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."