This past September 23 a Canadian 9/11 "truther" confronted me after a talk I gave at the University of Lethbridge. He turned out to be a professor there who had one of his students filming the “confrontation.” By early the next morning the video was online, complete with music, graphics, cutaways and edits apparently intended to make me appear deceptive (search YouTube for “Michael Shermer, Anthony J. Hall”). “You, sir, are not skeptical on that subject—you are gullible,” Hall raged. "We can see that the official conspiracy theory is discredited....It is very clear that the official story is a disgrace, and people who go along with it like you and who mix it in with this whole Martian/alien thing is discrediting and a shame and a disgrace to the economy and to the university." [sic]* Hall teaches globalization studies and believes that 9/11 is just one in a long line of conspiratorial actions by those in power to suppress liberties and control the world.

Conspiracy theories are a dollar a dozen. While in Calgary on that same trip, I met a politician who told me that he believes the fluoridation of water is the greatest scam ever perpetrated on the public. Others have regaled me for hours with their breathless tales of who really killed JFK, RFK, MLK, Jr., Jimmy Hoffa and Princess Diana, along with the nefarious goings on of the Federal Reserve, the New World Order, the Trilateral Commission, the Council on Foreign Relations, Yale University’s secret society Skull and Bones, the Knights Templar, the Freemasons, the Illuminati, the Bilderberg Group, the Rothschilds, the Rockefellers and the Learned Elders of Zion. It would take Madison Square Garden to hold them all for a world-domination meeting.

Nevertheless, we cannot just dismiss all such theories out of hand, because real conspiracies do sometimes happen. Instead we should look for signs that indicate a conspiracy theory is likely to be untrue. The more that it manifests the following characteristics, the less probable that the theory is grounded in reality:

Proof of the conspiracy supposedly emerges from a pattern of “connecting the dots” between events that need not be causally connected. When no evidence supports these connections except the allegation of the conspiracy or when the evidence fits equally well to other causal connections—or to randomness—the conspiracy theory is likely to be false. The agents behind the pattern of the conspiracy would need nearly superhuman power to pull it off. People are usually not nearly so powerful as we think they are. The conspiracy is complex, and its successful completion demands a large number of elements. Similarly, the conspiracy involves large numbers of people who would all need to keep silent about their secrets. The more people involved, the less realistic it becomes. The conspiracy encompasses a grand ambition for control over a nation, economy or political system. If it suggests world domination, the theory is even less likely to be true. The conspiracy theory ratchets up from small events that might be true to much larger, much less probable events. The conspiracy theory assigns portentous, sinister meanings to what are most likely innocuous, insignificant events. The theory tends to commingle facts and speculations without distinguishing between the two and without assigning degrees of probability or of factuality. The theorist is indiscriminately suspicious of all government agencies or private groups, which suggests an inability to nuance differences between true and false conspiracies. The conspiracy theorist refuses to consider alternative explanations, rejecting all disconfirming evidence and blatantly seeking only confirmatory evidence to support what he or she has a priori determined to be the truth.

The fact that politicians sometimes lie or that corporations occasionally cheat does not mean that every event is the result of a tortuous conspiracy. Most of the time stuff just happens, and our brains connect the dots into meaningful patterns.

*Erratum (12/6/10): In this quote the word "economy" should be "academy". Also, the [sic] annotation should be disregarded.