“Mice are very anxious,” a psychologist named Robert Sorge told me. It takes them an hour or so to calm down before they’re ready for work. One day, Sorge and his undergraduate assistant, Kelsey Isbester, were working in a lab at McGill University, conducting pain tests by injecting a stimulant into the paws of mice that caused them to lick the wound for forty-five minutes. Sorge set up a camera to record each mouse’s expression, and the pain level would be measured according to how much the mouse grimaced. (The mouse-grimace scale is a standardized system for interpreting pain; the signs are narrowing eyes, bulging of the cheeks and the nose, shifting whiskers, and wiggling ears.) “When we started watching the videos, what we found was that the first animal wouldn’t start licking for about four minutes, but the last animal would start licking right away,” he said. By minute four, they had left the room. Sorge and Isbester came to suspect that the mice might be intimidated by their presence, so they took turns administering the chemical. “We started looking at the data,” he recalled. “They only show stress when I was in the room.”

Mice, and rats, it turned out, are made especially stressed out by men. One way to watch the rising anxiety and depression of a mouse is to force it to swim. “Mice can swim very well, and they can swim for a long time, but they don’t really like it,” Sorge said. Over a series of experiments, the team determined that even if a female scientist is working with a mouse, “just having a man in a room was similar to three minutes of forced swim.”

The lab where Sorge and Isbester had been conducting their research, which was published this week by the journal Nature, specializes in sex differences. Jeffrey Mogil, a neuroscientist who oversees the lab, said that all men had an anxiety-inducing effect on the mice, though there was some variation in degree. “The man who produced the least effect got some ribbing, because he was the least manly.”

This effect could be seen on a mouse’s cringing face. The more stress, the more resistant to pain tests—like dipping its tail in hot water, or poking its foot with fibers—and hence less of a grimace. “The idea is, if you’re a deer and you’re being chased by a wolf, and the deer sprains its ankle, under normal circumstances the deer would stop and heal its ankle,” Mogil explained. “But if it did, it would get eaten. So our bodies don’t want to know this.” High-stress scenarios mask the pain. The man is the wolf.

Sorge recruited several colleagues—four men, four women—to meet the mice. “We can’t castrate people, so we can’t remove their testosterone,” he said. “What we did was, we used a prepubescent boy—Jeff’s son, actually—so he sat in the room. We never tested testosterone levels, but he’s young, I think about twelve. The suppression that he showed was about halfway between the men and women. We also had an older faculty member in the department who is sixty-nine. At that age, they should be declining in testosterone, and he was about halfway between the suppression of the average man and the average woman.”

The team determined that the rodents were responding to the scent of men, not the sight. “Because mice are nocturnal, they don’t see very well, so most of their interactions have to do with smell,” Sorge explained. “Someone brought this in—it wasn’t me!—but we have a life-size Paris Hilton cutout in the lab. We tried putting that in the room to see what would happen. That had no effect. We also have a William Shatner cutout.”

To confirm the smell theory, Sorge went on, “we’d have the man wear a T-shirt overnight in bed, with no cologne or anything. We’d have him take the shirt, put it in a bag, and drape it over a chair.” Sorge couldn’t detect the smell, though others in the lab could; the stink depends on the dose, he explained.

The team conducted the same musk test with other males—guinea pigs and cats and dogs, fixed and unfixed. (All male mammals, including humans, produce testosterone.) “We just used the bedding,” Sorge said. He brought in the pillow that his daughter’s cat sleeps on. In each case, the masculine scent provoked the same response from the mice. And it wasn’t just the introduction of a strong odor: “We tried vanilla smell. We tried banana—none of them did anything. It didn’t have to do with being a novel smell. It had to do with a testosterone smell.”

“My joke all week is that men doing experiments need to be fired or at least monitored by women,” Mogil said. “But obviously, I’m not advocating that.” His proposal: identify the gender of researchers in the method sections of papers.

“It doesn’t necessarily need to change the dynamics of the lab,” Sorge said. “But it should change the way you do the testing. Just having a man in the room can change things.”

Beth Mitchneck, the program director of the National Science Foundation’s ADVANCE program, which aims to bolster women in science and engineering, said that many academics already take gender into account when designing and reporting their research. She was wary of drawing any profound conclusions from the rodents in question. “Women scientists are not lab rats,” she told me. “It is very hard to extrapolate from a study of rats in the lab to human behavior.”

Mogil intends to press on. He said his next step is to try the experiment on humans. His team will likely measure the levels of cortisol, the stress hormone found in saliva, by having male and female researchers hand subjects a tube in which to spit. He declined to speculate on the outcome. “Who has more reason to be afraid of strange men? Men or women? You could make an argument either way.”

Photograph by Robert F. Bukat/AP.