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.
That’s not exactly the way conspiracy theorists think. Either Barack Obama was actually born in Hawaii, or an international plot unfolded over multiple decades to conceal his Kenyan birthplace and install him in the presidency. Either vaccines are safe and effective, or every major hospital and health organization in the world is covering up the fact that they actually cause autism. Never mind the razor — conspiracy theories are nothing but extraneous assumptions.
The most common theories are the ones that follow the eddies of politics. As a broad rule, a party or group that’s out of power will be more inclined to believe in conspiracies than a group that’s in power.