New research from scholars at Northwestern University in Evanston, Illinois, has found that although the majority of studies funded by the National Institutes of Health (NIH) do consider sex as a biological variable, fewer than half of those projects analyze or report their results by sex.
The team at Northwestern reviewed 574 NIH-funded studies between 2017 and 2024, finding 61 percent did include both sexes in their research. Some 83 percent of the projects that included both sexes reported sample sizes by sex, but only 44 percent conducted sex-based analyses. Of the single-sex studies, only 34 percent focused on sex-specific topics. Studies with women as both first and last authors were more than twice as likely to conduct sex-based analyses.
“Just including women is not enough,” said corresponding author Nicole Woitowich, executive director of the Northwestern University Clinical and Translational Sciences Institute and a research associate professor of medical social sciences at Northwestern University Feinberg School of Medicine. “When scientists simply check a box to say they included women, but fail to analyze the data by sex, we lose the ability to understand whether or how treatments affect men and women differently. That limits our capacity to advance precision medicine and improve care for everyone.”
Notably, studies that include both sexes were much more likely to have human samples, rather than animal samples, which often rely on only one sex.
“When drugs are developed and tested in animal models using only one sex, they can still move forward into clinical trials involving both men and women,” said Dr. Woitowich. “Without understanding sex-specific effects early on, we increase the risk of adverse outcomes once testing moves into humans.”