Anti-vaccination beliefs can cause real, substantive harm, as shown by the recent outbreak of measles in the US. These developments are as shocking and distressing as their consequences are predictable.
But if the consequences are so predictable, why do the beliefs persist?
It is not simply that anti-vaxxers don't understand how vaccines work (some of them may not, but not all of them). Neither are anti-vaxxers simply resistant to all of modern medicine (I'm sure that many of them still take pain killers when they need to). So the matter is not as simple as plain stupidity. Some anti-vaxxers are not that stupid, and some stupid people are not anti-vaxxers. There is something more subtle going on.
We all have what psychologists call "folk" theories, or "naïve" theories, of how the world works. You do not need to learn Newton's laws to believe that an object will fall to the floor if there is nothing to support it. This is just something you "know" by virtue of being human. It is part of our naïve physics, and it gives us good predictions of what will happen to medium-sized objects on planet earth.
Naïve physics is not such a good guide outside of this environment. Academic physics, which deals with very large and very small objects, and with the universe beyond our own planet, often produces findings that are an affront to common sense.
As well as physics, we also have naïve theories about the natural world (naïve biology) and the social world (naïve psychology). An example of naïve biology is "vitalistic causality" - the intuitive belief that a vital power or life force, acquired from food and water, is what makes humans active, prevents them from being taken ill, and enables them to grow. Children have this belief from a very young age.
Naïve theories of all kinds tend to persist even in the face of contradictory arguments and evidence. Interestingly, they persist even in the minds of those who, at a more reflexive level of understanding, know them to be false.
In one study, adults were asked to determine, as quickly as possible, whether a statement was scientifically true or false. These statements were either scientifically true and naïvely true ("A moving bullet loses speed"), scientifically true but naïvely false ("A moving bullet loses height"), scientifically false but naïvely true ("A moving bullet loses force"), or scientifically false and naïvely false ("A moving bullet loses weight").
Adults with a high degree of science education got the questions right, but were significantly slower to answer when the naïve theory contradicted their scientific understanding. Scientific understanding does not replace naïve theories, it just suppresses them.
As ideas spread through a population, some stick and become common, while others do not. The science of how and why ideas spread through populations is called cultural epidemiology. More and more results in this area are showing how naïve theories play a major role in making some ideas stickier than others. Just as we have a natural biological vulnerability to some bacteria and not others, we have a natural psychological vulnerability to some ideas and not others. Some beliefs, good and bad, are just plain infectious.
Here is an example. Bloodletting persisted in the West for centuries, even though it was more often than not harmful to the patient. A recent survey of the ethnographic data showed that bloodletting has been practiced in one form or another in many unrelated cultures, across the whole world.
A follow-up experiment showed how stories that do not originally have any mention of bloodletting (for instance, about an accidental cut) can, when repeated over and over again, become stories about bloodletting, even among individuals with no cultural experience of bloodletting.
These results cannot be explained by bloodletting's medical efficiency (since it is harmful), or by the perceived prestige of western physicians (since many of the populations surveyed had no exposure to them). Instead, the cultural success of bloodletting is due to the fact that it chimes with our naïve biology, and in particular with our intuitive ideas of vitalistic causality.
Bloodletting is a natural response to a naïve belief that the individual's life force has been polluted in some way, and that this pollution must be removed. Anti-vaccination beliefs are a natural complement to this: vaccinations are a potential poison that must be kept from the body at all costs.
At an intuitive, naïve level we can all identify with these beliefs. That is why they can satirised in mainstream entertainment.
In Stanley Kubrick's great comedy Dr. Strangelove, the American general Jack D. Ripper explains to Lionel Mandrake, a group captain in the Royal Air Force, that he only drinks "distilled water, or rainwater, and only pure grain alcohol", because, he believes, tap water is being deliberately infected by Communists to "sap and impurify all of our precious bodily fluids". The joke works because Ripper's paranoia is directed at something we all recognise: the need to keep our bodies free from harmful, alien substances. Anti-vaxxers think they are doing the same.
Thom Scott-Phillips is a Research Fellow, Cognitive and Evolutionary Anthropology at Durham University.