The Debunker: Do People Go Crazy During A Full Moon?

by Ken Jennings

In honor of the late Neil Armstrong and the Apollo astronauts, we’ve asked Jeopardy! champ Ken Jennings to run down some common misconceptions that lots of people still believe about the moon. Did you know it’s not even made out of green cheese? Science ruins everything.

Moon Myth #2: Human Behavior Gets Nuts During a Full Moon.

Fully 45 percent of American college students, according to one survey, believe the old saw that a full moon makes people a little wacky. This idea dates back to the ancient Greeks. They observed the near-correlation between the length of the lunar cycle and women’s menstrual cycles (a link that’s still controversial, but which many statisticians now believe to be an evolutionary coincidence), and they knew that the Moon’s orbit tugged at the ocean tides. In that case, why couldn’t it tug on the fluids in human brains and bodies as well? In fact, the Moon’s phases were so closely tied to the way the ancients saw mental illness that our word “lunatic” comes from the Latin “luna,” meaning Moon.

Occasionally some researcher will spot a rise in crime or hospital admissions correlated to the full moon, and the study will make headlines. But this malarkey was dispelled in 1985, when three scientists (two psychologists and one astronomer) collaborated on a meta-analysis for Psychological Bulletin, combining the data from 37 different studies of “moonstruck” humans. They found that the “full moon effect,” in short, doesn’t exist, and pointed out methodological errors in the studies that disagreed.

I know this is unlikely to convince those who have heard friend-of-a-friend stories of packed police precincts and emergency rooms on full moon nights, but there’s no evidence for the phenomenon. It’s mostly a result of confirmation bias: we’ve been conditioned by years of movies and myths to expect “full moon fever,” so when crazy stuff happens during a full moon, we remember that connection, and forget all the crazy stuff that happened at other times. As scientists have pointed out, the Moon’s gravity is a nonissue here: a wall six feet away or a mosquito both “pull” on you with more gravitational force than the Moon does. It’s possible, some scientists think, that in the days before modern lighting, a full moon could have been bright enough to change sleep patterns and cause some statistical rise in erratic behavior. But that hasn’t been true for centuries. Today, anyone who blames human problems on Luna is just loony.

Quick Quiz: What future star got an early break playing Cher’s one-handed love interest in the Oscar-winning 1987 movie Moonstruck?

Ken Jennings is the author of Brainiac, Ken Jennings's Trivia Almanac, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at or on Twitter as @KenJennings.

Photo by Flickr member billbord99. Used under a Creative Commons License.