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Reductio ad Absurdum

Kyle Hill

July 31, 2013

When the police began the booking process the handcuffed man blamed everything on a mosquito. The bloodsucker landed on his arm and he went berserk. The obvious change in gravity changed an otherwise calm man into a lunatic. Once every month—though sometimes twice a month—the police have it even worse. No cop wants to walk his or her beat at night when a full moon really has the power to change the populace.

Our moon has about as much gravitational effect on the body of any one person as a mosquito sitting on someone’s arm. If the amount of gravity the moon commands really did affect rates of crime, suicide, homicide, depression, and accidents, any swarm of the pests would be a flying disaster waiting to happen. The same gravitational truth exists for anything larger than the tiny insects. The laptop or monitor or smartphone that you are reading this on right now has more gravitational influence on you than our moon. Hold any of these too close to you and you might go on a rampage if the assumptions of “lunacy” were true.

If the moon itself, and not its gravity, was the instigator of madness, the famous Apollo landing missions should have worried about than just rocket trajectory and weight restrictions. Neil Armstrong would have made a giant leap towards insanity.

The mysterious power of the moon—if it did influence the violent crimes that mental health professionals and police officers dread each month—would increase the number of homicides around the time it shines brightest. But does it? I usually like to continue down hypothetical tangents in this column, but we actually have data that can answer this particular question. (Frankly, the presumptions of the full moon effect are so bizarre—grave misunderstandings about gravity and tides, for example—that I think it is too abstract to even think of a world where these presumptions were true.)

In 2012, the city of Chicago recorded 512 first-degree murders. Using the only public database with daily updates that I could find, I took each case and plotted it on a graph showing the date and number of homicides on each recorded date. If the moon truly could create a madness all its own, we would expect a visualization of these crimes to look something like this—a full moon atop each rise in homicide:

Each of the moons in the graphic is placed on or near the highest incidences of homicide across the months. A real full-moon effect should predict these positions to coincide with the actual full moon dates of 2012. But when we place the full moons in accordance with the dates they actually occurred, we see this:

On most of the dates of the full moon, there were no recorded homicides. This is small-scale analysis, but it goes against exactly what should happen over a year of violent crime in a major American city if the full moon had some effect. Remember, the moon is always full; it is just lit differently throughout the month.

Confirmation bias may keep it alive, but at larger scales like national studies and smaller scales like homicides in Chicago last year, the “full-moon effect” is reduced to the absurd.

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Kyle Hill

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Kyle Hill is a science writer who specializes in finding the secret science in your favorite fandom. He writes for the Scientific American Blog Network at his blog, But Not Simpler. Hill also contributes to Slate, Wired, Nature Education, Popular Science, and io9. He manages Nature Education's Student Voices blog, is a contributor to Al Jazeera America’s science show TechKnow, and you can follow him on Twitter under @Sci_Phile.