"Conspiracy theory" is usually used as a pejorative label, meaning paranoid,
nutty, marginal, and certainly untrue. The power of this pejorative
is that it discounts a theory by attacking the motivations and mental
competence of those who advocate the theory.
By labeling an explanation of events "conspiracy theory,"
evidence and argument are dismissed because
they come from a mentally or morally deficient personality,
not because they have been shown to be incorrect.
Calling an explanation of events "conspiracy theory" means, in effect,
"We don't like you, and no one should listen to your explanation."
In earlier eras other pejorative labels, such as "heresy," "witchery,"
and "communism" also worked like this.
The charge of "conspiracy theory" is not so severe as these other labels,
but in its way is many times worse. Heresy, witchcraft, and communism
at least retain some sense of potency. They designate ideas to be feared.
"Conspiracy theory" implies that the ideas and their advocates
are simple-minded or insane.