Toxoplasmosis and risky behavior

Once upon a time I read a science fiction story that suggested a sentient species acquired intelligence through neurochemical interactions between their brains and a parasitical worm. The worm made them physically ill, but it also gave them self awareness, comprehension, and radically changed their behavior as a species. The story ends with a punchline that this alien species gained the knowledge of Good and Evil by eating Adam & Eve’s forbidden fruit, but in their case their apple has a worm in it.

It seems something similar might be going on in our human brain, and in our case (at least for men) the parasite seems to make us a little more evil. I read a fascinating discussion on Toxoplasmosis and the effect this parasite has on the mammalian limbic system.

Toxoplasmosis gondii is the protozoan that famously infects cats, and which pregnant women avoid because it does bad things to the developing fetus. Rats and other small critters are commonly infected by Toxo. Rats normally respond fearfully to the smell of cat urine and cat pheromones, but Toxo modifies rat behavior so that rats are no longer afraid of cat smells; in fact infected male rats are sexually stimulated by the smell of cat urine. This changed behavior increases the chances the rat will be eaten by cats, and so the Toxo bugs are ingested and can reproduce in the cat’s gut.

The interesting thing: the behavior is very subtly selective. The rat’s don’t go all insane and crazy — they still act like normal rats and do normal rat things, they just suddenly get turned on by the smell of cat, and only by the smell of cats.

Now, it turns out Toxo does things to the human brain as well. As this study blithely observes, “The changes cannot influence the risk of predation in modern humans,” but apparently researchers have known for a while that motorcycle accident victims, for example, have about twice the chance of Toxo infection as the rest of the population.

Toxo seems to be genetically wired to take up residence in the mammal amygdala, or the ’emotional’ center of the brain. In humans, and especially in men, it appears to make us more impulsive and a little more ‘stupid.’ This research seems to show that toxo infected men have “lower superego strength” — this means we’re less likely to adhere to group norms or morals — and “higher vigilance” — making us “more expedient, suspicious, jealous, and dogmatic.” Paradoxically, toxo infected women tend to be more “warm hearted, outgoing, conscientious, persistent, and moralistic” though infected women also seem to be more promiscuous.

The next time you hear about a jackass who impulsively drives like a maniac or attacks cyclists on the trail or the road, maybe it wasn’t the devil who made him do it, but a protozoan parasite.



  1. =v= Hmm. Cat persons, particularly male cat persons, are not stereotypically impulsive. However, movies like Willard do suggest an impulsive streak amongst rat persons.

  2. …kinda scary stuff…we're prob'ly all playing host to a number of nasty little parasitic interlopers no matter how healthy we think we are…

    …& when you think of simple yet 'would be' illogical behavior patterns in general, maybe our quirky-ness is affected by more than our social upbringing…

    …the term “madhatter” came about in years past because of the mercury poisoning within the hat making industry & while that would be a straight chemical inducement, perhaps there is a greater percentage of the population under the influence of toxoplasmosis than might be expected…

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