Human beings have been blaming strange behavior on the full moon for centuries. In the Middle Ages, for example, people claimed that a full moon could turn humans into werewolves. In the 1700s, it was common to believe that a full moon could cause epilepsy or feverish temperatures. We even changed our language to match our beliefs. The word lunatic comes from the Latin root luna, which means moon. 
Today, we have (mostly) come to our senses. While we no longer blame sickness and disease on the phases of the moon, you will hear people use it as a casual explanation for crazy behavior. For example, a common story in medical circles is that during a chaotic evening at the hospital one of the nurses will often say, "Must be a full moon tonight."
There is little evidence that a full moon actually impacts our behaviors. A complete analysis of more than 30 peer-reviewed studies found no correlation between a full moon and hospital admissions, casino payouts, suicides, traffic accidents, crime rates, and many other common events. 
But here's the interesting thing: Even though the research says otherwise, a 2005 study revealed that 7 out of 10 nurses still believed that "a full moon led to more chaos and patients that night." 
What's going on here?
The nurses who swear that a full moon causes strange behavior aren't stupid. They are simply falling victim to a common mental error that plagues all of us. Psychologists refer to this little brain mistake as an "illusory correlation."
Here's how it works...
How We Fool Ourselves Without Realizing It
An illusory correlation happens when we mistakenly over-emphasize one outcome and ignore the others. For example, let's say you visit New York City and someone cuts you off as you're boarding the subway train. Then, you go to a restaurant and the waiter is rude to you. Finally, you ask someone on the street for directions and they blow you off.
When you think back on your trip to New York it is easy to remember these experiences and conclude that "people from New York are rude" or "people in big cities are rude."
However, you are forgetting about all of the meals you ate when the waiter acted perfectly normal or the hundreds of people you passed on the Subway platform who didn't cut you off. These were literally non-events because nothing notable happened. As a result, it is easier to remember the times someone acted rudely toward you than the times when you dined happily or took the subway in peace.
Here's where the brain science comes into play:
Hundreds of psychology studies have proven that we tend to overestimate the importance of events we can easily recall and underestimate the importance of events we have trouble recalling. The easier it is to remember, the more likely we are to create a strong relationship between two things that are weakly related or not related at all. 
How to Spot an Illusory Correlation
There is a simple strategy you can use to spot your hidden assumptions and prevent yourself from making an illusory correlation. It's called a contingency table and it forces you to recognize the non-events that are easy to ignore in daily life.
Let's break down the possibilities for having a full moon and a crazy night of hospital admissions.
- Cell A: Full moon and a busy night. This is a very memorable combination and is over-emphasized in our memory because it is easy to recall.
- Cell B: Full moon, but nothing happens. This is a non-event and is under-emphasized in our memory because nothing really happened. It is hard to remember something not happening and we tend to ignore this cell.
- Cell C: No full moon, but it is a busy night. This is easy to dismiss as a "crazy day at work."
- Cell D: No full moon and a normal night. Nothing memorable happens on either end, so these events are easy to ignore as well.
This contingency table helps reveal what is happening inside the minds of nurses during a full moon. The nurses quickly remember the one time when there was a full moon and the hospital was overflowing, but simply forget the many times there was a full moon and the patient load was normal. Because they can easily retrieve a memory about a full moon and a crazy night and so they incorrectly assume that the two events are related.
I first learned about this contingency table strategy while reading 50 Great Myths of Popular Psychology and I find that this simple table can be adapted to many different situations. Ideally, you would plug in a number into each cell so that you can compare the actually frequency of each event, which will often be much different than the frequency you easily remember for each event.
How to Fix Your Misguided 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 over-value that story in your head. Meanwhile, you never hear about all of the college dropouts that fail to start a successful company. You only hear about the hits and never hear about the misses even though the misses far outnumber the hits.
- You see someone of a particular ethnic or racial background getting arrested and so you assume all people with that background are more likely to be involved in crime. You never hear about the 99 percent of people who don't get arrested because it is a non-event.
- You hear about a shark attack on the news and refuse to go into the ocean during your next beach vacation. The odds of a shark attack have not increased since you went in the ocean last time, but you never hear about the millions of people swimming safely each day. The news is never going to run a story titled, "Millions of Tourists Float in the Ocean Each Day." You over-emphasize the story you hear on the news and make an illusory correlation.
Most of us are unaware of how our selective memory of events influences the beliefs we carry around with us on a daily basis. We are incredibly poor at remembering things that do not happen. If we don't see it, we assume it has no impact or rarely happens.
If you understand how these errors in thinking occur and use strategies like the Contingency Table Test mentioned above, you can reveal the hidden assumptions you didn't even know you had and correct the misguided thinking that plagues our everyday lives.
James Clear writes at JamesClear.com, where he shares self-improvement tips based on proven scientific research. You can read his best articles or join his free newsletter to learn how to build habits that stick.
This article was originally published on JamesClear.com.
- Even Shakespeare blamed our occasional craziness on the moon. In his play Othello he wrote, "It is the very error of the moon. She comes more near the earth than she was wont. And makes men mad."
- Rotton, James, and Ivan W. Kelly. "Much ado about the full moon: A meta-analysis of lunar-lunacy research." Psychological Bulletin 97.2 (1985): 286.
- Technically, 69 percent of nurses believed the full moon impacted hospital admission rates according to the study mentioned in this ABC News article.
- For lovers of psychology, this phenomenon is often referred to as the Availability Heuristic. The more easily we can retrieve a certain memory or thought -- that is, the more available it is in our brains -- the more likely we are to overestimate it's frequency and importance. The Illusory Correlation is sort of a combination of the Availability Heuristic and Confirmation Bias. You can easily recall the one instance when something happened (Availability Heuristic), which makes you think it happens often. Then, when it happens again -- like the next full moon, for example -- your Confirmation Bias kicks in and confirms your previous belief.