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Latent toxoplasmosis affects up to 1/3 of the human population and up to 40% of the population in Australia. I have heard that latent toxoplasmosis has many significant undesirable effects on personality, such as decreased novelty-seeking behaviour, and decreased intelligence quotient.

While these changes are statistically significant, I'm wondering if anyone knows the magnitude of these personality and cognitive changes, since a statistically significant result doesn't necessarily imply that there is a large difference in personality between those who are infected and those who aren't infected. Also, can one succeed in an academic career path if one becomes infected with toxoplasmosis?

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    $\begingroup$ Do you have some sources for this wide-spread infection? Would be interesting to read. $\endgroup$ – Chris Dec 20 '13 at 8:07
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    $\begingroup$ Heaps of sources, have to go now, but i'll post later. Just search on google or google scholar in the mean time if you want to. $\endgroup$ – Kenshin Dec 20 '13 at 8:47
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I believe its the other way around - humans may engage in riskier and more novel behavior when infected... which is the most interesting part of the story. Its not cut and dried, but I think its quite possible that risky behavior increases.

T. gondii is a parasite whose typical life cycle includes both mice and cats as hosts. The transmission involves the mice being eaten by a cat. Mice infected with T gondii will tend to lose some interest in being sheltered in close spaces and wander out into open spaces. They also lose their fear of cats. That is a permanent change even if the mice are cleared of the parasite.

Other animals, such as humans are secondary hosts - they can become infected by handling or close proximity to cat feces, who have a nearly 50% (45.6%) infection rate themselves. As a result toxoplasmosis is very common in humans.

Toxoplasmosis has been found to increase novelty seeking and increases risk seeking behavior at least in men. Its been shown to contribute to higher automotive accident rates, schitzophrenia and suicide.

Thus, the men were more likely to disregard rules and were more expedient, suspicious, jealous, and dogmatic. The personality of infected women, by contrast, showed higher warmth and higher superego strength (factors A and G on Cattell's 16PF), suggesting that they were more warm hearted, outgoing, conscientious, persistent, and moralistic. Both men and women had significantly higher apprehension (factor O) compared with the uninfected controls.

The correlation between toxoplasmosis and lower intelligence is a matter of debate, at least in this review I cite. Its still not clear whether it is a sociological correlation - do smart people just handle their cat poo more carefully or not at all? This more recent study shows that some factors might make infected humans more intelligent and some researchers see toxoplasmosis as a transformative factor in humans ability to generate and use technologies.

I do see those links suggesting less risky behavior too. Some say that the increase in traffic accidents is related to lower muscular coordination for instance. There is evidence on both sides, but looking at what T gondii does to mice, I would bet that some risk behavior will increase. As such, I'd guess that researchers as such might have a slightly higher rate of toxoplasmosis, but maybe you will do a study and help figure this out!

That being said - anyone who tries to link a single phenomenon; host-parasite interactions; a drug; a gene; a mutation to a phenotype with a simple description is definitely oversimplifying. The mapping from biological activity to english (or any other language) is hopefully conscious of the caveats of such talk.

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