Cass Sunstein and Adrian Vermeule have posted an excellent new paper on belief in conspiracy theory. As they point out, belief in highly dubious conspiracy theories about key political events is widespread. For example, they cite survey data showing that some one third of Americans believe that federal government officials either carried out the 9/11 attacks themselves or deliberately allowed them to happen. Large numbers of people also believe that John F. Kennedy's assassination was the result of a wideranging conspiracy in the government, that the AIDS virus was secretly produced in a government laboratory for the purpose of infecting blacks, and that the government is covering up evidence of alien visitation of Earth.

Why are such irrational beliefs so widespread in an open society where information refuting them is easily accessible? Sunstein and Vermeule present some possible answers. But they fail to consider a crucial question: Why is belief in bogus political conspiracies so much more widespread than comparably irrational beliefs about conspiracies in our daily lives? Far more people believe that the CIA killed Kennedy or engineered the 9/11 attacks than believe that a dark conspiracy is out to get them personally or that their associates and co-workers are plotting against them. Millions of people who embrace absurd conspiracy theories about political events are generally rational in their everyday lives.

In my view, the disjunction has to do with the rationality of political ignorance. As I describe in more detail in several of my works (e.g. - here and here), it is perfectly rational for most people to know very little about politics and public policy - and indeed most people are quite ignorant about even basic aspects of these subjects. Because the chance of your vote influencing the outcome of an election is infinitesmally small, there is little payoff to becoming informed about politics if your only reason for doing so is to be a better voter. By contrast, there are very strong incentives to be well-informed about issues in our personal and professional lives, where our choices are likely to be individually decisive. The person who (falsely) believes that a dark conspiracy is out to get him will impose tremendous costs on himself if he bases his decisions on that assumption; he's likely to end up a paranoid recluse like Bobby Fischer (who, of course, embraced political conspiracy theories as well).

In the political realm, on the other hand, widespread rational ignorance helps to spread conspiracy theory in two ways. First, the more ignorant you are about politics and economics, the more plausible simple conspiracy theory explanations of events are likely to seem. If you don't understand basic economics, you are more likely to believe that rising oil prices are caused by a conspiracy among oil companies or that the subprime crisis was caused by a conspiracy among banks. If you don't understand the basic workings of our political system, you are more likely to swallow the idea that the federal government could carry out something like the 9/11 attack and then (falsely) blame it on Osama Bin Laden without the truth being quickly exposed through leaks and hostile media coverage.

Second, the rationality of political ignorance implies that even people who do have considerable knowledge are likely to be more susceptible to conspiracy theories about political events than in their personal lives. As I explain in this paper (see also Bryan Caplan's excellent book), the rationality of political ignorance not only reduces people's incentives to acquire political information, it also undercuts incentives to rationally evaluate the information they do learn. As a result, we are more likely to be highly biased in the way we evaluate political information than information about most other subjects. Many people embrace political conspiracy theories because they are more entertaining and emotionally satisfying than alternative, more prosaic explanations of events. Unlike in our nonpolitical lives, most people have little incentive to critically evaluate their political beliefs in order to weed out biases and and ensure their truth.

That is not to say that people are uniformly rational in their nonpolitical decisions. Far from it. But they are likely to be a great deal less irrational than they are about politics.