People often behave strangely and inconsistently. Psychologists, criminologists, and economists typically study these unpredictable behaviors and search for their motives. However, science is increasingly finding evidence that bacteria and parasites can influence human behavior.
Behavioral control by an outside force appears to be a real threat to humans. From a biological perspective, what makes us any better than other species, even if they're less intelligent? Nature has many examples of how controlling another organism becomes a survival strategy for parasitic species.
For example, the Cordyceps fungus parasitizes ants, causing their death. But before dying, the insects rise into the tree canopy, where the fungal spores are carried by the wind.
Horsehair worm larvae most often parasitize terrestrial insects, snails, and crustaceans. Upon completion of their developmental cycle, the parasite forces the host into water, where the horsehair worm gnaws through the host's skin (usually killing the host) and emerges into the water.
As the larva develops, it may change hosts to larger ones. For example, while inside a snail, the hairworm changes the color of its eyes on its antennae from black to red, blue, or yellow, making the eyes themselves large and bulging. This attracts the attention of birds, which peck at the snails.
Although horsehair worms are the subject of popular horror stories, they are harmless to humans, but we have no immunity to other parasites.
The sexual phase of the Toxoplasma parasite's life occurs exclusively in cats, while its asexual phase can occur in any warm-blooded animal, including birds, rats, and humans. According to various estimates, 30 to 40% of people worldwide are carriers of Toxoplasma, and even more in some countries—in France, for example, 81%.
Toxoplasma parasites can alter the behavior of their hosts, causing the death of approximately 4% of all birds. Rats and mice lose the scent of cat urine, become bolder, and spend more time in the light, making it easier for cats to hunt.
Men infected with the parasite become more aggressive and jealous. Their risky behavior increases their risk of dying in car accidents. Women are more likely to commit suicide. It is believed that toxoplasmosis may cause dementia, mental disorders, and autism.
How exactly these tiny organisms influence human behavior remains unclear. Researchers have only established that the presence of Toxoplasma affects levels of neurotransmitters such as dopamine.
Researchers from the California Institute of Technology in Pasadena have discovered that human behavior can be controlled not only by parasites, but also by bacteria, of which there are a great many in the human body.
In the symbiosis between humans and bacteria, there are eight times more bacteria than cells in our body. Research has shown that bacteria regulate not only digestion but many other processes as well. Changes in the microbiome can cause diabetes, neurological diseases, cancer, and asthma.
According to Dutch researchers, gut microbes can directly influence the production of the neurotransmitter serotonin, which in turn regulates sociability, anxiety, and the perception of external stimuli. It's possible that in the future, depression and anxiety could be treated by adjusting the composition of the gut microbiome.
“Future research will attempt to understand how these microscopic overlords manipulate our decisions,” writes Robbie Rae, a geneticist at Liverpool John Moores University. “Their influence on society, culture, and politics should not be underestimated.”