Of course, understanding that many conspiracy theories are false and believed for not fully rational reasons does not show that there never are any actual conspiracies. Some of the more infamous and true conspiracy theories in the United States include:
During Prohibition (1920-1933), the U.S. Treasury Department poisoned industrial alcohol in an attempt to discourage bootleggers from using it to make alcoholic beverages. But apparently not all bootleggers were concerned with public health, and they produced and sold the beverages anyway, resulting in thousands of deaths. The government secretly continued the practice until the end of Prohibition, despite knowing its effects.
In 1932, the U.S. Public Health Service conducted an experimental trial of a treatment for syphilis on several hundred African-American men in Tuskegee, Alabama, without securing their informed consent. Men with the disease were never given adequate treatment for it and were never fully informed of their role in the experiment.
In 1972, President Richard Nixon authorized a break-in at the headquarters of the Democratic National Committee in the Watergate Office Building and then unsuccessfully tried to cover up his administration’s involvement.
In each of these cases, portions of the U.S. government conspired to commit harmful actions while keeping their role secret. This meets the letter definition of a “conspiracy” which is when any group of people have a secret plan to do something illegal or harmful. Given that definition, we can plausibly suspect that many actions by many governments result from conspiracies or secret plans to cause harm, particularly in the areas of espionage and counter-intelligence. Coming up with these conspiracy theories— and determining how well they are supported by available evidence—is the job of watchdog organizations, investigative reporters, and (later) historians.
So, there most definitely are conspiracies, and some conspiracy theories are true. But not all of them are. So, we are brought once again to the difficult epistemic challenge of trying to sort out the true from the false or the reasonable from the unreasonable. Are there any rules or indicators to help us distinguish between plausible and implausible conspiracy theories?
There are no rules that will reliably sort the true conspiracy theories from the false ones, but the following rules may serve as a set of helpful indicators:
1. The bigger and more powerful the conspiracy is supposed to be, the less likely it is real. Anyone who has managed a sizable group project knows how hard it is to get people to coordinate their efforts. The task becomes even harder, or impossible, if the shared effort is to cover up some harmful or immoral secret. So, the more people who must be included in the conspiracy, the bigger the lie that must be told, and the more harmful the thing being kept secret is, the more unlikely it is that a conspiracy will succeed.
2. Hanlon’s Razor: “Never attribute to malice that which is adequately explained by stupidity.” Many times, big events happen for stupid reasons or for not really any single reason at all. We tend to think that significant events must have significant causes, but in fact, the universe does not pay attention to what we regard as significant. For example, the explosion of the Hindenburg was a horrific disaster. A huge, gas-filled airship burst into flames, killing dozens of people. Some think it must have been sabotage because the event was so horrific. But it is more likely that the explosion was caused by static electricity, lightning, or engine failure. Sometimes, significant events happen for relatively unimpressive reasons.
3. Beware of claims of conspiracy that cannot be falsified. The frustrating aspect of thorough-going conspiracy theories is their seeming unfalsifiability. No matter what happens or whatever is uncovered, it will end up being used as proof either of the theory or of just how crafty and manipulative the conspirators are. There is practically nothing that could prove to a Flat-Earther that the Earth is not flat, for example, as every contrary bit of evidence is rejected as mere propaganda or as improperly-interpreted data.
4. Positing a conspiracy should be an explanation of last resort. Given how difficult it is to maintain any sizable conspiracy and how common it is that significant events happen for insignificant reasons, positing a conspiracy should be an explanation of last resort. If there is no more natural or plausible explanation of some event, and if we are sure that all the evidence is genuine, then perhaps we must posit that some conspiracy is at work. But one must work with great honesty and objectivity to determine whether the evidence to be explained is genuine and whether there really is no more plausible explanation. One of the most popular quotes employed by conspiracy theorists themselves is from Arthur Conan Doyle’s character Sherlock Holmes: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” Conspiracy theorists proudly proclaim this dictum of a fictional detective as their justifying principle. The problem is that most conspiracy theorists are actually eager to reach for the improbable, and the “impossible” explanations they have eliminated are perfectly possible, at least, once one has sorted out the real evidence from false or implausible reports.
These four considerations may help alert us to false conspiracy theories in some cases. But, again, sometimes there are real conspiracies, and the theories about them are true. In the end, the best advice that can be given is to take each theory on its own merits, assessing the plausibility of its claims given our prior beliefs, our general experience of the world, and the basic attitude in Bayesian reasoning. We need to compare the likelihood that we would be seeing the so-called evidence for the theory if the theory were true against the likelihood that we would be seeing it anyway even if the theory were false.