How Studying Sex as a Biological Variable Is Revolutionizing Science and Medicine
8 min read
Imagine a world where medications are prescribed based not just on your diagnosis, but on your fundamental biological characteristics—including your sex. This isn't science fiction; it's the cutting edge of modern biomedical research. For decades, scientific studies predominantly used male subjects, assuming that findings would apply equally to females. But from brain aging to drug metabolism, biological sex influences fundamental physiological processes in ways we're only beginning to understand 2 3 .
Women have experienced adverse drug reactions at higher rates than men, often because medications were predominantly tested on male cells, male animals, and male human subjects 6 .
The story of the sleep drug zolpidem (Ambien) exemplifies this problem—years after its approval, researchers discovered that women metabolize the drug more slowly than men, leading to dangerous morning drowsiness and thousands of impaired driving incidents 6 .
Today, a scientific revolution is underway as researchers increasingly recognize sex as a critical biological variable. This shift promises more precise treatments and deeper understanding of health and disease for everyone. Let's explore how this transformation is happening, what we've learned, and what challenges remain.
In scientific contexts, biological sex refers to the classification of organisms as male or female based on their reproductive organs, functions, and chromosomal complement. Gender, by contrast, encompasses an individual's self-representation and how social institutions respond to that presentation 4 5 . While this article focuses primarily on biological sex, researchers recognize that both factors can influence health outcomes.
Biological sex differences originate from genetic and physiological factors that interact with environmental influences across the lifespan. As noted by the National Academies of Science, "Being male or female is an important fundamental variable that should be considered when designing and analyzing basic and clinical research" 4 .
Sex differences arise from multiple biological mechanisms:
Every cell in the body has a biological sex that influences its vulnerability or resilience to aging and disease 3 . The XX and XY chromosomal complements create differences in gene dosage and regulation.
Estrogens, androgens, and progestins are present in substantially different levels in males and females, influencing everything from brain development to metabolic function 5 .
Experiences and exposures throughout life interact with biological factors to create health outcomes 4 .
These mechanisms are not mutually exclusive but interact in complex ways throughout development and aging.
The scientific community's approach to studying sex has evolved significantly, driven largely by policy changes. In 1993, the NIH Revitalization Act mandated the inclusion of women in clinical trials, though it didn't require analysis of data by sex 3 . More recently, in 2014, the National Institutes of Health announced it would "require applicants to consider sex as a biological variable (SABV) in the design and analysis of NIH-funded research involving animals and cells" 5 .
NIH Revitalization Act mandates inclusion of women in clinical trials 3
NIH requires applicants to consider sex as a biological variable in preclinical research 5
49% of studies report using both male and female research subjects, up from 28% in 2009 8
These policy changes have had a measurable impact. A 10-year follow-up study examining sex inclusion across nine biological disciplines found that in 2019, 49% of studies reported using both male and female research subjects, a significant increase from 28% in 2009 8 . The most dramatic improvements occurred in neuroscience (29% to 63%) and immunology (16% to 46%).
| Discipline | 2009 (% studies including both sexes) | 2019 (% studies including both sexes) | Change |
|---|---|---|---|
| Neuroscience | 29% | 63% | +34% |
| Immunology | 16% | 46% | +30% |
| Endocrinology | 30% | 56% | +26% |
| Physiology | 13% | 36% | +23% |
| Pharmacology | 33% | 29% | -4% |
| Reproduction | 10% | 14% | +4% |
| Behavior | 70% | 81% | +11% |
Despite this progress, important gaps remain. The same study found that only 42% of studies that included both sexes actually analyzed their data by sex, and 27% failed to even describe their sample sizes by sex 8 . This indicates that while researchers are increasingly including both sexes, they're not always extracting the full scientific value from doing so.
One of the most fascinating areas of sex differences research involves brain aging and neurodegeneration. Groundbreaking research has revealed that biological sex significantly influences how brains age, with women showing remarkable resilience in certain cognitive domains 3 .
A key study from the Mayo Clinic Study of Aging followed over 1,200 individuals and found that females showed approximately 10-25% better cognition in aging compared to men 50 years and older 3 . This female advantage was particularly evident in carefully phenotyped studies that excluded dementia.
To understand what might explain these differences, researchers have employed multiple approaches:
These findings may provide mechanistic insights into observations that in many populations, women show resilience to cognitive deficits, often with better baseline memory and verbal fluency than men in typical aging 3 .
| Metric | Female Advantage | Sample Size | Significance |
|---|---|---|---|
| Metabolic brain age | ~4 years younger | 200+ individuals | Persistent youth pattern |
| Epigenetic aging | ~1 year slower | 2,000+ brain samples | Slower biological aging |
| Structural brain age | ~7 years younger | 2,000+ individuals | Reduced age-related atrophy |
Remarkably, these patterns aren't unique to humans. Research in C. elegans—tiny worms with nervous systems that serve as model organisms for aging studies—has revealed parallel patterns of sex differences in cognitive aging 3 .
In a fascinating study, aging XX hermaphrodite worms showed higher indices of learning and memory, neural resilience to age-induced morphological demise, and younger metabolic gene expression compared to aging XO males 3 . Furthermore, despite mechanisms of dosage compensation that equalize the X chromosome dose between the sexes, aging increased expression of the X chromosome preferentially in neurons of XX hermaphrodites.
Since the X chromosome is enriched for neural genes in worms, increased X chromosome expression in XX animals might contribute to their neural resilience during aging 3 . This finding is particularly significant because it suggests that evolutionarily conserved mechanisms may underlie sex differences in brain aging across species.
Researchers investigating sex differences employ specialized methods and reagents to ensure rigorous, reproducible science. Here are some key tools:
| Tool/Reagent | Function | Application Example |
|---|---|---|
| Four Core Genotypes mouse model | Separates chromosomal from hormonal sex effects | Studying sex chromosome contributions to phenotypes |
| Hormonal assays (ELISA, Mass Spec) | Measures circulating hormone levels | Correlating hormonal status with physiological measures |
| Estrous cycle tracking methods | Determines phase of female reproductive cycle | Controlling for cyclic hormonal variations |
| Sex-specific cell lines | Allows in vitro study of sex differences | Examining cellular mechanisms without whole-body systems |
| CRISPR/Cas9 sex chromosome editing | Manipulates sex chromosome genes | Testing specific genetic contributions to sex differences |
Despite growing recognition of the importance of studying sex differences, researchers still face challenges and misconceptions.
A persistent myth suggests that female subjects increase experimental variability due to their hormonal cycles. However, multiple studies have debunked this notion. In fact, a comprehensive review found that data from females were no more variable than data from males across hundreds of traits 9 . In some cases, female data showed even less variability than male data.
Researchers have developed practical strategies for incorporating sex as a biological variable:
The default option should always be to include both sexes in exploratory studies with a 1:1 ratio 9 .
Studies should be powered to detect sex differences when they exist, which doesn't necessarily require larger sample sizes if properly planned 9 .
When relevant, researchers should track and account for hormonal status in both males and females, as male hormones can also fluctuate 9 .
While significant progress has been made in recognizing sex as a biological variable, important challenges remain:
The most pressing issue is the gap between inclusion and analysis. As noted in the 10-year follow-up study, "improvements in the inclusion of both sexes over the past decade have not been accompanied by general improvement in sex-based analyses" 8 . Simply including both sexes isn't enough—researchers must analyze their data by sex to uncover potential differences.
Most research to date has treated sex as a binary variable, but biological reality is more complex. Future research needs to account for intersex variations and understand how sex interacts with other variables like age, ethnicity, and environmental factors 1 .
Addressing these complex questions will require interdisciplinary collaboration across fields as diverse as genetics, endocrinology, neuroscience, and social science 1 . As our understanding evolves, we're moving beyond binary conceptualizations of sex and gender to appreciate their dynamic, intersecting nature 1 .
Ultimately, considering sex as a biological variable will be crucial for realizing the promise of precision medicine. As one researcher noted, "the previously held notion of one patient one treatment is no longer realistic, and new therapeutics must be designed for a heterogeneous patient population" 9 .
The journey to fully incorporate sex as a biological variable in research is far from complete, but the progress made in recent years is remarkable. From policy changes that mandate consideration of sex to groundbreaking discoveries about sex differences in brain aging, the scientific community is increasingly recognizing that one size does not fit all when it comes to biology and medicine.
As we continue to unravel the complex interplay between sex, gender, health, and disease, we move closer to a future where medical treatments are tailored to individual characteristics—including biological sex—for better outcomes for everyone.
The scientific revolution sparked by considering sex as a biological variable isn't just about making science more accurate; it's about making medicine more effective and equitable for all people.
As consumers of healthcare and beneficiaries of scientific research, we all have a stake in ensuring that this transformation continues. The future of medicine depends on it—for women, for men, and for all of us.