Conspiracy theories have generally had a bad press. They conjure up images of eccentrics in tinfoil hats who believe that aliens have landed and the government is hushing up the news. And maybe it’s statistically true that most conspiracy theories belong on the harmless fringe of the credibility spectrum.
On the other hand, the historical record contains some conspiracy theories that have had profound effects. Take the “stab in the back” myth, widely believed in Germany after 1918, which held that the German army did not lose the First World War on the battlefield but was betrayed by civilians on the home front. When the Nazis came to power in 1933 the theory was incorporated in their revisionist narrative of the 1920s: the Weimar Republic was the creation of the “November criminals” who stabbed the nation in the back to seize power while betraying it. So a conspiracy theory became the inspiration for the political changes that led to a second global conflict.
More recent examples relate to the alleged dangers of the MMR jab and other vaccinations and the various conspiracy theories fuelling denial of climate change.