"Even someone whose ideas are deviant can produce a Web site that looks sleek and professional," said Barkun. "Those who have conspiracy theories to peddle can do it very easily. And if they're reasonably sophisticated, [they] can do it in a way that gives those ideas the appearance of validity."
Whether those who make conspiracy theories for the masses are themselves mentally ill, Barkun said it depends on the conspiracy theorist.
"I don't think you can generalize," he said. "Certainly, in terms of conspiracy theorists, they're all quite different."
As for Icke, he has long heard people accusing him of being a mentally-disturbed conspiracy theorist.
"I went out on a limb," he said. "What's happened within a period of 20 years is that the world has come to me. ... As people have seen it coming to pass, the laughter has stopped.
"I take a different view," he added, "and if that's at odds with mainstream society to the point of being crazy, then I'm fine with that."