I just attended a conference that was both illuminating and depressing. It was co-sponsored by the Center for Strategic Studies at the University of Jordan and the American Enterprise Institute, and the idea was to get Americans and moderate Arab reformers together to talk about Iraq, Iran, and any remaining prospects for democracy in the Middle East.
As it happened, though, the Arab speakers mainly wanted to talk about the Israel lobby. One described a book edited in the mid-1990s by the Jewish policy analyst David Wurmser as the secret blueprint for American foreign policy over the past decade. A pollster showed that large majorities in Arab countries believe that the Israel lobby has more influence over American policy than the Bush administration. Speaker after speaker triumphantly cited the work of Stephen Walt, John Mearsheimer and Jimmy Carter as proof that even Americans were coming to admit that the Israel lobby controls their government.
It seems that a significant reason that people believe conspiracy theories, whether about "Big Pharma," 9/11, the Bush Administration, the "Israel Lobby," or anything else, is that the theories in question provide what appear to be "rational" support for views that were already deeply held from "the gut". And it doesn't hurt if the conspiracy theory advances the believers' self-interest.