Cognitive Dissonance

Ryan Sager, in his excellent blog, Neuroworld, explains how a belief unsupported by any credible evidence (in this case, the belief that vaccines cause autism) forms and perpetuates despite overwhelming evidence to the contrary. I encourage you to read the whole thing. Here he is explaining why people won't give up their previously held beliefs in the face of contradictory evidence:

When the evidence comes in, though, shouldn’t these folks change their minds?

...Unfortunately, that’s not how humans work. I explained in another post the story of Marian Keech and the Seekers. In short, Keech headed up a UFO cult in the 1950s. Earth was supposed to be destroyed, and she and her followers were supposed to be rescued by a flying saucer on December 21. The day came and went, but no destruction, no UFO. Instead of giving up their beliefs, however, most of the Seekers glommed onto a new narrative — that the Seekers’ belief had saved the Earth — and began to try to win converts.

The ones who redoubled their commitment were the ones who’d invested the most in the theory — quitting their jobs, selling their houses. The UFO not showing up created a feeling of what’s been termed “cognitive dissonance.” How could I have given up my job if there’s really no UFO? The answer their brains came up with: Because what I did saved the world!

Cialdini calls this phenomenon "consistency drive." The idea is that humans are really bad at admitting when they are wrong, so the more they invest in a belief, the greater than belief will be. If one has invested a lot in a belief, the consequences of being wrong are so bad that people will willfully blind themselves to the truth. They teach you about "sunk cost" in business school to fight against this impulse. When people have contradictory beliefs, Instead of abandoning one of them, people will come up with ridiculous explanations for how the beliefs are not actually contradictory, and then try not to think about it too hard.

In fact, thinking about it too hard can restart the process all over, and lead one to continue manufacturing ridiculous explanations, until one abandons the need for explanations altogether. Almost every conversation I have with a person of faith ends in some variation of "reason isn't everything." This is just a nicer-sounding way of saying "I don't have a good reason to believe what I believe." In a rational world, saying that sort of thing would be equivalent to conceding the point. If a person doesn't have a good reason to believe something, s/he should stop believing it. But people have invested so much in their beliefs that they can't do that. Instead, they convince themselves that reasons aren't neccessary. And thus begins the total abandonment of rationality.