects of neuromodulators such as estrogen and progesterone and make female animals more like males. But meticulous evaluations of variation within rodents have concluded that females are no more variable than males. On the contrary, male rodents had a significantly higher degree of variability in every criterion surveyed, including metabolism, morphology, gene expression, and cardiac measures. Brain-specific factors such as neurochemistry and the electrical activity of neurons were also significantly more variable among male rodents than females.

Unfortunately, transitioning to a scientific world where all studies include both sexes is more difficult than it seems. To accurately assess female subjects, we must rethink how we conduct experiments, especially those in the discipline of neuroscience. Traditional criteria for experimental success and failure are based on male brains and do not accurately reflect the contributions of sex steroid hormones. For instance, it is considered a well-known fact that male animals learn navigation-based tasks more quickly than females. However, we now have reason to believe that males simply use a different strategy to navigate, relying more on nearby landmarks than on their own position in space. Because of this, when moving from point A to point B, male rodents tend to reach their destinations faster, but females acquire more information about their environment along the way. 

Despite these findings, most experimental designs remain engineered toward male physiology. The effects of this disparity are not immaterial. Women are more likely to be misdiagnosed with a host of neurological conditions, ranging from autism spectrum disorder to stroke. The generalization of male science onto female bodies is a detriment to both scientific research and public health, yet the path to an equitable scientific future remains poorly defined. In a post-pandemic world where taxpayer-funded scientists are facing greater public scrutiny, it is more essential than ever that the answers to scientific questions are accessible for everyone and applicable to all. 

Nora Wolcott is a PhD student in the Molecular, Cellular, and Developmental Biology department at the University of California, Santa Barbara. Reach out to her on Twitter @NoraWolcott.