The fallacy of positive instances in conspiracy theory

Why do people believe in conspiracy theories? 

To understand, we look no farther than the prophet Nostradamus. In the 16th century, he wrote: 

 "Beasts wild with hunger will cross the rivers, 
The greater part of the battle will be against Hister,

He will cause great men to be dragged in a cage of iron,

When the son of Germany obeys no law."

and another prophecy:

"Out of the deepest part of the west of Europe,

from poor people a young child shall be born,

Who with his tongue shall seduce many people,

His fame shall increase in the Eastern Kingdom."


"He shall come to tyrannize the land.

He shall raise up a hatred that had long been dormant.

The child of Germany observes no law.

Cries, and tears, fire, blood, and battle."

Hundreds of people have read these words and reached the conclusion that Nostradamus predicted the rise and fall of Hitler thousands of years before his birth. Even the name "Hister" is almost identical to Hitler. Hitler was born to impoverished parents, became a great orator, and tyrannized the land, increasing hatred of the Jewish people. What are the odds? 

If this seems too amazing to be a coincidence, you might be falling prey to the fallacy of positive instances. Let me explain.

Imagine you go on a blind date, and discover that the other person owns the same model of car that you own. You dismiss it as a neat coincidence. 

Later on you learn that they have a sister with the same name as your sister, and both sisters were born in April. You start to wonder if this is confirmation that it's meant to be. 

As the evening continues, you discover that you both own all six Star Wars movies, and the entire Lord of the Rings trilogy on dvd. You loved the same television shows as a child. You both love ice cream, you both hate spam. You are made for each other.

Before you run off to Vegas to get married, let's take a moment to put things into perspective.

How many people on Earth own that model of car? How common is your sister's name? Since you grew up in the same decade as this person, is it that unusual that you watched the same television shows as a child? Star Wars and Lord of the Rings are very culturally relevant and most people your age have at least seen the movies. Almost everyone loves ice cream. Many people hate spam. 

Now think about the things you don't share an interest in; the reason you spent most of the evening talking about the things mentioned above is because you have them in common. You weren't able to discuss music at all because they like country and you like hip-hop. Your mothers have different names, and are born in different months. Your fathers have different names and are born in different months. You don't even have a brother. 

We tend to remember or notice information that supports our desires, and forget discrepancies. This is known as the fallacy of positive instances. 

When you are amazed by the idea of Nostradamus predicting Hitler, you ignore the fact that he wrote almost a thousand vague predictions, most of which have not "come true." When you discover that Hister is the Latin name for the Danube River, not the name of a person, his prediction seems even less dazzling. 

The most compelling argument against Nostradamus' psychic abilities is that his "true" predictions are the result of random chance, and creative interpretation. Because of the sheer volume of his predictions, it's certainly possible that some events would seem to fit, simply by coincidence. Not to mention that most of his quatrains refer to deaths, wars, or natural disasters - events that are guaranteed to happen over and over again throughout history.

So next time you come across a coincidence that's too good to be true, take a moment to weigh the number of positive correlations against the number of facts that don't correlate at all. Many a conspiracy theory is easily debunked if you take into account all the facts that don't correlate.