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Full Moons and Friday the 13th: Weird Stuff or Just Weird Psychology?

It’s a folk legend that the lunatics run wild when there is a full moon. Crime rates supposedly increase, as do the rates of hospital visits and suicides. Tonight, there is a full moon on Friday the 13th. Is all hell going to break loose? The interesting thing is, it very well might seem that way, but that’s only because of a particular quirk in human psychology.

Let’s start with the obvious: Are full moons related to things like crime and suicide rates? Quoting from a sourced piece in Scientific American:

“By combining the results of multiple studies and treating them as though they were one huge study—a statistical procedure called meta-analysis—[psychologist James Rotton, astronomer Roger Culver, and psychologist Ivan W. Kelly] have found that full moons are entirely unrelated to a host of events, including crimes, suicides, psychiatric problems and crisis center calls.”

There isn’t any evidence to support the idea that people go crazy when the Moon shows a full face, but many of us have had experiences that seem to indicate precisely the opposite. Why? The short answer is bias.

Psychologists have cataloged well over 100 different cognitive biases that reliably distort our thinking without our knowledge. Confirmation bias—the tendency to focus on and emphasize information that confirms our preconceptions and discredit or discard information that does not—is a huge one. At the more innocent end, it helps us add meaning to odd coincidences. Wasn’t it weird how your friend called you just when you were dialing her number? At the more serious end, it seems like an uptick in homicides can be blamed on the Moon.

But knowing about confirmation bias, is it more likely that an orbiting rock compels bad behavior, or that we are selectively remembering the times that bad stuff happened during a full moon and forgetting about all the times when the full moon was out and nothing happened? Psychology and the available data support the latter.

Of course, the Moon does have an effect on our planet. The tides slosh around our blue marble thanks in large part to the gravitational tugs of the Moon. But those tugs are dependent on mass and distacne. The Moon’s gravitational influence you (and maybe change your behavior), for example, is far less than the gravitational influence from the device you are reading this post on.

Whether you are triskaidekaphobic or not, take solace in knowing that tonight will be no more dangerous than any other night. Remember, the moon is always full; it is just illuminated differently throughout the month.

IMAGE: Morning Full Moon by Tim Fields

Kyle Hill is the Chief Science Officer of the Nerdist enterprise. Follow the geekery on Twitter @Sci_Phile.

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