I can think of two common and closely related patterns of delusional conspiracy theories that are nevertheless plausible to great numbers of people. (These are not the only ones, of course.)
The first pattern occurs when large groups of people express beliefs and perform acts that are systematically biased in a certain direction and effectively further some clearly recognizable interests and goals, and yet there is no consciously organized coordination among them. Such situations often occur because humans are instinctively attracted towards opinions and acts that signal high status and affiliation with prestigious groups. Yet for many people it's hard to believe that such spontaneous decentralized coordination is possible, so they seek explanation in conspiracy theories.
So, for example, the mainstream media often exhibit strong and practically uniform bias in their coverage of politicians, events, and issues. Thus, for many people it's tempting to assume that they are indeed under some centralized editorial command. Depending on their ideological inclinations, they may imagine these commands coming from boardrooms of nefarious media tycoons, or perhaps (for even loonier types) from scheming cabals of Jews. Yet in reality, even the strongest and most uniform biases simply reflect those beliefs that are considered high-status among the sort of people that tend to work in the media -- and the prevalence of this sort of people is itself not a matter of any conspiracy or centralized control, but a spontaneously originating and self-perpetuating artifact of normal human social behavior.
The second, closely related pattern occurs when people detect correctly that they are getting cheated, lied to, or screwed over by the elites in charge, but are unable to analyze accurately what's going on. (These actions may actually be conspiratorial to some degree, but more often it's a matter of spontaneous coordination as in the above example.) In many instances, an accurate analysis of the situation would be too complicated for the average person, while intellectuals risk their own status if they realize the truth and speak it openly (attacking the elite opinion is by definition a status-lowering move). Thus, people end up creating conspiracy theories that have some vague kernel of truth in them, but are a gross and simplistic caricature of what's really going on. In turn, intellectuals are happy to use these laughable caricatures as convenient strawmen for attacking their more sophisticated critics.
For example, the conspiracy theories that flourished in the Second Red Scare (a.k.a. "McCarthyist") period in the U.S. were a consequence of a very real record of misdeeds, blunders, covert dealings, ideological delusions, and sometimes even real espionage and treason among the American political and intellectual elites. Yet the popular awareness of these problems found its expression in naive and grotesque grand conspiracy theories à la General Ripper, which were a ridiculous caricature of the complicated and bizarre reality -- in which actual conspiracies were far from absent, but a relatively minor factor in the big picture, which was as usual determined by the regular human patterns of social behavior and status-seeking, and the systematic biases and spontaneously coordinated actions following from them.
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.)