Fewer than half of NIH-funded studies break down findings by sex

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Fewer than half of NIH-funded studies break down findings by sex

Lumping male, female data together creates a false impression that results apply equally to everyone

• May obscure critical differences affecting diagnosis, treatment, health outcomes for millions
• Studies led by women are more than twice as likely to analyze data by sex
• Significant economic implications in drug development if data is not analyzed by sex
• 'Just including women is not enough'

CHICAGO --- A decade ago, the National Institutes of Health (NIH) began requiring scientists to "consider sex as a biological variable" in order to receive NIH grant funding - a policy intended to encourage scientists to think ahead about whether being biologically male or female could influence the results of their research.

Today, 61% of NIH-funded studies do consider sex as a biological variable, reports a new Northwestern Medicine study, which is up from 49% in 2019. Less than half (44%) of those studies, however, analyze or report their results by sex, even when both sexes were included, the new study found. As a result, scientists may be missing important sex differences that could affect diagnosis, treatment, dosing and health outcomes for millions of people.

"Just including women is not enough," said corresponding author Nicole Woitowich, executive director of the Northwestern University Clinical and Translational Sciences Institute (NUCATS) 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."

The study was published April 27 in Nature Communications Medicine.

When data from males and females are lumped together, important sex-specific effects can disappear, creating the false impression that the results apply equally to everyone.

"It takes larger studies to conduct robust sex-specific analyses, but making the effort means that we have a better shot at drug development - and ultimately patient care - that benefits everyone," said study author Leah Welty, director of the Biostatistics Collaboration Center at Northwestern and a professor of preventive medicine (biostatistics and informatics) and psychiatry and behavioral sciences at Feinberg.

More study findings

The study also found human studies are much more likely to include both sexes and analyze data by sex than animal studies, which may be slowing scientific progress. Many animal studies still rely on only one sex (often males), limiting the rigor, reproducibility and real-world relevance of the science that ultimately informs human medicine, Woitowich said.

"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," Woitowich said. "Without understanding sex-specific effects early on, we increase the risk of adverse outcomes once testing moves into humans."

This also has significant economic implications, she added. On average, it takes more than 10 years and millions of dollars to bring a new drug to market.

"A drug may work better in one sex than the other, but that insight can be lost early during development if data aren't analyzed by sex," Woitowich said. "We miss opportunities to improve medicines and ultimately, we waste both time and money."

Lastly, studies with women as both first and last authors were more than twice as likely to conduct sex-based analyses. That finding underscores how the composition of scientific teams can influence research practices, with direct consequences for public health.

More about the study

The study analyzed 574 NIH-funded studies between 2017 and 2024. Using publicly accessible data, the team identified R01 grants - the NIH's most common funding mechanism for independent research projects - and examined one publication per grant. They assessed whether the study included both sexes, whether results were analyzed by sex and the gender composition of the study authors.

Per the NIH policy, scientists don't have to include both males and females in their research, but they do have to consider whether sex is relevant and account for it in their study design and reporting, Woitowich said. For instance, a prostate cancer study would appropriately only include men while an ovarian cancer study would include only women.

Among studies that included both sexes, only 83% reported the number of male and female subjects in their sample sizes, which Woitowich said hinders another scientist's ability to repeat a study or gain meaningful results.

"The NIH emphasizes rigor and reproducibility," Woitowich said. "But if we don't know basic information, such as the number of male and females in a study, is that research truly rigorous? Is it even possible to reproduce their work? I would argue no."

The study is titled, "Incorporation of the National Institute of Health (NIH) Sex as a Biological Variable Policy by R01 Grant Awardees."

Other Northwestern authors on the study include Joelle H. Warden and Mia Parangalan.