Pardon my re-post, but it’s relevant:

Conspiracy theories become “contagious” when seemingly respectable authority figures endorse them. The opinions of politicians, authors, preachers or celebrities can have authority for susceptible people. Their endorsement of a conspiracy theory authorizes the believer to accepting the delusion.

The believer, being in on the “truth” now possesses a special secret power which provides a sense of security previously lacking. This need for security explains the reluctance of a believer to give up the conspiracy theory in the face of fact, reason and evidence. The believer initially felt powerless and insecure. The conspiracy theory explained the danger in a way which made the believer feel powerful and secure. Any fact which contradicts the conspiracy theory is therefore seen as a direct and dangerous threat to the psyche of the believer. This is how contrary evidence is perceived as further proof of the conspiracy.

To insist upon the veracity of contrary factual evidence will elicit a fear/threat response in the believer. Point out that fire does indeed melt steel, or that Obama’s Hawaiian birth certificate is a real birth certificate: it does not get the conspiracy theory believer to open his mind and consider other possibilities. Rather, it threatens and enrages them into still greater levels of irrationality.