Let's talk world conspiracy theory
The basic idea: the whole world is managed by a small group of people (who maybe are not even human but alien). This theory, called world conspiracy, gives these individuals full control over all resources and politica activities of the planet.
World conspiracy theory is not harmless
Believing in the world conspiracy theory may seem completely harmless, indeed it is very insidious because of the damage caused to education. According to this theory, in fact, nobody can be believed: neither to teachers, nor to scientific research, nor to doctors' advice, not even to themselves (we could be controlled without our knowledge).
Studies show that those who believe in the conspiracy theory tends to believe that AIDS is part of a conspiracy for the control of the world population and therefore tend not to adhere to therapeutic programs or preventive measures, as well as those who think that pharmaceutical companies are hiding evidence that vaccines cause autism will tend to refuse vaccinations more frequently.
Conspiracy theories linked to less civic engagement
More generally, conspiracy theories are associable to a diminished civic commitment and sometimes to prejudices, racism, radicalism and violence. The fact that the scientist and mathematician David Grimes has long ago obtained a formula which shows how a world conspiracy is impossible, since it would have been discovered a long time ago, seems to serve a little.
According to his calculations, for example, a possible “staging” of the Moon landing could have remained secret only 3.7 years, while the “conspiracy” against the realization of a vaccine to fight cancer no more than 3.2 years.
Also heuristic representativeness weighs
As demonstrated Tversky and Kahneman, also heuristic representativeness (that is intuitive evaluation based on the temporary state of the circumstances) can play a role in the joint error, a specific probabilistic reasoning error based on which typically from 50% to 90% people overestimate the likelihood of concurrent events.
In short: rationally there is no evidence of the existence of any world conspiracy of any kind, indeed there are opposing proofs of impossibility that such a conspiracy exists. Nonetheless many still believe in it, trusting one's instinct more than rationality.