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Male Lab Rats Have Outnumbered Females, And That's Bad News For Science

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Science has a gender gap—and we're not just talking about the scientists themselves. Research has uncovered the fact that when it comes to the animals used in scientific experiments, males greatly outnumber females. That's not just a problem for science; it's a problem for those who benefit from scientific research. That is to say, it's a problem for all of us.

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No Girls Allowed

Science has historically been a white man's game, but in the 1990s, steps were taken to make it more inclusive. In 1994, the U.S. National Institutes of Health (NIH) recognized that many researchers were excluding women and minorities from their clinical trials, and began requiring that these groups be included. Today, women make up about half of clinical trial participants. But in the animal experiments that lead up to those clinical trials, the gender split hasn't been so balanced.

In 2011, Annaliese K. Beery and Irving Zucker published the results of a survey they conducted into this animal gender imbalance. What they found was dramatic: in 8 out of the 10 biological disciplines they looked at, male animals outnumbered females. That was most pronounced in neuroscience (a 5.5 to 1 male-to-female ratio), pharmacology (5 to 1), and physiology (3.7 to 1). And that's just the studies that specified the sex of the animals—75 percent of studies didn't say one way or another.

The basic reason female animals are left out of experiments is the same reason experts say they should be included: males and females are biologically different. According to Beery and Zucker, researchers who recognize that difference ("Some consider males representative of the human species," they write) use only male animals out of fear that a female animal's hormonal cycles would screw up their results. But research, including this 2014 meta-analysis, has shown that female animals are no more variable than males. Still, they are different: studies show that hypersensitivity to pain works differently in male and female mice, for example. Women also suffer more than men from conditions like depression, anxiety, and stroke, but most animal studies on these traits focus mostly on males.

Related: Male Mice Sing Songs To Woo Their Mates

What's Being Done

So how do we fix it? In a report on their research in the journal Nature, Beery and Zucker recommended that journal editors require authors to at least disclose whether they used male and female animals in their studies, and that funding agencies should favor studies that include males and females. By being transparent about the sexes of the lab animals, scientists can better understand which results apply to whom instead of painting everyone with a broad brush.

Since then, progress has been made: in 2014, the NIH instituted a new policy that requires scientists funded by the NIH to use equal numbers of male and female animals in their studies. Not everyone agreed with this decision. Writing for Scientific American, R. Douglas Fields called it a "major step in the wrong direction," especially since using animals of two sexes adds research cost without any additional funding. It remains to be seen what the new policy will mean for clinical research, but the issue does call many past studies into question. Either way, putting this once-invisible problem in the spotlight at least makes it easier to address.

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