Michael Barkun's book "A Culture of Conspiracy" traces the history of certain UFO and "New World Order" conspiracy theories. He finds that these theories and communities were originally distinct, but that certain bookshops and magazines would sell/advertise books of both genres. He traces, over the years, the gradual synthesis as ideas from one milieu start to invade another until both communities routinely refer to both Area 51 and black helicopters as part of the same conspiracy canon, for example. He describes the process as Improvisational Millennialism, where people select from existing conspiracy theories to invent their own synthesis. But largely posits that the synthesis as driven by the theories sharing the same transmission channels.