One reason to suspect this is that a country’s level of Toxoplasma infection seems to be related to the level of neuroticism displayed by its population. Another is that those infected seem to have poor reaction times and are more likely to be involved in road accidents. A third is that they have short attention spans and little interest in seeking out novelty. A fourth, possibly the most worrying, is that those who suffer from schizophrenia are more likely than those who do not to have been exposed to Toxoplasma.
Apparently, the protozoa known as Toxoplasma gondii carries genes for the production of dopamine, which serves no purpose to single celled organisms. It does have significant effects in higher organisms, such as humans, and cats.
Cats transmit toxoplasmosis to rats and mice. It seems that this disease then causes rodents to behave in self-destructive ways, which includes being attracted to the scent of cats. Toxoplasmosis needs infected rodents to be consumed by cats so that it can continue its life cycle. Now the fact that this disease affects rodents in a way that promotes its own life-cycle is nothing short of amazing to me.
Dopamine levels are central to a number of psychological problems in humans. I’m no doctor, but the presence of a foreign source of dopamine in the system seems problematic. We know that giving too much dopamine in treatment of diseases such as Parkinson’s can lead to dopamine dysregulation syndrome, which “is characterized by self-control problems such as addiction to medication, gambling, or hypersexuality.”
This looks like an area for medical research that is pretty wide open. I’d love to see more research being done on this.