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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.

Is this really me? Parasites and other humans’ cells in our brains change our psychologyby Nadira FaberJuly 24, 2015July 24, 2015Many 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. In their highly recommendable paper “Humans as Superorganism: How Microbes, Viruses, Imprinted Genes, and Other Selfish Entities Shape Our Behavior”, published in Perspectives on Psychological Science this week, Peter Kramer and Paola Bressan make a convincing case that we human beings are not unitary individuals, but superorganisms – “collections of human and non-human elements that […] jointly define who we are.” They describe a variety of foreign entities we coexist with and how they can influence our behaviour. I want to give two astonishing examples here: microbes and body cells of other human beings. Microbes Human cell infected with Toxoplasma gondii 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. Th...

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