A stock image of the holiday season is a night scene of Santa and his reindeers silhouetted across a full moon, his sleigh packed with presents ready to be delivered throughout the evening. While this joyous image fits some of our romantic notions of being moonstruck, it contradicts some widely held beliefs about the negative effects of full moons. (And never mind that the odds of experiencing a full moon on Christmas Eve itself are very small: the last one was in 2007 and the next may not appear until 2026.)
A teacher I know, complaining about her students’ boisterous behavior in the classroom, quipped that there must have been a full moon, while tales of rising crimes and emergency room visits during a full moon are common.
Let’s take a look at some of these myths — and don’t forget to bring along your healthy skepticism. Perhaps a good place to start is “Twilight,” so to speak.
At least as far back as ancient Roman and Greek mythology, stories of lycanthropy – the ability of a human to transform into an animal – speak of the role of a full moon in creating werewolves. Given what we now know about the effect of the moon’s force on many aspects of life, such beliefs are understandable. Consider tides and the motion of the Earth in relation to the moon’s gravitational pull. Or the monthly menstrual cycle that some associated with the moon’s cycle: the words "menstruation" and "menses" are based on the Latin word for “month” (mensis) and derived from the Greek “mene,” a female divinity who presided over the months.
A key word that points to some of our contemporary myths: lunatic, derived from the Latin lunaticus. Clearly the lunar connection is evident here; even Aristotle and Pliny the Elder held to the belief that, due to the moisture in the brain and the tidal effects on water, the moon created insanity in many people.
Superstitions also make good use of a full moon. Blowing on a wart nine times in the light of a full moon and placing a knotted string over your shoulder when the moon is full are folk remedies for removing warts. Or maybe just for removing our critical thinking skills!
But a long history of associations with a full moon does not explain our contemporary beliefs in today’s more scientific age. One of the most persistent myths, held even by health care professionals and police officers, is the connection between a full moon and increases in crimes, aggressive behavior, and trauma.
Stories of emergency rooms reaching higher caseloads on full moon nights are common, along with increases in arrests for crimes and assaults. It’s even said that dogs and cats have higher incidences of illness and injury during a full moon.
An obvious way to critically investigate these beliefs is to look at the numbers on emergency rooms and police reports. About 15 years ago, researchers reviewed more than 100 studies on lunar effects and concluded there was no reliable correlation between a full moon and such outcomes as epilepsy, domestic violence, homicides, or psychiatric admissions. Or even lycanthropy.
Nor has the new millennia changed the moon’s power. A more recent study from India found no difference in the number of crimes reported on full moon days compared with non-full moon days.
One explanation for the persistence of the myth of increased crime on full moon days has to do with the amount of light available. Crimes do increase as sunset is delayed, according to another study, yet there was no statistical correlation between lunar cycles and incidents of crime.
In days before outdoor lighting, full moons provided more light to carry on night time activities; people sleep less when it’s brighter, giving people more opportunities to stay outdoors and possibly leading to more injuries, assaults, and other increased activities, including time spent with pets. But our explanations demonstrate the common problem of mixing up causation with correlation and the power of post hoc explanations: the problematic behaviors witnessed must have been caused by the full moon, or so we say post hoc, after the fact.
Selective attention also plays a part in our recollection of events. Attentive to our beliefs in many of these myths, we selectively notice, on full moon nights, stories about crimes, students acting out in class, and other undesirable events. Do we remember the good things that happened that day or the romantic stroll under a full moon? Do we compare how many assaults or misbehaviors occurred a few days before when the moon was not full? And do crimes and injuries increase when the moon is full and it’s too cloudy to notice it?
Although this year you won’t be seeing Santa and his team of reindeers flying across a full moon, when people insist they experienced strange effects, had warts disappear, and witnessed unusually high incidences of injurious activities or a bit of excessive facial hair growth when the moon was full, just give out a hearty “ho, ho, ho.”