I had heard about this line of research before through casual reading, but until now I hadn't realized how strong some of the data actually were or the more subtle and far-reaching effects infection with toxo could have.
Flegr became curious about toxo after incidentally discovering he carried the parasite himself. He wondered if infection with toxo could explain some of his own quirks, specifically his lack of fear and irrational calmness in the face of danger. He knew that in rats toxoplasmosis caused confrontational and overtly dangerous behavior: a rat with toxo will completely lose it's natural fear of cats and will seek out interactions with them.
So he set out to study toxoplasmosis infected people. He discovered that there were subtle but significant differences in the personalities of people who carried the parasite, but the differences were based upon gender. Infected men were cautious and suspicious, socially withdrawn sloppy dressers. Women with toxo were more extroverted, meticulous dressers. Infected humans as a group were also more than two and a half times more likely to get into car accidents---a difference that might be due to both fearlessness and slower reaction times seen in infected people.
Then there was the relationship to psychiatric disorders, the aspect I had already read about. Some neuroimaging studies have shown that people with schizophrenia who show reduced grey matter volume are almost all also positive for toxoplasmosis. This is particularly striking given that toxoplasmosis has two genes which can increase the production of dopamine.
So now when I read articles purporting that psych meds shrink the brain I'll know what question to ask first: "Did they control for the cat?"
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