In the debate over which is the stronger sex, the so-called “man flu” inevitably comes up. It’s a popular theory that men get sicker—or at least act sicker—when they contract the virus, while women soldier on with work, childcare and life.

The scientific evidence for this is far from conclusive, but some research has shown that male and female immune cells do react differently to invading viruses. Now, a recent study in mice adds more fuel to the fire, suggesting that the male sex really does get hit harder by certain illnesses—and that physiology, not psychology, may be at least partially to blame.

In the recent study, published in the journal Brain, Behavior and Immunity, adult male mice displayed more symptoms of sickness than females when they were exposed to bacteria that cause an illness with symptoms similar to the flu. The males also had more fluctuations in body temperature, fever and signs of inflammation, and took longer to recover.