Why, despite all the evidence to the contrary, do so many people remain adamant in their belief that vaccines are responsible for harming hundreds of thousands of otherwise healthy children? Why was the media so inclined to air their views? Why were so many others so readily convinced? Why, in other words, are we willing to believe things that are, according to all available evidence, false?…

…the granddaddy of all cognitive biases, kicks into action — which is to say, it’s at the precise moment when we should be looking for reasons that we might be wrong that we begin to overvalue any indication that points to our being right. Misapprehensions about medicine are particularly vulnerable to the effects of confirmation bias, because the process by which a given intervention works is so often contra-logical: It makes no intuitive sense that re-breaking a bone would help a fracture to heal or that using chemotherapy to kill living tissue would help a person survive cancer. Now consider vaccines. Injecting a healthy child with a virus in order to protect him from a disease that has all but disappeared just feels somehow wrong. The fact that getting vaccinated is so obviously painful and that infants can’t understand that you want to help and not hurt them only makes matters worse.