I've just read (via the NZ Skeptics page on FB) a fascinating article on Slate about the psychology of conspiracy theorists. In it, Will Saletan describes a series of studies from the past 20 years, that attempted to understand why a fair proportion of people seem to incline towards conspiracy theories (for example, a 2007 poll found that only 64% of adults in the US believed that the 9/11 attacks caught their government off-guard: most of the remainder believed that the powers-that-be either knew in advance or were actually actively involved).
The experimental data Saletan discusses seem to show that the distrust (or at the very least, cynicism) that many participants demonstrated is based on how those participants perceived the character of others:
... it's a common weakness known as the fundamental attribution error - ascribing others' behaviour to personality traits and objectives, forgetting the importance of situational factors and chance. Suspicion, imagination, and fantasy are closely related.
He goes on to say that
The more you see the world this way - full of malice and planning instead of circumstance and coincidence - the more likely you are to accept conspiracy theories of all kinds. Once you buy into the first theory, with its premises of coordination, efficacy, and secrecy, the next seems that much more plausible
and presents additional data to support that contention. (Orac has also written about this from time to time.)
I have to say, some of the anti-fluoride commenters we get on Making Sense of Fluoride certainly appear to fall into the fundamental attribution error. How else can one interpret the assumptions that fluoridation is the result of one big (global) conspiracy theory (linked, in the minds of at least some commenters, with a nebulous depopulation program), and that those discussing the science in favour of fluoridation are naturally being paid to do so.
And given that at least some groups who oppose water fluoridation (with statements such as "The problem is that the research and information is used to educate medical practitioners is completely false, they have literally been brainwashed when it comes to fluoride") also oppose vaccination (with talk of hoaxes), then I have to agree with Saletan that
Conspiracy believers are the ultimate motivated skeptics. Their curse is that they apply this selective scrutiny not to the left or right, but to the mainstream. They tell themselves that they're the ones who see the lies, and the rest of us are sheep. But believing that everybody's lying is just another kind of gullibility.