Why do people hold so tightly to ideas that are not supported by the data?

This morning I was listening to a radio story about measles and how there is a measles outbreak somewhere in the US because parents refused to allow their children to be vaccinated. Some people believe that kids should get measles in order to boost their natural immunity, believing that measles is a fairly harmless disease. Others think that vaccinations cause autism.

Both of these ideas have no support in the scientific literature. I believe a lot of it comes through “alternative” sources. Some might call it new age thinking. In any case, the belief is harmful because it allows a disease we had eliminated in the US to spread again, and to hurt and possibly kill other people’s children. Yes. Measles can kill. It’s not as benign as people think. Not being vaccinated potentially has serious consequences for a large population.

I’d like to understand better where these beliefs come from and why they are so compelling that people maintain the beliefs in the face of overwhelming scientific evidence to the contrary. I’m not really interested in discussing the science behind such beliefs, except insofar as it explains why people believe these things so fervently. There are probably other examples of this kind of belief, and if you can think of some, please add them to the discussion.