By stigmatized knowledge I mean claims to truth that the claimants regard as verified despite the marginalization of those claims by the institutions that conventionally distinguish between knowledge and error — universities, communities of scientific researchers, and the like. Although this definition encompasses rejected knowledge in both [James] Webb’s and [Colin] Campbell’s senses, it also includes a broader range of outsider ideas. The domain of stigmatized-knowledge claims may be divided into five varieties:
Forgotten knowledge: knowledge once allegedly known but lost through faulty memory, cataclysm, or some other interrupting factor (e.g., beliefs about ancient wisdom once possessed by inhabitants of Atlantis).
Superseded knowledge: claims that once were authoritatively recognized as knowledge but lost that status because they came to be regarded as false or less valid than other claims (e.g., astrology and alchemy).
Ignored knowledge: knowledge claims that persist in low-prestige social groups but are not taken seriously by others (e.g., folk medicine). Rejected knowledge: knowledge claims that are explicitly rejected as false from the outset (e.g., UFO abductions).
Suppressed knowledge: claims that are allegedly known to be valid by authoritative institutions but are suppressed because the institutions fear the consequences of public knowledge or have some evil or selfish motive for hiding the truth (e.g., the alien origins of UFOs and suppressed cancer cures).
Two characteristics of the stigmatized-knowledge domain require particular attention: the special place accorded to suppressed knowledge and the empirical nature of the claims. The suppressed knowledge category tends to absorb the others, because believers assume that when their own ideas about knowledge conflict with some orthodoxy, the forces of orthodoxy will necessarily try to perpetuate error out of self-interest or some other evil motive. The consequence is to attribute all forms of knowledge stigmatization to the machinations of a conspiracy. Conspiracy theories therefore function both as a part of suppressed knowledge and as a basis for stigmatization. At one level, conspiracy theories are an example of suppressed knowledge, because those who believe in conspiracy theories are convinced that only they know the true manner in which power is held and decisions made. The conspiracy is believed to have used its power to keep the rest of the populace in ignorance. At another level, conspiracy theories explain why all forms of stigmatized-knowledge claims have been marginalized—allegedly the conspiracy has utilized its power to keep the truth from being known. So the distinction between hidden knowledge on the one hand, which is “true,” and orthodoxy on the other, which is “false,” acts to push believers in stigmatized-knowledge claims toward beliefs about plots to suppress the truth, and hence in the direction of conspiracism.
– A Culture of Conspiracy: Apocalyptic Visions in Contemporary America (2013), pp. 26-7