What are we talking about?
A belief that some covert but influential organization is responsible for an unexplained event.
Why do people believe conspiracy theories?
Often times, things happen in the world that are difficult for us to comprehend because they have no obvious meaning or causality. These events are typically quite frightening either because they represent a direct danger to us, or happen so infrequently that we question whether or not it could (or should) happen at all.
Leon Festinger (1957) proposed cognitive dissonance theory, which states that a powerful motive to maintain cognitive consistency can give rise to irrational and sometimes maladaptive behavior. According to Festinger, we hold many cognitions about the world and ourselves; when they clash, a discrepancy is evoked, resulting in a state of tension known as cognitive dissonance. As the experience of dissonance is unpleasant, we are motivated to reduce or eliminate it, and achieve consonance (i.e. agreement).
The following is from an article in Scientific American, written by Michael Shermer;
Why do people believe in highly improbable conspiracies? In previous columns I have provided partial answers, citing patternicity (the tendency to find meaningful patterns in random noise) and agenticity (the bent to believe the world is controlled by invisible intentional agents). Conspiracy theories connect the dots of random events into meaningful patterns and then infuse those patterns with intentional agency. Add to those propensities the confirmation bias (which seeks and finds confirmatory evidence for what we already believe) and the hindsight bias (which tailors after-the-fact explanations to what we already know happened), and we have the foundation for conspiratorial cognition.
Are conspiracy theories always wrong?
Nope. Julius Caesar, Abraham Lincoln and Franz Ferdinand (Austro-Hungarian prince), just to name a few, were all assassinated by groups of conspirators. Governments have been in on it too – here’s a list of amazing-but-true projects that the US Government has undertaken. (from cracked.com)
Even more here.