Scientists’ inability to replicate research findings using mice and rats has contributed to mounting concern over the reliability of such studies. Pain researchers have now found that the gender of experimenters has a big impact on the stress levels of rodents used in research. 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.