Olfactory exposure to males, including men, causes stress and related analgesia in rodents

The presence of male experimenters induces stress and stress-induced analgesia, affecting behavioral assays in mice. We found that exposure of mice and rats to male but not female experimenters produces pain inhibition. Male-related stimuli induced a robust physiological stress response that results in stress-induced analgesia. This effect could be replicated with T-shirts worn by men, bedding material from gonadally intact and unfamiliar male mammals, and presentation of compounds secreted from the human axilla. Experimenter sex can thus affect apparent baseline responses in behavioral testing.