Religion as Parasite, Parasite as Religion

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This essay won joint third prize in a competition in The Skeptic magazine.


Sceptics sometimes describe religion as a parasite on the human mind. In the first part of the article, Religion as Parasite, I look at some of the implications of this way of thinking for our understanding of religion. In the second part, Parasite as Religion, I consider a more literal and perhaps surprising possibility: that biological parasitism may actually play a part in the formation of religious belief. Finally, in the third part, I bring out some of the implications of these ideas for our understanding of why religion exists.

Religion as Parasite

A parasite is an organism that has adapted to live in or on another organism, its host. We usually think of parasites as fairly large creatures—worms or flukes, perhaps—but microscopic organisms such as bacteria and viruses are also parasites. It is mainly these smaller parasites that I have in mind here. The approach I take owes something to Richard Dawkins's idea of memes but even more to Terrence Deacon's view of language as a virus-like parasite that has evolved to be particularly well adapted to the brains of children.

Deacon's theory of language

The fact that young children are able to learn languages with apparent ease, Deacon suggests, does not mean that they have some extraordinary innate linguistic ability, as some believe, but rather that languages have evolved to be learnt easily by immature minds. He finds this more plausible than Noam Chomsky's idea of a 'language organ' in the brain. It is quite often said today that there is a 'God module' or 'religion organ' in the brain, but this idea encounters many difficulties from an evolutionary perspective, and the suggestion that religions are parasites that have evolved to colonise the brains of children seems to fit the facts better. Many religions enhance this effect by encouraging their adherents to have large families: this is true of Judaism, Islam, and Christianity, and of course the Roman Catholic proscription of artificial contraception reinforces the tendency.

Similarities between language and religion

There are numerous similarities between language and religion. Both are usually learnt in early childhood, literally at the mother's knee. Religious beliefs inculcated in this way can be difficult to shake off, just as one's 'mother tongue' is more persistent in the face of disuse than are languages learnt in later life. Seen in this way, the well-known if apocryphal Jesuit saying 'Give me a boy until he's seven and he's mine for life' takes on a new significance.

Evolutionary aspects of language and religion

Species evolve, of course, and so do languages. Changes in vocabulary and syntax mean that eventually new linguistic 'species' emerge which are mutually incomprehensible. Something similar happens in religion: Judaism, Christianity, and Islam share a common ancestry but conversion from one to another is fairly rare—they can no longer 'interbreed'.

Religion and language both acquired in childhood

But the connection between language and religion goes deeper than this, for religion depends on language for its continued existence. And, once again, the association with childhood emerges here. Religious people are often reproved by the non-religious, and even by some co-religionists, for having a 'childish' view of God; and this is in a sense reflected in references to God the Father (today often transformed by feminists into God the Mother). If religion has evolved to be easily learnt by children, this makes good sense. Is this perhaps what Jesus meant when he said 'Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.' (Matthew 18, 3)?

Religious language

Acquiring a religion involves to some extent learning a different vocabulary and syntax: for example, the old Quaker use of 'thee' and, in some Christian circles, phraseology such as believing 'on' Jesus instead of the vernacular believing 'in'. Many religions take this further by having a sacred language (Hebrew for Judaism, classical Arabic for Islam, Sanskrit for Hinduism, Pali for Theravada Buddhism). Because religions are generally ancient the languages they use are often partially or wholly unintelligible to the laity and sometimes even the clergy, but contrary to what religious modernisers suppose, this linguistic remoteness is a strength, not a weakness. It adds to the impressiveness of the religion for its adherents, and misguided attempts to make religion more user-friendly by updating the language often have the opposite effect from what is intended.

Different patterns of transmission

The acquisition of religion in childhood is an example of 'vertical' transmission—transmission from parent to child. 'Horizontal' transmission, from adult to adult by conversion, also occurs but is less frequent. According to the late Ben Cullen, horizontal transmission of parasites usually gives rise to more virulent infections than does vertical transmission, and he thinks this is true of religion. So, for example, the religions of traditional Australian societies, transmitted vertically, were generally harmless or even beneficial, whereas a cult like Aum Shinrikyo, transmitted horizontally, was lethal. This may explain the fanaticism sometimes shown by converts to a religion: some prominent Islamic terrorists have been converts.

Parasites not always bad

We generally think of parasites as something we want to avoid, but not all parasites are harmful to their hosts. They may be neutral in their effects or even beneficial, in which case they are usually referred to as symbionts. In fact, symbiosis is extremely common—probably much commoner than the production of disease. For example, termites have bacteria in their gut which are needed to allow the termites to digest cellulose. The bacteria are essential to termite life and the insects ensure their survival: when a growing termite sheds its skin it is immediately eaten either by the owner or by other members of the colony.

Symbiosis in humans

Whether symbiosis occurs in humans is less certain, but it probably does. There is a theory, admittedly not accepted by everyone, which holds that the remarkable increase in allergic diseases in many industrialised countries is related to our excessive hygiene. This reduces our childhood exposure to parasites. Young children who encounter a lot of viruses as a result of contact with other children are less likely to develop allergies later, and other parasites may have similar effects. Throughout most of our evolutionary prehistory we would have been infested with parasites, and our immune systems evolved to keep them under control though not to eliminate them. Perhaps, now that our immune systems find themselves with time on their hands, they start to misbehave, reacting excessively against things that they would normally have ignored, such as dead dust mites.

Some experimenters have tested such ideas on themselves. By deliberately infecting themselves with intestinal worms they have reduced the severity of their asthma. And there is currently a plan to infect patients suffering from multiple sclerosis with hookworm, since that parasite appears to reduce the tendency to relapse in this disease.

Can religion be a beneficial parasite?

If it is true that exposure to biological parasites—in moderation—in one's youth is protective against some diseases later in life, can we apply the same logic to religion? Perhaps it is good for us to be exposed to religion as children—always provided, of course, that it is not in too virulent a form. So fanatical religions may be bad for you, but less extreme varieties may be beneficial. Although it is disputed, some evidence seems to show that having a religion tends towards psychological and even physical health. Although a rigid fundamentalist upbringing may be psychologically damaging, perhaps youthful exposure to religious ideas, even if these are abandoned later, may be good for you.

Mitochondria as parasites

Symbiosis probably began very early in our evolutionary history. The mitochondria in our cells are structures—organelles, as they are rather poetically known, meaning tiny organs—that were almost certainly once free-living bacteria. At some time in the remote past they became permanent denizens of 'advanced' cells (that is, cell possessing a separate nucleus). They produce the energy which our cells need in order to work; we could not exist without them. But this was a later development in their function. Originally, they had a different job to do: detoxifying oxygen.

Today we think of oxygen as essential to life, but it was not always so. When oxygen first appeared on the planet it was a poison for most forms of life. But certain bacteria acquired the ability to process it and make it harmless. Later, some of these bacteria were incorporated into other kinds of cells, where they eventually became, not merely useful, but essential—not just as detoxifiers but as producers of energy. That is where our mitochondria came from and why we still have them today. They are symbionts. Now we cannot do without either mitochondria or oxygen. We would die without them, and there are diseases caused by malfunction of the mitochondria.

Religions: our psychological parasites?

Have religions become our psychological mitochondria—parasites that we now cannot do without? If so, what function do they serve? Perhaps it is to detoxify the fear of death. There is no general agreement about why religions exist, but one popular idea is that they help people to cope with the thought of death. Not everyone accepts this, but even if fear of death is not what brought about religion at its inception it is probably a major reason why it still exists today. We are presumably unique among species on the planet in having an awareness of our coming death, and Deacon thinks that this may be one of the most maladaptive consequences of the way our brains have developed. Most people try to put that awareness to the back of their minds and to distract themselves in all kinds of ways. But those who truly believe in a religion usually find at least a measure of consolation in their faith. Fear of death can be seen as a psychological toxin, and infection with the religion parasite may help us to detoxify it.

Parasites that induce religion?

To call religion a mind parasite is, at least partly, metaphorical. But there may be a completely literal way which parasitism—biological parasitism —is relevant to religion.

Parasites influence behaviour in various animals

Biological parasites can influence behaviour in animals and probably also in humans in remarkable ways. In her book, 'Riddled with Life', Marlene Zuk provides numerous examples of how this works. Here is just one among many.

There is a tiny parasitic wasp in the forests of Costa Rica which uses a certain type of spider as a host for its larva. The spider normally spins an orb-type web like the kind you can see in your garden in Britain. The wasp stings the spider, paralysing it temporarily, while it lays an egg on its abdomen. After about half an hour the spider recovers and carries on with its life as if nothing had happened. But during the next two weeks the wasp larva hatches out and sucks the spider's blood, until it is ready for the next stage in its career.

The night before the larva is due to form a pupa, the spider's behaviour changes. Instead of spinning its usual spiral web, it produces something like the top of a circus tent. In this the wasp pupa hangs upside down, preparing to hatch out and continue its life cycle by infecting another unfortunate spider.

This is too bad for the spider, but you may think that you have nothing to worry about. You are a long way removed from arachnids in evolutionary terms, after all. But wait: what about mammals? May these, too, may have their behaviour modified by parasites? Almost certainly, yes.

There is a well-known single-cell parasite called toxoplasma which infects predators, such as cats. Cats get it by eating infected rats and mice, who have in turn acquired it from the soil, where it arrived via cat faeces.

Rats have an innate fear of cats, for obvious reasons; it has been selected for in evolution. When exposed to the smell of cat urine, uninfected rats show a sensible aversion to it. Not so rats infected with toxoplasma. Describing the experiments in which this altered behaviour was demonstrated, Zuk says she found that watching a video of the infected rats as they wandered into an area sprayed with cat urine 'was like seeing the heroine in a horror movie open the door to the deserted barn while the maniac with the ax lurks behind it'.

Parasites affect human behaviour too?

So the toxoplasma parasite appears to be able to alter the rats' brains so as to make them behave in a foolhardy way, which is bad for the rats but good for the parasite. But now comes the really worrying bit. Human infection rates with toxoplasmosis range from 22 to 84 per cent in different countries. The parasite is acquired by eating undercooked infected meat. As a rule it does not seem to cause obvious harm in humans, though it can produce foetal abnormalities in pregnant women. But are there any effects on human behaviour?

It seems there may be. Infected people are more accident-prone. Men who are infected are more reserved, less trusting, and more likely to break rules. Women, in contrast, are more out-going, trusting, and self-assured. It is still not clear whether these differences are due to the toxoplasma or are personality features that make certain people more liable to infection. But if toxoplamosis is indeed responsible it suggests that humans, though not the 'intended' target, can show the same personality changes as do infected rats.

It is no doubt surprising, and potentially disturbing, to think that our personality may be in part the result of our parasites. But should it be? Many factors go to make us the kinds of people we are: our genes, our upbringing, our early experiences, to name just a few. If someone is better-natured as a result of infection with a parasite, is she really any different from another person whose pleasant temperament is the product of her genes? And would we wish to 'cure' the person if eliminating their parasite made them mistrustful and unsociable?

If toxoplasmosis can make people kinder, it doesn't seem out of the question that a parasite could make them religious. This might be an unknown infection or it might even be toxoplasma itself. It would be interesting to carry out a study to see if there is a connection between a religious temperament and infection with toxoplasma. If there is, the religion-as-parasite theory would be shown to be more literally correct than anyone had supposed! Religious believers would be horrified at the thought, but it doesn't seem to be out of the question.

Some implications for religion

Religion may be a form of mind parasite, as some secular critics believe, but if it is as deeply inscribed in our brains, and perhaps even in our nuclei, as the ideas I have described imply it is, the attempt to eliminate it by pointing out its illogicality and lack of credibility is unlikely to succeed. This is not necessarily, as many seem to believe, because religion has been selected for during evolution. Perhaps religion is simply a passenger, not usually doing us much harm but not much good either.

There is an evolutionary analogy for this idea. It is likely that at least some of our genes began life as viruses—parasites—and got written into our cell nuclei during evolution. But many genes don't appear to do anything. One of the most surprising discoveries in modern genetics is that the function, if any, of most of our genes is unknown. Only about 5 per cent of the human genome (the whole collection of genes that we inherit from our parents) makes proteins that are used by the organism. The other 95 per cent has no known function and has been called junk DNA, although this is no longer a very respectable scientific term, because perhaps it does have functions which we have not yet discovered. Some of these 'junk DNA' genes do make proteins, but these don't seem to do anything useful. The great majority of our DNA is designated non-coding, which means that it apparently does nothing—it is just a passenger. Perhaps religion is like this; perhaps it is hitching a ride in our brains but not contributing much, either good or bad. It is not essential to find a function for religion to explain its existence.

Like our genomes, our minds are not carefully designed instruments that function perfectly in all circumstances to give us objective truth, but rather a collection of cobbled-together parts that don't always work as we might expect. If some of the parts are really made from parasites that have take up residence in us, it is hardly surprising if their manifestations are sometimes rather difficult to understand.


  1. Chomsky N. (1972). Language and Mind. Pantheon.
  2. Cullen B. (1996). Parasite Ecology and the Evolution of Religion.
  3. Deacon T. (1997). The Symbolic Species: The Co-evolution of Language with the Human Brain. Allen Lane: The Penguin Press.
  4. Zuk, M. (2007) Riddled With Life: Friendly Worms, Ladybug Sex, And The Parasites That Make Us Who We Are. Harcourt.