Cats and alterations in personality

‘If the mind is a machine, then anything can control it – anything, that is, that understands the code and has access to the machinery’

Let us take for example the orb spider. Typically, the orb spider weaves a web that is no less than an engineering marvel, a mosaic of spiral non-sticky web and a final sticky web line to hold the entire web together. It must be a matter of great pride for the spider to achieve such a feat in a matter of hours. However, these plans can be foiled by the Polysphincta gutfreundi, a small tropical wasp whose entire image is built on what it does to the orb-weaver spider. The female wasp lays an egg in the abdomen on the spider, after which a tiny larvae emerges out of it, deriving nutrition from its host’s body. However, that is not enough for the larva. The larva needs a safe spot to undergo metamorphosis and emerge a wasp, and what better than to use the extraordinary intellect of the spider in building itself a home. The larva imparts certain chemicals in the spider that makes it weave the web just a little differently. This “drugged” spider is under the influence of the larva, weaving to its tunes and constructing a web for the benefit of the larva. These webs are strong and specifically designed to keep the larva’s cocoon suspended and away from the sight of larger predators.



If the larva is removed from the spider’s body by human intervention, the spider lives and returns to its normal web-making abilities soon enough.

The rabies virus, too, evokes the feelings of rage in its host so that the host bites another living being, and transmits the virus to more hosts.

Such a parasite exists closer to home, and much more elusive than a rabies virus. Let us talk about the infamous Toxoplasma gondii.

This protozoan has a fairly complicated life cycle which begins inside a very common animal, and only inside this animal.


Inside cats, the protozoan spends time reproducing and generally having a gala time, after which zygote-filled cysts are released along with the cat’s poo. From here on, it can go anywhere it wants – water, soil, food, other hosts, and even humans (but we will get to that later). Depending on who ingests this infection that is now free to roam the world, the infection is concentrated in various parts of the body. In pigs (if that is the intermediate host), they are mostly in the muscles, and in the case of rodents, they are mostly in the brain. Inside this intermediate host, the protozoan can’t really reproduce, so it yearns for a way out and into a cat again, where it can continue to reproduce more protozoa. However, this might not always be possible.

In rodents, this protozoan has evolutionarily built a great way of escaping. It has been proven, that rats infected with Toxoplasma doesn’t hate cats quite as much. In fact, the smell of cat’s urine even sexually arouses the rodent towards the source. It also makes the rodents wary of predators around it, and makes for very easy prey for cats. Most of what the rodents do after being infected is a way for the cat to devour the host, and as such makes it the protozoan’s doing. It can happily reproduce again in a cat’s body.

When the passage from the intermediate host to the cat’s body is not really possible, the protozoan just chills in the host’s body for as long as it can, which is generally the lifespan of the animal. Globally, 30-50% of humans are infected with Toxoplasma gondii. And it alters the personalities of humans too.

Other than decreased reaction times, the infected humans showed a lot of changes in terms of their behaviour. The interesting part is that it shows up differently in both men and women. To quote a study,

Compared with uninfected men, males who had the parasite were more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules. Infected women, on the other hand, presented in exactly the opposite way: they were more outgoing, trusting, image-conscious, and rule-abiding than uninfected women.

Compared with uninfected people of the same sex, infected men were more likely to wear rumpled old clothes; infected women tended to be more meticulously attired, many showing up for the study in expensive, designer-brand clothing. Infected men tended to have fewer friends, while infected women tended to have more. 


The underlying difference that the study found was how differently genders handled anxiety and emotional strain, and that is the kind of alteration that affected people when they were infected with the protozoan. Of course, if a woman is an introvert, Toxo would not turn her into a raving extrovert, just a little less of an introvert. But over a larger sample size, the evidence is shocking.

This brings up the question that if Toxo can affect and alter the personalities of so many without them even realizing it, would it affect the entire human culture as a whole? When 30-50% of the human population is affected by this protozoan, does it seep into the cultural and societal aspects of our lives?


Flegr, J (Jan 2013). “Influence of latent Toxoplasmainfection on human personality, physiology and morphology: Pros and cons of the Toxoplasma-human model in studying the manipulation hypothesis”. The Journal of Experimental Biology. 216 (Pt 1): 127–33.

Webster JP, Kaushik M, Bristow GC, McConkey GA (Jan 2013). Toxoplasma gondii infection, from predation to schizophrenia: can animal behaviour help us understand human behaviour?”. The Journal of Experimental Biology. 216 (Pt 1): 99–112.

Lafferty, Kevin D. “Can the common brain parasite, Toxoplasma gondii, influence human culture?.” Proceedings of the Royal Society of London B: Biological Sciences 273.1602 (2006): 2749-2755.