- A new flu pandemic has appeared that's especially nasty. About 1% of all people who contract it will die. (This is not unrealistic because such flu pandemics have happened, at even higher mortality rates.) You are pretty much guaranteed to get the flu unless you live a really isolated life.
- A vaccine is developed that prevents contracting the flu completely.
- Later it's discovered that approximately 0.1% of people get a serious chronic disease from the vaccine.
The highly contradictory reaction that many people have to this becomes even clearer if we assume two alternative hypothetical situations:
- Instead of the vaccine giving you a chronic disease (with a 0.1% probability) it simply is slightly ineffective in that it simply reduces the probability of contracting the flu to 10%. This means that if you take the vaccine, your probability of dying from the flu drops from 1% to 0.1%.
- The vaccine protects you from the flu completely, but it has a 0.1% probability of killing you outright.
If you think about it, that kind of thinking makes no sense. The two situations are completely identical: If you don't take the vaccine you have a 1% chance of dying, and if you take it, you have a 0.1% probability of dying. It doesn't matter what exactly is it that kills you, the probability is still exactly the same. Yet in the first case people would not have a problem taking the vaccine but in the second case they do.
Why do so many people prefer taking a 1% chance of dying instead of taking a 0.1% chance of dying (or even just getting a disease) in one situation but not the other?
Also, in the original hypothetical situation some people could argue that they would prefer death over a chronic disease. However, that explanation doesn't make sense because they are effectively ready to commit suicide by not taking the vaccine (with a 1% probability), but not if they take the vaccine and get the disease. Such people are ready to die from the flu (which is effectively suicide) but not ready to commit suicide if they contract the disease from the vaccine (even though the chances of having to commit it are much smaller.) Why? What's the difference?
This is, I believe, caused by a very primitive underlying psychological phenomenon related to a very instinctive aversion to assigning fault and blame to oneself. It's the difference between harming oneself actively vs. harming oneself passively, by inaction.
In other words, "if I die from the flu, it's not something that I actively did to myself, therefore it's not my fault." However, "if I contract a disease or die from the vaccine, it will be because of something I actively did to myself, and therefore it's my own fault."
There's a curious, deep-ingrained need in the human psyche to avoid doing anything that can be blamed on oneself. It's preferable to take a higher risk of self-harm by inaction ("not my fault") than a smaller risk of self-harm by actively doing something ("it's my fault".)
Deep down, when we get to the very bottom of the psychology behind it, it's all about shame, and avoiding it at all costs. ("It's more shameful if I harm myself via something I did than if I receive some harm by something that I didn't do to myself directly.")
This, of course, doesn't make much sense, and in these situations it's quite detrimental. It causes people to take unneeded risks that could be avoided or greatly diminished, and they can't even fully explain why they are taking these unneeded risks. And no amount of explaining probabilities is going to help. (Of course most people will come up with excuses, but they don't even themselves understand the true underlying psychological reason for their behavior. In fact, some people go to incredible lengths to rationalize these excuses, up to a point of it becoming outright ridiculous.)
This is precisely the reason why certain claims, for example about vaccines, trump any amount of actual scientific research. For instance, you can have hundreds and hundreds of extensive clinical trials with thousands and thousands of test subjects, hundreds of published and peer-reviewed papers, and the basically unanimous agreement of the scientific community that vaccines are safe... and all that's needed to trump all that is a couple of nobodys on the internet claiming that vaccines are dangerous. They don't need actual evidence, they don't need actual research, they don't need actual clinical trials. The mere claim that they are dangerous is enough to completely nullify the enormous amount of actual research done by actual scientists.