With the measles outbreak at DisneyLand beginning in December of 2014, the debate on the need and safety of vaccines has been reignited. While I am not going to get into the science of vaccine safety and effectiveness, I am going to cover the psychology of the anti-vaccine movement, and some potential factors that lead to people denying scientific facts.
We tend not to think much about what we don't experience. For example, most of us (in the United States) take for granted that we have clean water to drink, adequate shelter, and the fact that we can criticize our President without getting shot. These are simply not issues that concern us and as a result, we spend very little cognitive energy on them. Likewise, we don't think about all the diseases we don't have thanks to vaccines, such as Smallpox, Diphtheria, Whooping cough, Measles, Neonatal tetanus, Hepatitis B, Tuberculosis, and Polio, but we do focus on the statistically rare problems we do see as a result of vaccines. Because of this, our mental calculation of the pros and cons of vaccines tends to be severely inaccurate.
Related to out of sight and out of mind, the availability heuristic is the mental shortcut one takes when forming an inaccurate view of reality based on what information is more readily available. For example, most people are more worried about dying in a plane crash than an automobile accident, even though by some estimates, you are 22 times more likely to die in an automobile accident. One of the reasons for this misplaced fear is the attention plane crashes get compared to automobile accidents. The more rare the event, the more attention (media, Hollywood, gossip, etc.) it is likely to get. Serious problems that are undoubtedly the result of a reaction to a vaccine are rare, a topic of great interest, and therefore both memorable and salient. It is this salience that leads to a highly biased evaluation of all the available information on vaccine safety, including the millions of cases of people who receive their vaccinations without any problems.
A good emotional plea can be far more effective than a collection of facts and accurate data. Charities know this secret well and don't bombard potential donors with statistics on how many people die of starvation, but rather provide a picture of a single starving child and give that child a name, with a plea to help this one child. The fact that vaccines have prevented hundreds of millions of people from contracting the measles is of much less emotional value than a story told by two grieving parents that anyone with heart would empathize, of one beautiful child named Amy who died as a result of a rare negative reaction to the vaccine . Emotional appeals have strong effects on our perceptions, but no effects on the facts.
Having children is both a wonderful, but terrifying process. Given all the possible things that can go wrong over which we have no control, those things which we can control comfort us. Autism Spectrum Disorder is a psychological condition in which there is no known cure and no known causes (despite what some may claim) that currently affects and estimated 1 out of 68 children. Believing one has control over autism can be a comforting thought, but it is nothing more than wishful thinking. The belief that the severity of autism can be reduced or even that autism can be prevented by foregoing a few shots contributes to motivated reasoning, or a method of reasoning based on one's motivations rather than facts.
The appeal to nature is a logical fallacy people make when they believe or suggest that "natural" is better than "unnatural," without evidence for the assertion. Besides the ambiguity involved in the terms, reality provides us with countless examples of things in nature that are deadly (viruses, volcanoes, many species of plants, etc.) and human-made things that are life-saving (dental care, climate control, surgery, etc.). Many who are against vaccines don't like the idea of putting "unnatural chemicals" or "toxins" in our bodies, by default associating the unnatural with "bad," regardless of any evidence supporting that association. This is simply fallacious reasoning.
The post-hoc fallacy is a common fallacy based on how we perceive the world. If one event proceeds another, we tend to make a causal attribution where none exists. Since the vast majority of vaccines (including MMR) are given before but close to the time when most psychologists could confidently diagnose a child with autism spectrum disorder (at around 18–24 months), it is understandable that parents' would be tempted to infer causality (i.e., the vaccine caused the autism). However, controlled studies are not subject to this bias, and the overwhelming scientific consensus shows no evidence for this causal connection.
Once people have a reason to hold an anti-vaccination position, whether it is because they believe that they are protecting their children from harm, seeking justice against an evil industry, or being a hero by exposing a government conspiracy, the confirmation bias kicks in, and information supporting their position is sought out and information that disconfirms their position (i.e., science) is ignored. Selective exposure magnifies this process by allowing people to customize their news and get information only from those sources that agree with their position.
This is not meant to be an exhaustive list of the decision process when it comes to the level of support one has for vaccines; it is meant to showcase just some of the common thoughts of generally rational thinkers that lead to irrational conclusions. While there are many people who are anti-vaccine that are simply unaware of the facts and being fed anti-scientific misinformation, there are also those who are aware of the science and because of motivated reasoning, argue their anti-vaccine position even stronger (known as the backfire effect). People can change their minds and may eventually accept the scientific facts of vaccine safety, but with this issue especially, time is a luxury we don't have.
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