Public Release: 

Study: More neuroscience research articles are reporting the sex of laboratory animals

North Carolina State University

In the largest-ever survey of neuroscience research, scientists from North Carolina State University found that the number of research studies reporting the sex of lab animals increased significantly in the current decade, though sex bias remains present. The findings demonstrate progress in addressing both males and females in neuroscience research but also highlight the continued need for improved research design.

Historically, neuroscience research has not reported the sex of the laboratory rats and mice used in many experiments, a phenomenon known as sex omission. Neuroscience research has also historically exhibited sex bias, meaning male animals are used more often than females. Recent recognition of this experimental pattern generated fierce debate regarding the impact upon experimental replication and findings that may differ by sex.

John Meitzen, assistant professor of biological sciences at NC State, set out to determine whether this recognition within the neuroscience community had affected sex omission and bias in neuroscience literature. Meitzen and his 11-member research team reviewed over 10,000 research articles published between 2010-2014 in six journals known for publishing top-tier neuroscience research. Of the articles surveyed, 6,636 articles using mice or rats were further evaluated.

The team found that the number of manuscripts reporting the sex of mice and rats increased by about 30 percent over this four-year period. Despite this increase, sex omission remained in about 20 percent of all analyzed articles. They also found that while the number of articles using both males and females increased, overall sex bias toward male animals persisted, particularly in rats. The results varied by both animal species and academic journal.

"Omitting sex data can affect experimental results and interpretation," Meitzen says. "You can't be sure that you're replicating an experiment if you don't know what sex the animals were in the initial research.

"In neuroscience particularly, using both sexes can have a significant impact on findings, depending upon the questions being posed. You cannot assume that what is true in one sex is true in the other. The increases in both reporting of sex data and the use of both male and female animals is encouraging. More can be done to motivate the consideration of both males and females, and empower neuroscientists to report the sex of laboratory animals"


The research appears in eNeuro. Meitzen is corresponding author. NC State undergraduate students Tyler Will, Lindsay Kunz, Kelly Thompson, Laura Ginnari, Clay Jones, Elizabeth Reavis; graduate students Stephanie Proaño and Anly Thomas; and research technician David Dorris evaluated the articles.

Note to editors: An abstract follows.

"Problems and progress regarding sex bias and omission in neuroscience research"

DOI: 10.1523/ENEURO.0278-17.2017

Authors: Tyler R. Will, Stephanie B. Proaño, Anly M. Thomas, Lindsey M. Kunz, Kelly C. Thompson, Laura A. Ginnari, Clay H. Jones, Sarah-Catherine Lucas, Elizabeth M. Reavis, David M. Dorris, John Meitzen, North Carolina State University

Published: Online in eNeuro


Neuroscience research has historically ignored female animals. This neglect comes in two general forms. The first is sex bias, defined as favoring one sex over another; in this case, male over female. The second is sex omission, which is the lack of reporting sex. The recognition of this phenomenon has generated fierce debate across the sciences. Here we test whether sex bias and omission are still present in the neuroscience literature, whether studies employing both males and females neglect sex as an experimental variable, and whether sex bias and omission differs between animal models and journals. To accomplish this we analyzed the largest ever number of neuroscience articles for sex bias and omission: 6,636 articles using mice or rats in 6 journals published 2010-2014. Sex omission is declining, as increasing numbers of articles report sex. Sex bias remains present, as increasing numbers of articles report the sole use of males. Articles using both males and females are also increasing, but few report assessing sex as an experimental variable. Sex bias and omission varies substantially by animal model and journal. These findings are essential for understanding the complex status of sex bias and omission in neuroscience research and may inform effective decisions regarding policy action.

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