Believers in conspiracy theories are usually written off immediately as weirdos and idiots, but as a new survey by YouGov for Statista reveals, in some cases, these are actually widely-held beliefs and far from the bizarre, fringe opinion that you might have come to expect.According to the survey, one of the most commonly believed conspiracy theories among U.S. adults is that Lee Harvey Oswald did not act alone in the assassination of JFK - 47 percent believe either strongly or somewhat that there was in fact another shooter behind the grassy knoll. President Trump's oft-touted theory of the "deep state" has also to a fair degree made it into common discourse, with 29 percent believing it to some extent.Meanwhile, what might be considered as the 'mother of all conspiracy theories' - that the 1969 moon landing was faked - seems to have fallen out of favour, with only 11 percent getting behind the idea 50 years on.