What causes people to believe in conspiracy theories?



I'm sorry if this post doesn't seem that high quality, but I do feel this might be the best place to ask. The point of this post is to inspire discussion, hopefully discussion that might be useful for answering certain questions I had.

On another board, I gathered evidence of the existence of  "mainstream" conspiracy theories with the goal of figuring out why those conspiracy theories are, well, mainstream. Part of the problem is that, because they're mainstream, many people here may believe in them and may even contest the idea that they are even conspiracy theories. I don't really want to get into arguments over if a conspiracy theory is true or not, so just remember "Politics Is The Mindkiller".

1) JFK was assassinated in a conspiracy. (75% of Americans believe this according to a 2003 Gallup poll.)

2) Martin Luther King, Jr. was assassinated in a conspiracy. (58% of Americans believe this according to a 2008 CNN/Essence poll.)

3) Bush lied about WMDs. (43% of Americans according to a 2005 Pew Survey, only 41% disagreed with this statement, according to a 2005 Pew Survey.)

4) No international consensus on who did 9/11, with 49% of Mexicans, 66% of Egyptians, 40% of Turks, 52% of Jordanians and 55% of Palestinians naming a suspect other than al-Qaeda. This is from a 2008 World Public Opinion poll (graph below).

It's clear that at least some conspiracy theories are treated as mainstream in at least some polities, but other conspiracy theories, like "Americans hoaxed the moon landing" are fringe (only 6% of Americans believe this, according to a 2001 Gallup poll [link here]). In fact, many bloggers, including the economist Robin Hanson, labor under the idea that all conspiracy theories are fringe and wonder why are these individuals so different from the "mainstream". So here's two questions that I would like answered, because these results had been bugging me:

1) There is some sort of method by which an individual can 'filter' out the false and "fringe" conspiracy theory while then selecting a 'true' and "mainstream" conspiracy theory to be accepted. What factors play into an individual's decision-making process to determine what conspiracy theories to accept and what to reject?

2) Is the process of believing in conspiracy theories impacted by some form of rationality? Does Bayesian logic plays a role here as well...do individuals unconsciously rate the likelihood of a conspiracy theory and accept conspiracy theories with a high probability of it occurring (while rejecting conspiracy theories with a low probability of it occurring)?

Addendum: (I tried to select examples that could generally be agreed to be "conspiracy theories" to avoid arguments over definitions that I'd lose, but I may have failed in this sort of thing. To reveal bias, I believe that a conspiracy theory is a hypothesis about a covert plot by more than one individual.)