A study published in the European Journal of Personality suggests that humans infected with toxoplasmosis gondii are more extroverted compared to non-infected individuals. According to the CDC, roughly 22.5% of Americans over 12 are infected. The effects of the parasite are attributed to increased levels of dopamine in the brains of infected individuals. People can become infected with the parasite through exposure to undercooked contaminated meat, unwashed fruits and vegetables from contaminated soil as well as infected cat litter.
Lindová J, Příplatová L, Flegr J. Higher Extraversion and Lower Conscientiousness in Humans Infected with Toxoplasma. European Journal of Personality. 25(3): 285-291, 2012.