Illusory Correlation: a Common Mental Mistake That Leads to Erroneous Thinking
We all use silly logic to help us rationalize a confused world. Take the full moon, for example. For centuries, people have blamed the full moon for an inexplicable coincidence with them. But this is an illusory correlation — we trick ourselves into believing something based on what stands out the most in our memories.
This post originally appeared on James Clear’s blog .
People have blamed the full moon for strange behavior for centuries. For example, in the Middle Ages, people argued that the full moon could turn people into werewolves. In the 1700s, it was widely believed that a full moon could cause epilepsy or fever. We even changed our language to fit our beliefs. The word lunatic comes from the Latin root luna , which means moon.
Today we (mostly) came to our senses. While we no longer blame sickness and illness for the phases of the moon, you’ll hear people use this as a casual explanation for insane behavior. For example, in the medical community, there is a story that during a hectic evening in the hospital, one of the nurses often says, “There must be a full moon today.” There is little evidence that the full moon actually affects our behavior. A comprehensive analysis of over 30 peer-reviewed studies found no correlation between full moon and hospitalization, casino payouts, suicide, road traffic accidents, crime rates, and many other common events.
But here’s what’s interesting: While the study suggests otherwise, a 2005 study found that 7 out of 10 nurses still believed that “the full moon that night caused more chaos and more patients.”
What’s going on here?
Nurses who swear that the full moon causes weird behavior are not stupid. They simply fall prey to a common mental error that we all suffer from. Psychologists call this little brain error “illusory correlation.”
This is how it works.
How we deceive ourselves without realizing it
Illusory correlation occurs when we mistakenly overestimate one outcome and ignore others. For example, suppose you are in New York and someone interrupts you while boarding a subway train. Then you go to a restaurant and the waiter is rude to you. Finally, you ask someone on the street how to make a road, and they carry you away.
When you think back to your trip to New York, it’s easy to remember those experiences and conclude that “people from New York are rude” or “people in big cities are rude.”
However, you forget about all the dishes you ate when the waiter was acting perfectly normal, or the hundreds of people you passed on the Metro platform who didn’t cut you off. These were literally no events, because nothing remarkable happened. As a result, it is easier to remember times when someone was rude to you than times when you were happy to have lunch or were calmly traveling on the subway.
This is where brain science comes into play:
Hundreds of psychological studies have proven that we tend to overestimate the importance of events that we can easily remember and underestimate the importance of events that we find difficult to remember. The easier it is to remember, the more likely we are to create a strong connection between two things that are loosely connected or not connected at all.
How to spot illusory correlation
There is a simple strategy you can use to discover your hidden assumptions and prevent yourself from creating an illusory correlation. This is called a contingency table, and it forces you to recognize non-events that are easy to ignore in everyday life.
Let’s take a look at the possibilities of a full moon and a crazy night of hospitalization.
- Cell A: Full moon and rich night. This is a very catchy combination, and in our memory it is too stressed because it is easy to remember.
- Cell B: Full moon, but nothing is happening. This is not an event, and in our memory it is not given due attention, because in fact nothing happened. It’s hard to remember that something is n’t happening, and we tend to ignore this cell.
- Cell C: No full moon, but tense night. This can easily be attributed to a “crazy working day.”
- Cell D: No full moon or normal night. Nothing memorable happens on either side, so these events are also easy to ignore.
This contingency chart helps you understand what is happening in the minds of nurses during a full moon. The nurses quickly recall the time when there was a full moon and the hospital was overcrowded, but they simply forget that many times there was a full moon and the burden on patients was normal. Because they can easily recall the memories of the full moon and crazy night, and therefore mistakenly assume that the two events are related.
I first learned about this contingency table strategy by reading The 50 Great Myths of Popular Psychology and found that this simple table can be adapted to many different situations. Ideally, you should enter a number in each cell so that you can compare the actual frequency of each event, which will often be very different from the frequency you can easily remember for each event.
How to fix erroneous thinking
We make illusory correlations in many areas of life:
- You hear about Bill Gates or Mark Zuckerberg dropping out of college to start a billion dollar business, and you reevaluate the story in your head. Meanwhile, you will never hear of all college dropouts who have failed to build a successful company. You only hear about hits and never hear about misses, although the number of misses far exceeds the number of hits.
- You see someone of a certain ethnic or racial background being arrested, and therefore assume that all people with that background are more likely to be involved in the crime. You will never hear about 99 percent of people who are not arrested because this is not an event.
- You hear about shark attacks on the news and refuse to go to the ocean on your next beach vacation. The chances of a shark attack have not increased since you last entered the ocean, but you never hear of millions of people swimming safe every day. There will never be a story in the news called “Millions of tourists swim in the ocean every day.” You overestimate the story you hear on the news and create an illusory correlation.
Most of us are unaware of how our selective memory of events affects the beliefs we carry with us on a daily basis. We are incredibly poor at remembering things that don’t happen. If we do not see it, we assume that it does not affect or rarely happens.
If you understand how these thinking errors occur and use strategies like the contingency table test mentioned above, you can uncover hidden assumptions that you didn’t even know were and correct the erroneous thinking that plagues our daily lives.