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Beyond genes: How cold viruses, the ‘cat litter’ parasite and other microbes may affect personality

"The freakiest of the behavioral-warping microbes" most commonly enters the human body through direct contact with infected cat feces.
REUTERS/Radu Sigheti
“The freakiest of the behavioral-warping microbes” most commonly enters the human body through direct contact with infected cat feces.

Can viruses, bacteria and parasites influence our personality and behavior?

Quite possibly, and in not-so-obvious ways, according to an intriguing article published last week in the Washington Post.

First, an obvious example of a microbe-altering behavior: the rabies virus. As science reporter Ben Harder points out in the Post piece, people infected with the rabies virus sometimes become belligerent during the infection’s early stages. But, Harder adds,

“[n]ot all microbes are so obvious about influencing our behavior. If the effect is subtle, it could be hard to tell whether a behavior is coming from the person or from the thing inside them. Cold viruses, for instance, were recently found to make people friendlier, especially during the period before symptoms appear but when the soon-to-be-sick person is highly infectious to others. Evolutionarily, that helps the virus survive, because a gregarious host is a host who’s likely to spread the illness. Advanced syphilis has been reported to sometimes trigger behavioral changes including an exaggerated desire for sex.”

Then there is what Harder calls “the freakiest of the behavioral-warping microbes,” Toxoplasma gondii. This parasite, whose most common entrance into the human body is through direct contact with infected cat feces, causes toxoplasmosis. Most infected people do not get sick, but sometimes the illness causes flu-like symptoms. And it can cause serious complications in individuals with weakened immune systems and in babies whose mothers become infected during pregnancy.

Some 60 million Americans have the Toxoplasma gondii parasite, according to the National Institutes of Health.

Until recently, latent cases were thought to be symptom-free. But that attitude may change, says Harder:

Research in recent years has identified several personality traits that appear to be associated with latent toxoplasmosis. Infected men are more willing to disregard social norms, for example, and are more jealous and dogmatic. Infected women are more conscientious, warm, easygoing and attentive to others. Both sexes, when infected, are more apprehensive and insecure.

“Latent toxoplasmosis also slows a person’s reaction time, which may explain why multiple studies have found that infected people get into more car accidents. One prominent researcher speculated that toxoplasmosis indirectly kills a million drivers and pedestrians a year worldwide.

Another researcher summed up the personality patterns by saying that infected men are alley cats — in other words, loners and scrappy fighters — and infected women are sex kittens. A third scientist has hypothesized that the high prevalence of toxoplasmosis in certain countries, including France and Brazil, may influence cultural stereotypes about those nations.

Bacteria’s dominance
Hmmm. … I agree with Harder. That’s a lot to blame on a parasite. Still, these studies raise an interesting question: When we consider personality and behavior, are we putting too much emphasis on genetics and ignoring microbes? Of our body’s 100 trillion or so cells, only one-tenth contain DNA, notes Harder. The rest are bacteria — mostly helpful ones, fortunately.

Earlier this year, an international team of researchers found that humans can be divided into three basic categories based on which kind of bacteria is most prominent in their guts. Could those categories be a marker for personality types as well? asks Harder.

Perhaps. Or perhaps not. After all, humans can also be sorted by blood type, which has no effect on personality.

Still, this is an intriguing line of research.

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Comments (2)

  1. Submitted by Beth Hawkins on 06/20/2011 - 10:26 am.

    Fascinating indeed. It’s easy to imagine the little bugs scheming for their own survival!

  2. Submitted by Dimitri Drekonja on 06/20/2011 - 04:54 pm.

    A point of clarification- bacterial cells contain DNA too- perhaps that quote was meant to be “Of our body’s 100 trillion or so cells, only one-tenth contain HUMAN DNA, notes Harder”

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