As Ars Technica reports:
Someone in a lab that studies pain perception noted that “Our laboratory personnel have reported anecdotally that pain behavior appears to be blunted while experimenters are present.” At some point, they decided to see if this was biasing their results. The researchers gave some mice a painful injection, left them alone for a bit, and then sent a researcher back in the room for a few minutes. The mice’s pain was then assessed using “the mouse grimace scale” (yes, that’s a formal measure of pain).
It turns out the presence of a researcher could deaden the pain—but not just any researcher. It had to be a male. The male could be replaced with a bit of dirty laundry they’d created, and it would have the same effect. The same was true for bedding used by other mammals, although this failed when castrated dogs were tested.
This made them wonder whether their own work might have been biased by male researchers, so they went back and collected lots of archival data that was performed by known lab workers. Sure enough, the sex of the person working with the mice did affect their results.
And this, let’s stress, are the good ones – researchers that actually care enough about the scientific process that they look at this. Lots of others will just take the results they get, especially when they agree with what they’ve hoped for, and not think about the confounders.
Time to point towards The weirdest people in the world?(.pdf) and “Control” laboratory rodents are metabolically morbid: Why it matters again.