The sex of animals used in medical research may be having an influence on the outcomes of studies. This could have a significant impact on the design of future experiments, and may raise questions about whether or not results based on studies that used only a single sex can be fully applied to the opposite sex.

Many of the experiments that have occurred on laboratory animals, particularly mice and rats, tend to be done mainly on males. This is especially true of older research, but is still occurring today, with one study finding that 80 percent of papers that recorded the sex of the animals used only tested males. It has generally been assumed that as men and women are biologically similar, the results gained from animal models in general investigations can be applied to both sexes.  

But it seems that sex may have a much more important role to play and may influence the results in some cases. The study, published in Nature Communications, looked at how sex influences lab mice in general and then when they have certain genes knocked out. By comparing 234 physical traits in 14,000 male and female mice, they found that the sex of the animals influenced 56.6 percent of what are known as quantitative traits (such as bone mass) and 9.9 percent of qualitative traits (like head shape).

They then took 40,000 mutant mice with a gene switched off and observed what impact their sex had on them. They found that their sex altered the effect of the mutations in 17.7 percent of all quantitative traits and 13.3 percent of all qualitative traits.

“This study illustrates how often sex differences occur in traits that we would otherwise assume to be the same in males and females,” explains co-author Professor Judith Mank in a statement. “More importantly, the fact that a mouse's sex influenced the effects of genetic modification indicates that males and females differ right down to the underlying genetics behind many traits. This means that only studying males paints half the picture.”

The assumption that women are so biologically close to men that they don’t need separate studies even extends into some clinical trials that test the efficacy and safety of new drugs. While it has been a requirement of all trials for the last 20 years to include women in them, they are still under-represented, with only 41 percent of participants in 2006 being women.

The scientists, from both the Wellcome Trust Sanger Institute and the International Mouse Phenotyping Consortium, suggest that the results could impact more than half of all their studies, and as such likely many conducted elsewhere. It is a little known and often ignored problem that needs to be rectified for more representative science to be conducted.