William of Occam would have hated conspiracy theories. A 14th-century philosopher and Franciscan friar, William is celebrated for developing the “law of parsimony,” better known today as “Occam’s razor.” According to the razor principle, the simplest explanation for an event is almost always the best; shave away any extraneous assumptions, and what you’ve got left is usually the truth.
The question is, why do so many people believe in them? One reason we are so keen to believe in conspiracy theories is that we are social animals and our status in that society is much more important than being right. Consequently, we constantly compare our actions and beliefs to those of our peers and then alter them to fit in. This means that if our social group believes something, we are more likely to follow the herd. If more people believe a piece of information, then we are more likely to accept it as true. In short social proof is a much more effective persuasion technique than purely evidence-based proof. Social proof is just one of a host of logical fallacies that also cause us to overlook evidence. A related issue is the ever-present confirmation bias, that tendency for folks to seek out and believe the data that supports their views while discounting the stuff that doesn’t. Confirmation bias also manifests as a tendency to select information from sources that already agree with our views (which probably comes from the social group that we relate too).