In Understanding Power Noam Chomsky discusses conspiracy theories:
. . . the real question is, are there groupings well outside the structures of the major institutions of the society which go around, hijack them, undermine them . . . [etc]. . . do significant things happen because groups or subgroups are acting in secret outside the main structures of institutional power?
Well as I look over history, I don't find much of that. . . And if you look at the place where investigation of 'conspiracies' has absolutely flourished, modern American history, I think what's notable is the ABSENCE of such cases—at least as I read the record, they almost never happen.
Chomsky then goes on to assess various conspiracies such as JFK's assassination, etc, before concluding,
As soon as you look into the various theories, they always collapse, there's just nothing there. But in many places, the left has fallen apart on the basis of these sheer cults.
Few people on earth have a better working knowledge than Chomsky of the CIA and State Department's lifelong machinations all through Central and South America (supporting coups; training paramilitary terrorists; subverting democracy, etc, etc), yet Chomsky assesses conspiracy theories to be bunk. And this points to something much more disturbing than conspiracy theories; namely, reality. There's a lot we do know. The truth is out there. But no one gives a fuck, because it would require acknowledging the reality of American power and its stake in global capitalism.
Conspiracy theories are more trim and tidy than the above. This is their irony and their appeal.
In social theory Fredric Jameson has suggested,
Conspiracy is the poor person's cognitive mapping in the postmodern age; it is a degraded figure of the total logic of late capital, a desperate attempt to represent the latter's system.
Jameson suggests that conspiracy theories are an attempt to account for events within the fragmented experience of modern life. Conspiracies provide an illusion of identity and location for the theorist (I/we are being fucked with here in this way) and attempt to capture or localize power, extracting it from the ever more circuitous and opaque systems through which it operates. I don't know how far to take this kind of social psychoanalysis, but Jameson sees conspiracy theories as a quasi-hysterical and symptomatic reaction to the aporias of (post)modernity.
Jameson is a very smart guy and a Marxist, and he too has no truck with conspiracy theories.
Conspiracy theories are perhaps the one area of understanding power on which people as diverse as Chomsky, Jameson, and Foucault are likely to agree: crap.