Christopher French, a professor of psychology at Goldsmiths, University of London, explains:
Although conspiracy beliefs can occasionally be based on a rational analysis of the evidence, most of the time they are not. As a species, one of our greatest strengths is our ability to find meaningful patterns in the world around us and to make causal inferences. We sometimes, however, see patterns and causal connections that are not there, especially when we feel that events are beyond our control.
The attractiveness of conspiracy theories may arise from a number of cognitive biases that characterize the way we process information. “Confirmation bias” is the most pervasive cognitive bias and a powerful driver of belief in conspiracies. We all have a natural inclination to give more weight to evidence that supports what we already believe and ignore evidence that contradicts our beliefs. The real-world events that often become the subject of conspiracy theories tend to be intrinsically complex and unclear. Early reports may contain errors, contradictions and ambiguities, and those wishing to find evidence of a cover-up will focus on such inconsistencies to bolster their claims.
“Proportionality bias,” our innate tendency to assume that big events have big causes, may also explain our tendency to accept conspiracies. This is one reason many people were uncomfortable with the idea that President John F. Kennedy was the victim of a deranged lone gunman and found it easier to accept the theory that he was the victim of a large-scale conspiracy.
Another relevant cognitive bias is “projection.” People who endorse conspiracy theories may be more likely to engage in conspiratorial behaviors themselves, such as spreading rumors or tending to be suspicious of others' motives. If you would engage in such behavior, it may seem natural that other people would as well, making conspiracies appear more plausible and widespread. Furthermore, people who are strongly inclined toward conspiratorial thinking will be more likely to endorse mutually contradictory theories. For example, if you believe that Osama bin Laden was killed many years before the American government officially announced his death, you are also more likely to believe that he is still alive.
None of the above should indicate that all conspiracy theories are false. Some may indeed turn out to be true. The point is that some individuals may have a tendency to find such theories attractive. The crux of the matter is that conspiracists are not really sure what the true explanation of an event is—they are simply certain that the “official story” is a cover-up.