The prevalence of Toxoplasma gondii, a single-celled parasite, accounts for some cultural differences.

You are not the only one controlling your mind.

Approximately one-quarter of Americans host a parasite that has been shown to affect personality in both rodents and humans. According to a recent study, this single-celled organism may be able to shape entire cultures.

In a paper published in the online edition of Proceedings of the Royal Society, United States Geological Survey researcher Kevin Lafferty argues that a significant factor in why some countries exhibit higher levels of neuroticism than others may be the prevalence of the parasite Toxoplasma gondii. The study also indicates that it may influence a society’s preference for strict laws, an expression of uncertainty avoidance, and its valuation of ‘masculine’ priorities such as competitiveness and financial success over ‘feminine’ values like relationship-building.

“Toxoplasma appears to explain 30% of the variation in neuroticism among countries, 15% of the uncertainty avoidance among Western nations and 30% of the sex role differences among Western nations,” Lafferty said via e-mail.

Lafferty analyzed preexisting data on Toxoplasma prevalence and mean trait levels in 39 countries. He found a significant linear correlation between latent Toxoplasma prevalence and neuroticism with a few outliers, including the unusually neurotic nations of Hungary and China and the notably easygoing Turkey.

Links between Toxoplasma, uncertainty avoidance and concerns about masculinity initially appeared to be insignificant but later emerged when Lafferty focused on Western nations.

Lafferty based his analysis on earlier research by Jaroslav Flegr, a parasitologist at Prague’s Charles University, which showed that in humans, Toxoplasma infection correlates highly with certain personality traits: Infected men tended to have lower levels of intelligence, superego strength and novelty-seeking, while infected women exhibited higher levels of intelligence, superego strength and warmth. Infected people of both sexes tend to be susceptible to feelings of guilt.

Lafferty chose to analyze cultural neuroticism because Toxoplasma appears to influence neuroticism-related traits equally in both sexes, he said, unlike, say superego strength.

“Given the previous results from the rodent models and Flegr’s human studies, I’m not sure I would have chosen ‘neurotism”/‘neurotic’ elements of human cultures as the measure here, particularly across genders, but that is a matter for debate,” said Imperial College London epidemiologist Joanne Webster in an email.

She noted that uncertainties remain as to why the link between Toxoplasma and cultural dimensions known to be associated with neuroticism are so evident in Western nations. 

In 2000, Webster reported that rats infected with Toxoplasma are less fearful of and, in some cases, can even be attracted to their feline predators. She surmised that the parasite subtly manipulates a rat’s behavior to increase the rodent’s chances of being eaten by a cat—the only animal in which it can reproduce—thereby upping the odds of the parasite reproducing.

Lafferty acknowledges that his data set alone does not necessarily imply that latent Toxoplasmosis creates cultural neuroticism.

“For any correlation, it is possible that you have cause and effect mixed up,” he said. “However, for this study, I can only think of a logical mechanism for the possibility that Toxoplasma affects culture—not the reverse.”

Flegr, who advised Lafferty on his analysis, said in an e-mail that the new study jives with some of his own lab’s unpublished results, especially with respect to masculinity.

“We have the data showing that Toxoplasma-infected men are scored as more dominant and more masculine than Toxoplasma-free men by female observers.”

Originally published August 15, 2006

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