Sex as a Biological Variable

What it is

The policy is the latest step in a sequence that the NIH Office of Research on Women's Health dates. In 1986 the NIH urged researchers applying for funding to include women and minorities in studies of human subjects. A 1990 General Accounting Office study of NIH grant applications found that about 20 per cent gave no information about the sex of the study population. The NIH Revitalization Act of 1993 required the inclusion of women and minorities in NIH-funded clinical research. The NIH Policy on Sex as a Biological Variable went into effect in 2016.

The gap the policy addresses was measured by Annaliese Beery and Irving Zucker in 2011. Reviewing research on mammals in ten biological fields for 2009, they found male bias in eight, and single-sex studies of male animals outnumbering those of females by 5.5 to 1 in neuroscience. Studies of both sexes often failed to analyse results by sex. They also wrote that the belief that female animals are too variable for routine inclusion is without foundation.

In effect

Analysing by sex can change what a study finds. Zucker and Brian Prendergast's 2020 analysis of 86 drugs found that 76 reached higher blood concentrations or cleared more slowly in women, and that these differences predicted more adverse reactions in women. Differences of that kind are only visible when results are broken down by sex.

What it does not say

The principle does not claim that women and men differ on every measure, or that a difference found in one study is large or important. It asks that sex be considered and reported, so that differences can be found or ruled out. Nor does it tell any individual how to use a medicine; findings about sex differences in drug levels are matters for regulators, prescribers and further trials.

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