Brain parasite drives human culture

The brain parasite, Toxoplasma gondii, is spread by cats and affects a huge proportion of the world’s population. It’s effects on our behaviour make it a potent driving force of human culture.

Toxoplasma, a brain parasite that drives human cultureWe like to think that we are masters of our own fates. The thought that others might be instead controlling our actions makes us uneasy. We rail against nanny states, we react badly to media hype and we are appalled at the idea of brainwashing.

But words and images are not the only things that can affect our brains and thoughts. Other animals – parasites – can do this too.

Now, Kevin Lafferty from the University of California, Santa Barbara, has found startling evidence that a common brain parasite, Toxoplasma gondii, could be influencing human culture across the globe.

Toxoplasma gondii is a single-celled brain parasite spread by cats. Our feline companions are its preferred home and only there can it mature and reproduce. So like most parasites, T.gondii has a complex life cycle designed to get it into its final host.

If it finds itself in another animal, it travels to the brain and changes the host’s behaviour to maximise its chances of ending up in a cat. For rodents, this means being eaten and infected individuals are less fearful of cats and more active, making them easier prey.

Humans can become infected with Toxoplasma after contact with cats. Humans can also contract the parasite, through contact with soil contaminated by the faeces of carriers or through eating infected meat. But since cats are very unlikely to eat humans, in our bodies, T.gondii reaches a cul-de-sac. Still, there is nothing to stop the parasite, evolutionarily speaking, from trying out the strategies that work so well in other hosts.

In rare cases, T.gondii infection causes a disease called toxoplasmosis that produces mild flu-like symptoms and only really threatens foetuses and those with weak immune systems. In most instances, the parasite acts more subtly.

Carriers tend to show long-term personality changes. Women tend to be more intelligent, affectionate, social and more likely to stick to rules. Men on the other hand tend to be less intelligent, but are more loyal, frugal and mild-tempered. The one trait that carriers of both genders share is a higher level of neuroticism – they are more prone to guilt, self-doubt and insecurity.

In individuals cases, these effects may seem quirky or even charming but across populations, they can have a global power. T.gondii infection is extremely common and rates vary greatly from country to country.

While only 7% of Brits carry the parasite, a much larger 67% of Brazilians are infected. Given that the parasite alters behaviour, infection on this scale could lead to sizeable differences in the general personalities of people of different nationalities. This is exactly what Lafferty found.

Neuroticism is one of the most widely-studied of all psychological traits and Lafferty found that levels in different countries correlated well with the levels of T.gondii infection. The parasites’ presence was also related to aspects of culture associated with neuroticism.

Countries where infection was common were more likely to have ‘masculine sex roles’, characterized by greater differences between the sexes and their part in society and a stronger focus on work, ambition and money rather than people and relationships. Strongly infected societies were also more likely to avoid risk and embrace strict rules and regulations.

Obviously, different countries are also not just uniform populations, and increasing rates of migration mean that many countries are very ethnically and culturally mixed. However, this works in favour of Lafferty’s theory as any mixing would serve to mask the link between infection and culture. If anything, the link is stronger than seen in this study.

It would be imprudent to suggest that T.gondii is the major driver of human culture. It is just one of a number of influences that include genes, our physical environment and our histories. And Lafferty himself is quick to point out caveats to his own results.

For a start, they do not imply that the parasite is causing these personality types; it could be that people with these traits are more likely to become infected. To establish the true direction of causality, Lafferty will need to find out how the parasite manipulates the mind. The general idea is that infection alters levels of the immune system’s communication chemicals – the cytokines – which in turn alter levels of neurotransmitters like dopamine. But the details remain a mystery.

Nonetheless, the results are striking and they suggest that climate could have a larger effect on culture than previously thought. Toxoplasma gondii’s eggs live longer in humid, low regions so variations in climate could influence the global distribution of cultural traits. Perhaps, this could explain why men and women perform more distinct roles in society in countries in warmer climates. Other factors can also affect the risk of infection, including cat ownership and national cuisines that include undercooked meat.

We like to think of culture as something governed by the collective actions of free-thinking and free-acting humans. But Lafferty’s analysis shows us that if environmental factors like parasites can affect our thoughts and actions, no matter how subtly, they can have a strong effect on national cultures.

In many cases, these effects could be much stronger than the agents that we normally believe to drive cultural trends. After all, more people around the world are infected with Toxoplasma than are connected to the internet.

Reference: Lafferty. 2006. Proc Roy Soc B 273: 2749 – 2755.

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Related posts on other parasites:

Worms track us down with a chemical trail
Genetically-modified mosquitoes fight malaria by outcompeting normal ones

Parasites can change the balance of entire communities

Viruses evolve to be more infectious in a well-connected population
The secret of drug-resistant bubonic plague

21 Responses

  1. Is the evidence all in? And was it completely examined? I ask because I can think of quite a few counterexamples throughout history.

    Take Rome, for instance. Cats really didn’t become domestic pets in Rome ntil after 1 AD — but you don’t have to go any further than the nobles of Rome to see the “cat toxoplasmosis syndrome” in full effect long before Rome had a significant number of cats anywhere.

    The Aztecs (and to a lesser degree the Mayans and Incans) had notable “cat toxoplasmosis syndrome” long before any domestic cats showed up.

    Ancient Egypt, where cats were worshipped, seems to have almost no symptoms of the “syndrome.” This, the first cat-loving society, should be a hotbed of all those symptoms… and is not.

    The Berbers, who do not own cats, exhibit many symptoms of the “syndrome.”

    The historical counterexamples are lengthy, as are personal examples of non-cat owners who show the “toxoplasmosis syndrome” and cat owners of multiple cats who undoubtedly test positive for the parasite but do not exhibit the “syndrome.”

    I have trouble buying into this one.

  2. Neither my article nor Lafferty’s paper suggests that the Toxoplasma parasite is the only, or even a major, driver of human culture. But from his data, it clearly has an effect.

    Finding counter-examples, be they individuals or populations does not void the basic idea. The study looks at very large-scale trends across different populations.

    In a similar way, smoking clearly causes lung cancer even though not all smokers develop lung cancer, and not all lung cancer patients were smokers.

  3. What should not be lost, is the realization that a parasite can affect a population in such ways. Remember the basic psychological annectdote of a patient having a religious experience when certain parts of their brain was stimulated,then wonder if all the parasites who can affect us and all the ways they can, are known?

  4. Oops…I think Mr. Yong got mixed up. Infection with Toxo tends to disinhibit rather than encourage more-compliant behavior patterns. More “straight-laced” cultures (British, Japanese) have lower (lowest) levels of infection, as opposed to Brazil and France where the party is always ON. Rats infected with Toxo exhibit riskier behavior, making them more vulnerable to cats; cats being the required reproductive host and main vector for the parasite.

    This is where I get mixed up. It has, for decades been a mystery why repeat studies have shown people with less-robust immune systems describe themselves as and are evaluated by investigators to be “shyer,less outgoing”. Now; here’s the chicken vs. egg query: Do babies who are born less outgoing, possibly born to “shyer” parents getting fewer challenges to their developing immune system than children who are more “out there”? As our species evolved did stronger immunity prevail where more individuals are gregarious and interactive (sociable persons with weak immune systems would be more vulnerable to contagion and thereby contribute less genetic material to subsequent generations)?
    I might also point out that infectious disease is always going to be more prevalant where populations are more dense. Populations are more dense where climate conditions are less challenging; that is, wetter, warmer, lower-lying environs. Add the centuries of invasion/integration through trade, etc that furthur diversifies a population’s gene pool and exposure to pathogens, you get folks with pretty tough disease resistance.
    What I’m (finally) getting to is that you should see fewer symptomatic infections (overtly sick persons) in populations with endemic or ubiquitous infections (except in immunosupressed individuals). If 80% of Brazil/France tests positive for Toxo they might be much less vulnerable to actually becoming sick from the infection. Whether or not Toxo can be linked to cultural characteristics or not is tricky. Insular, less diverse populations will have high resistence to local pathogens, but could be decimated by a pathogen they have not yet been exposed to (Native Americans/measles; European colonists/malaria).
    If Japan or Britain were subjected to a big exposure to Toxo, would there be a great deal of illness? Bet there would. Back in the days before antibiotics,would a Toxo epidemic in a previously unexposed population select for outgoing individuals who already had hardier immune systems because they were more outgoing, therefore self-exposed to more immune challenges?…What would really be interesting is if unexposed rats that were more risk-averse could made to exhibit riskier behavior by infecting them with Toxo. Has that experiment been done? I’d love to find out!
    Laura Wrzeski

  5. J King – indeed. In my mind, findings like this one, and the contribution of intestinal bacteria to obesity, pose some interesting questions about our notions of free will.

    Laura – thanks for this fascinating post. Some *really* interesting questions there, and I’d love to see some answers to them…

  6. “In a similar way, smoking clearly causes lung cancer even though not all smokers develop lung cancer, and not all lung cancer patients were smokers.”

    How do they know that its not a virus that causes lung cancer, and the smoking is a co-conspirator?

  7. “I have trouble buying into this one.”

    I find it plausible to some extent… Than again, I believe the majority of human diseases are directly or indirectly caused by germs…

    I can’t believe why they are paying so much attention genetics, MONEY I guess…

  8. “How do they know that its not a virus that causes lung cancer, and the smoking is a co-conspirator?”

    That would be the shedloads of evidence accumulated through over a half-century of research. Which includes hordes of epidemiology, biochemical studies of the effects of the carcinogens in smoke, analyses of the way cancer trends change with smoking trends, animal studies and so on. There are hundreds of websites out there that document all this undeniable evidence.

    Smoking causes lung cancer. Throwing a virus into the mix is a bit like saying that gunshot wounds don’t kill people – what actually happens is that the bullets activate a special virus that only reacts to bullets and which then multiples rapidly and kills the person who’s shot.

  9. I knew it! I knew those fuzzy, little freaks were taking over somehow.

  10. […] Not Exactly Rocket Science has a post up featuring “… the favourite stories which I [that is, Ed Yong] managed to write about over the last year.” […]

  11. […] Is human culture influenced by brain parasites? […]

  12. please email the name of the medicines used for this parasite in the brain of cats.

  13. So a couple of people have now asked for how to treat Toxoplasma and the simple answer is… I don’t know. Sorry. You’re going to have to ask a vet.

  14. Oh, dear. I was beguiled by this blog’s “mission statement” and the fact that it has apparently won a serious award, but I find this post quite disappointing. Surely one of the the main points should be why the researchers believed that the correlation between toxoplasmosis infection and certain personality traits is significant? How did they define and measure these supposed personality changes? How can they possibly believe that people with neurotic tendencies have more cats than people who don’t? If the central hypothesis is that parasites modify behaviour in order to spread more easily, shouldn’t the researchers have begun by finding out how many domestic cats there are in different parts of the world, and how popular they are as pets? In my limited experience, a British woman is much more likely to own a cat than a Brazilian woman. Furthermore, while one cannot of course rely on stereotypes, the Brit belongs to a culture that “sticks to the rules” more consistently than the Brazilian… so where does that leave the poor little parasites?

    Reader beware: there is too much flaky pseudo-science in our postmodern world, and unfortunately it contributes to many misconceptions, some of them downright dangerous. It’s no news that culture is strongly influenced by the physical environment; from that to the supposition that Brazilians are “prone to guilt, self-doubt and insecurity” due to toxoplasmosis infections, there is a long and questionable leap, which many casual readers may make.

    Good writing is indeed a wonderful weapon against ignorance… may I suggest including a critical outlook as the first element of a well-written blog? You never know, “its effects on our behaviour” might include a capacity to use apostrophes correctly.

  15. This is absolutely hilarious … human society controlled by cats. I mean …srsly.

  16. Yeah well I have had Toxoplasmosis for many years, probably since I was a child, I was diagnosed when I was 24 when I had problems with my eyes, unfortuantely the little blighters had settled in my retinas and started eating them. Treatment was similar to malaria treatment, and I was sick from the drugs. I have had several bouts in my life but now have sufficient immunity to fight them off and dont require any further treatment. As for behaviour, well does not life make us some what nuerotic and untrustworthy as well as guilty for the things we may or may not have done. It would be far to easy to just blame a little parasite for out own problems and mistakes instead of taking responsibility for them oursleves. K

  17. to laura wrzeski i saw a doco where a toxo infected rat crawled over a cat. the cat wasarthritic with no teeth but i don’t think the rat knew that. no unifected rat would ever do that.

  18. if a microscopic spore can invade an ants body,travel to a precise area of said ants brain and alter its behavior so that it will contrary to itsinsticts climb to several metres up a tree.There is a slight chance that mexicans could be encouraged to dance on their hats

  19. I believe I read in Parasite Rex that toxoplasmosis infection made men less likely to listen to, or take, advice.

    Don’t just blame cats. Bats are a source of Toxoplasma infections as well, which is why you’re supposed to wear a mask when cleaning out old barns, attics, and cottages.

  20. Laura Wrzeski, the point of this is that rat’s with toxo do not exhibit risk behaviour… They specifically like cats and the smell of cats. It’s a very specific change. The change is so specific that it dumbfounded scientists.


    toxo does things like this to rats.

    here is another organism and example of specific behaviors.

    Now such things may only affect a small group of people profoundly, but even slight changes in large groups of people show up as a cultural effect…
    As to some individuals being affected strongly by such things, remember the ruling class and intellectual class are rarely called crazy… Just eccentric.

    Quite the genius little organisms and that word “genius” is a Latin word.

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