Review
Sex bias in neuroscience and biomedical research

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Abstract

Female mammals have long been neglected in biomedical research. The NIH mandated enrollment of women in human clinical trials in 1993, but no similar initiatives exist to foster research on female animals. We reviewed sex bias in research on mammals in 10 biological fields for 2009 and their historical precedents. Male bias was evident in 8 disciplines and most prominent in neuroscience, with single-sex studies of male animals outnumbering those of females 5.5 to 1. In the past half-century, male bias in non-human studies has increased while declining in human studies. Studies of both sexes frequently fail to analyze results by sex. Underrepresentation of females in animal models of disease is also commonplace, and our understanding of female biology is compromised by these deficiencies. The majority of articles in several journals are conducted on rats and mice to the exclusion of other useful animal models. The belief that non-human female mammals are intrinsically more variable than males and too troublesome for routine inclusion in research protocols is without foundation. We recommend that when only one sex is studied, this should be indicated in article titles, and that funding agencies favor proposals that investigate both sexes and analyze data by sex.

Introduction

Women and non-human female mammals have been given short shrift in biomedical research, often on the assumption that results from males apply to females, or because of concern that hormonal cycles decrease the homogeneity of study populations and confound effects of experimental manipulations (Wizemann and Pardue, 2001). Some consider men representative of the human species and differences from the male norm as atypical or abnormal; others seek to protect women from adverse effects of drugs (Marts and Keitt, 2004), and generalize findings on males to females without justification.

Epidemiological and clinical studies of men often generate different results from those for women, exemplified by sex differences in response to many drugs (Soldin and Mattison, 2009), in cardiovascular disease (Barrett-Connor, 1997, Berger et al., 2009) and in autoimmune dysfunction (Fish, 2008, Lockshin, 2006, Whitacre, 2001). Widespread prevalence of sex differences in human disease and neglect of women in biological research negatively impacts the health of women (Correa-De-Araujo, 2006). Understanding of these consequences eventually led to the US National Institute of Health Revitalization Act of 1993, which required enrollment of female participants in federally supported phase III clinical trials. A European Union program was designed to provide basic researchers with practical tools and best practice examples regarding sex and gender differences in study design (Klinge, 2008). These interventions contributed to increased inclusion of women in clinical research in the USA, paralleled by similar changes in the European Union and Australia (Wetherington, 2007, Rogers and Ballantyne, 2008, Klinge, 2008, Soldin and Mattison, 2009), but most studies that enrolled both sexes did not provide sex-specific analysis (Hayes and Redberg, 2008). No funding agency stipulations presently require enrollment of females in non-human animal research, nor has there been movement to conduct basic science studies on both sexes (Sandberg and Ji, 2008).

A substantial portion of the NIH budget supports research on animal models, an enterprise that facilitated development of treatments to alleviate depredations of aging, autoimmune diseases, cancer, behavioral dysfunctions, cardiovascular disease, diabetes and other human afflictions (Council on Scientific Affairs, 1989). What remains in question is whether present-day research on animal models is sufficiently attentive to female subjects, and whether appropriate species are investigated. Previous analyses of a few journals and disciplines revealed a strong male sex bias (e.g., Berkley, 1992, Blanchard et al., 1995, Sechzer et al., 1994).

Section snippets

Literature survey

To gain a contemporary view of sex bias in the human and animal literatures, we systematically sampled journal articles from 2009 across a broad range of disciplines, including several that have not been canvassed for decades, and others never previously surveyed. We also tracked changes in sex bias and species utilization in five journals for six or more decades, for an historical perspective. Reports were categorized with respect to species studied, sex of subjects, whether both males and

Subject sex across fields: animal studies

The 2009 survey of research on non-human mammals revealed a male bias in 8 of the 10 fields surveyed, including neuroscience, physiology, pharmacology, endocrinology, zoology and, to a lesser extent, behavioral physiology and behavior (Fig. 1A). The ratio of articles reporting on only males versus only females was most skewed in neuroscience (5.5:1), pharmacology (5:1) and physiology (3.7:1). The weakest male biases were in behavior (1.4:1) and behavioral physiology (1.5:1). A female skew was

By sex: animal studies

79% of articles dealing with non-human mammals in the early 20th century failed to report subject sex in the Journal of Physiology (London) and the Journal of Pharmacology and Experimental Therapeutics (Fig. 4A). The percentage of articles reporting subject sex increased steadily through 1969, without substantially affecting the relative abundance of male and female single-sex studies. A marked increase in male-only reports after 1969 stabilized at around 50% (Fig. 4A). Less than one-third of

Present state and implications of sex bias

Single-sex studies of males still predominate in the biological literature, and neglect of females is widespread in many disciplines, including neuroscience, pharmacology, endocrinology, zoology, and physiology. One cannot assume that beyond the reproductive system, sex differences either do not exist or are irrelevant; despite this, a high proportion of studies failed to specify sex, and in experiments performed on both sexes data often were not analyzed by sex. Reporting of the sex of tissues

Implications for individual diseases

Women are diagnosed with anxiety disorders 2.25 times more often than men in a sample of 8 anxiety subtypes (Bekker and van Mens-Verhulst, 2007), but the majority of animal studies on anxiety and anxiolytic drugs focus on male rats (Palanza, 2001).

Women have more stroke events than men over the course of their lives, with poorer functional outcomes and more depression, but in some age groups stroke is more common in men (Reeves et al., 2008). Women have a lower incidence of hypertension than

Recommendations

The exclusion of females in much of non-human animal research can and should be corrected. When there is convincing evidence of the absence of sex differences in a particular trait, one can test equal numbers of male and female subjects and combine them into a single group for analysis (Mogil and Chanda, 2005). When the status of sex differences is unknown we recommend explicit comparison of the two sexes side by side within studies. This will facilitate the discovery of sex differences and

Benefits of studying multiple species

Scientifically valid comparisons between species and interspecific extrapolations require the use of common analytic procedures and similar dimensions. Meaningful comparisons between animal and human data must be based on functional outcomes, i.e., does the trait under consideration perform the same function in the animal model and human (Beach, 1978). The choice of the best animal models for advancing understanding of normal and abnormal human functions is constrained by disciplinary

Conclusions

Just as the absence of statistical analysis from biological research prior to the 1940s did not prevent major advances in biology, the neglect of females has not prevented progress in non-human animal research. Several arguments nevertheless can be made for abandoning the status quo ante for biomedical research. To understand the biology of women or develop safe treatments for diseases of women one must do more than study men. The plethora of sex differences at all levels of biological

Acknowledgements

Financial support was provided by NIMH Grant MH-61171 and the Robert Wood Johnson Foundation Health & Society Scholars program. We are grateful to Julia Heiman and Gregory Demas for encouraging us to write this article, to four anonymous reviewers as well as Sabra Klein, Rae Silver, Jill Becker, David Soergel, and Ellen Zucker for suggestions that improved the manuscript.

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