Fewer than half of NIH-funded studies analyze sex differences

Fewer than half of the studies funded by the US National Institutes of Health (NIH) examined sex differences, despite a 2016 policy requiring grantees to report on them, according to a study published in Nature Communications Medicine.

To assess implementation of the NIH sex as a biological variable (SABV) policy, researchers evaluated 574 NIH-funded studies published from 2017 to 2024 and found that, although most (61%) included both sexes, fewer than half (44%) analyzed results by sex, even when both sexes were included.

The SABV policy mandates that grantees take sex into account in designing, analyzing, and reporting studies.

“Sex is a fundamental biological variable that influences disease mechanisms, treatment responses, and health outcomes,” the researchers wrote. “Yet, gaps remain in how sex inclusion and sex-based analyses are implemented across different types of studies and by scientific discipline or medical specialty.”

The authors examined research supported by NIH R01 grants, the agency’s “flagship” mechanism for funding investigator-initiated research, and found that research teams led by women were more likely to analyze data by sex. The authors noted that “these findings are consistent with previous studies, which have found a strong positive correlation with women-led publications and sex inclusion and analysis, and in our own previous work exploring sex and gender analysis in publications regarding COVID-19. This suggests that gender dynamics within research teams may influence the prioritization of sex-based analyses, further underscoring the importance of diversity in scientific leadership.”

The study also revealed differences in sex-inclusion reporting across NIH research centers. For example, almost one-quarter of studies funded by the National Institute of Allergy and Infectious Diseases (NIAID) did not disclose the sex of their subjects. In contrast, studies funded by the National Institute on Aging (NIA) consistently reported the sex of their subjects and provided a breakdown of sample sizes by sex.

Studies using animal models were significantly less likely than human subject studies to include both sexes or to perform sex-based analyses, highlighting an area where the implementation of SABV remains limited.

The researchers noted that although the SABV policy aims to improve the “rigor and reproducibility” of biomedical research, funding agencies like the NIH should not be expected to closely oversee individual scientists' work. They emphasized that accountability and transparency in research practices rest with the broader scientific community.

Publishers, journal editors, and peer reviewers are important for upholding high research standards, the authors said. In addition, the peer-review process is vital for ensuring research quality; however, its success relies on reviewers' understanding of best practices in sex reporting and sex-based analyses.

Data for this project were obtained from the NIH RePORTER tool, a publicly accessible database that provides information on NIH-funded research projects. This includes details about awarded grants, project descriptions, and associated outcomes such as publications. The researchers specifically searched for R01 awards funded in FY2017 and FY2018 that reported project outcomes. The articles related to these projects were published in 403 different journals.

Study