A parasite that lives to change mouse behavior may also be altering the way humans act. The parasitic protozoa Toxoplasma gondii thrives by cycling through feline and rodent hosts. When it infects mice, the brain-dwelling parasite makes them more daring and, in particular, less afraid of the scent of cats (so it can get passed back to the feline hosts when they eat the infected, emboldened rodents).

The chemical changes brought on by the parasite appear to have some of the same effects on humans, who can be infected by ingesting parasite eggs from cat feces. Research by Kevin Lafferty, a research ecologist with the U.S. Geological Survey, has found that the parasite can cause women to act more moralistically, and men less so. And "when looking at human societies, those traits correlated pretty well with the prevalence of T. gondii infections," Levri says. Other research has shown a higher incidence of risky behavior in people who are infected.