Marion Cotillard is a French actress who won some kind of award recently. I mention this because back in 2006 she gave an interview on French television (see the U.K. Telegraph story here ) in which she said about the September 11 terrorist attacks, “I think we’re lied to about a number of thing. We see other towers of the same kind being hit by planes, are they burned? There was a tower, I believe it was in Spain, which burned for 24 hours. It never collapsed. None of these towers collapsed. And there [in New York], in a few minutes, the whole thing collapsed.” According to Ms. Cotillard, the Twin Towers were losing money, and it was cheaper to blow them up in a phony terrorist attack than to modernize them.

In one of those little gifts that life sometimes gives us, Ms. Cotillard went on to deliver her views on the United States space program. “Did a man really walk on the moon?” Ms. Cotillard asked. “I saw plenty of documentaries on it, and I really wondered. And in any case I don’t believe all they tell me, that’s for sure.”

It’s fun, of course, to laugh at people like Ms. Cotillard (who has, in any case, recently has recanted her remarks ), but it’s not charitable, and so I do in fact have a serious point here, which is this: although it’s obvious that Ms. Cotillard’s views are crazy, it’s far from obvious just why they’re crazy. That is, almost none of us have any direct, personal knowledge about the terrorist attacks of September 11. Even people like me, who watched the towers burn with my own eyes, have no personal knowledge of how the attacks were carried out and by whom. For almost all of us, what we know about the events of that day we learned from the news media, from the report of the September 11 Commission, from books we subsequently read, and so on. Why do we trust these sources? It’s clear that we don’t trust them absolutely; just like Ms. Cotillard, I “don’t believe all they tell me.” I do, however, believe them when they tell me that Twin Towers fell because Islamic fanatics crashed commercial airliners into them. When is it rational to believe the social sources of knowledge, and when is it rational to doubt them?

This, it turns out, is very far from a simple question. One thing is certain: you have to believe most (in some unquantifiable sense) of what the sources tell you—indeed, almost all of it—or else you’ll know virtually nothing. The question, then, is what special circumstances justify departing from the rule that you should believe what you’re told? Here’s a first cut at an answer: holding everything else you know constant, you should believe what the sources tell you unless what they are telling you is less plausible than the proposition that everyone who would have to be involved in a conspiracy to deceive you is in fact deceiving you.

In evaluating the plausibility of a conspiracy to deceive, it’s critical to understand just how many persons would have to be involved. With September 11, for example, the deception that Ms. Cotillard posits would involve thousands, perhaps hundreds of thousands of individuals, most of whom would have no motive at all to participate in the conspiracy. That’s why her views are so obviously crazy. That’s one reason, too, why a free press is so important: a large number of independent decisionmakers able to publish their views to the public makes perpetrating and maintaining a large deception extremely difficult. If the September 11 attacks had been faked, for example, just think of the opportunities for a cub reporter to become the next Woodward or Bernstein by breaking the story of the century. A free press thus provides a kind of guarantee of the veracity of the social sources of knowledge.

But even people smarter than Ms. Cotillard fall into similar mistakes often enough. I teach corporate law, and I always have a student or two who thinks that the “big corporations” are routinely engaging in serious wrongdoing. To be sure, this does happen sometimes, but it’s not common. Not only are most people (even those who work in corporate America) pretty honest, but those who think the big corporations are out to get the little guy fail to understand how many individuals would have to be involved in order to perpetrate such wrongdoing. We’re talking not just about corporate officers and directors but also scores or even hundreds of junior employees, as well as outside lawyers, accountants, and other professionals—maybe even government regulators, stock exchange officials, and others. Most of these people have very strong incentives not to participate in wrongdoing. For example, would outside counsel, who provides legal services to dozens of other clients, risk a criminal conviction and disbarment for the fees from one client? Not likely. Or would a low level corporate employee risk criminal conviction for the sake of a job? Many people choose to find alternative employment for much lesser reasons. Even CEOs, who are usually already very wealthy, would not find it worthwhile to boost the value of their stock options at the risk of imprisonment. Yes, in the right circumstances, the set of incentives necessary for such spectacular wrongdoing can be generated (when this happens, often it’s late in a market bubble, as happened with Enron and Worldcom), but this is the exception, not the rule.

As a matter of psychology, it’s worth noting that, even among those people who fall victim to conspiracy theories, just which theories they accept will depend on their other irrational prejudices. The people who think that the Clintons murdered Vince Forster, for example, are not usually the same people who think that President Bush was behind the September 11 attacks. As Madison says in The Federalist No. 50, “When men exercise their reason coolly and freely on a variety of distinct questions, they inevitably fall into different opinions on some of them. When they are governed by a common passion, their opinions, if they are so to be called, will be the same.”