How Microbes, Viruses, Imprinted Genes, and Other Selfish Entities Shape Our Behavior

Many people are suspicious about being manipulated in their emotions, thoughts or behaviour by external influences, may those be drugs or advertising. However, it seems that – unbeknown to most of us – within our own bodies exist a considerable number of foreign entities. These entities can change our psychology to a surprisingly large degree. And they pursue their own interests – which do not necessarily coincide with ours.


There are microbes that have specialised on living as parasites in the brains of animals, including us. One such microbe parasite is Toxoplasma gondii.

Typically, Toxoplasma’s aim is to get into cats, where it can reproduce and lay eggs in their intestines. To achieve this, Toxoplasma goes via rats, which serve as so-called intermediate hosts. When a cat eats an infected rat, the parasite gets passed on to the cat. So Toxoplasma manipulates the behaviour of rats such that the probability of being eaten by a cat increases. An infected rat not only loses its fear of cats, but even gets attracted to the smell of cat urine.

Toxoplasma also passes on to other mammals, birds, fish – and humans – which ingest infected animals, water or soil. Eating meat that has not been well cooked is the most common source of humans getting infected. And they do so extremely often, with a prevalence of around 50% in some European countries.

Once infected, the parasite remains within us for the rest of our lives and interacts with our nervous system. And it might well be one reason why we sometimes behave in ways that do not promote our own reproductive success or survival. Research suggests that Toxoplasma infection not only correlates with, but actually causally influences human behaviour.

When we get infected, Toxoplasma synthesises an enzyme that increases the production of the neurotransmitter dopamine in our brains. Also, our immune system responds to this microbe with breaking down the amino acid tryptophan. These reactions, it seems, can have a considerable impact on our mental health and behaviour. As Kramer and Bressan report “the infection has been associated with workplace and traffic accidents (possibly because it renders one less careful and slows down reaction time), depression, suicides, changes in personality, and various mental and neurological diseases, including bipolar and obsessive-compulsive disorders. […] In particular, Toxoplasma infection raises the probability of developing schizophrenia 2.7 times and is thereby the largest known single risk factor of the disorder— larger than any of the currently known genetic and environmental ones.”

Interestingly, medication that affects the dopaminergic system (like the antipsychotic haloperidol) has been shown to reduce Toxoplasma’s replication and invasion of brain cells. This might be taken as evidence that the antipsychotic effects of this medication may be related to its influence on the parasite.

It is not only microbes in the brain that can influence human behaviour, but also those in the guts– some for the better. Some strains seem to be, for example, associated with improvement in depression and anxiety symptoms.

Foreign human cells

The body cells of another person are another vivid example of foreign entities in our body that can influence our behaviour. These infiltrate our bodies most likely when we are foetuses. Some of these foreign cells can multiply and form large areas of our brains, but still be distinguished from our own cells, as they carry different genes.

Often, such foreign cells come from our mothers. While she is pregnant, the mother’s immune system can mistake the foetus for an intruder and react to it as it would to a parasite. This is especially likely to happen when the foetus is male, as his body cells carry proteins linked to the Y chromosome which the mother’s body (carrying no Y chromosome) can classify as “alien” and produce antibodies against. These antibodies can then deactivate molecules in the foetus’s brain. Some of these molecules seem to play a crucial role in the sexual differentiation of the brain, and their deactivation can lead to a redirection of the sexual orientation of this (male) brain towards the more female end.

The more often a woman has been exposed to male body cells, prominently by having been pregnant with boys, the more antibodies she has produced. Hence, male foetuses are more likely to be affected by these antibodies if the mother has been pregnant with other boys before. This phenomenon might be one route for men to develop a homosexual orientation. There is evidence that the more sons a woman has born, the higher the likelihood that her next male baby will be homosexual. Statistically, a man who has four older brothers is three times more likely to sexually prefer men than a man with no older brothers. That adopted children don’t have the same influence on their younger siblings adds credibility to the hypothesis that this indeed might be a biological, not a social, effect.

Such maternal antibodies can not only influence sexual orientation, but, problematically, also disturb the growth of the foetus’s brain in such a way that neurodevelopmental disorders like autism occur.

Microbes and cells of other humans are two examples of foreign entities that can have a significant impact on our psychology. And they illustrate that indeed we are less like unitary individuals and more like superorganisms, guided in our behaviour partly by built-in, though foreign, entities.

The findings Kramer and Bressan report not only are relevant for our understanding of human psychology, but also have practical implications. It seems that psychologists need to expand their collaborations with other disciplines beyond already “close friends” like neuroscience and philosophy to seemingly more unrelated ones like immunology or parasitology. And psychotherapists should be aware that a purely behavioural treatment of patients might be less successful when they are infected by foreign entities like microbes. At some point, a direct treatment of the “foreign visitors” in our bodies might even complement psychotherapy.

I have to leave it to philosophers to decide which implications, if any, such findings have for concepts like human nature, personhood, and agency. I wonder, though, how the current debate about human enhancement might be affected. Many people are sceptical about altering what is “genuinely ourselves”. But it seems we’re not genuinely ourselves in the first place – not entirely, at least.

Is this really me? Parasites and other humans’ cells in our brains change our psychology