If you are a young man with your heart set on medical research, the results of a new study may be heartbreaking.

An international team of pain researchers led by scientists from McGill University found that the gender of experimenters has a big impact on the stress levels of rodents, which are widely used in preclinical studies.

Researchers report that the presence of male experimenters produced a stress response in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.

Female experimenters produced no such effects.

“Scientists whisper to each other at conferences that their rodent research subjects appear to be aware of their presence, and that this might affect the results of experiments, but this has never been directly demonstrated until now,” Jeffrey Mogil, a psychology professor at McGill University and senior author of the paper, said in a statement.

For the study, researchers placed cotton T shirts, worn the previous night by male or female experimenters, alongside the mice; the effects were identical to those caused by the presence of the experimenters, themselves. This suggests that the effect of male experimenters on the rodents’ stress levels was due to smell.

Further experiments proved that the effects were caused by chemosignals, or pheromones, that men secrete from the armpit at higher concentrations than women. These chemosignals signal to rodents the presence of nearby male animals. (All mammals share the same chemosignals).