"A parasitic microbe commonly found in cats might have helped shape entire human cultures by manipulating the personalities of infected individuals, according to a new study.

Infection by a Toxoplasma gondii could make some individuals more prone to some forms of neuroticism and could lead to differences among cultures if enough people are infected, says Kevin Lafferty, a U.S. Geological Survey scientist at the University of California, Santa Barbara.

In a survey of different countries, Lafferty found that people living in those with higher rates of T. gondii infection scored higher on average for neuroticism, defined as an emotional or mental disorder characterized by high levels of anxiety, insecurity or depression."

http://www.ars.usda.gov/is/AR/archive/feb96/toxo0296.htm