It’s another busy night in an inner city hospital emergency department (ED) and patients keep pouring in with injuries from accidents, assaults and self-harm attempts. One veteran nurse turns to a junior doctor, rolls her eyes and mutters, “must be a full moon tonight.”
The junior doctor racks her sleep-deprived brain to remember what the moon looked like on the way to work. She can’t recall – and doesn’t bother checking – but files the incident away so she can bring it up at social gatherings and retell it as part of the folklore of the ED.
What we have just witnessed (or something like it) happens every week around the country. Despite the “lunar effect” being one of the most easily studied and thoroughly debunked myths in medicine, it provides a fascinating look at the psychology of rational people who work in jobs where superstitious beliefs can easily develop.
But first, the science.
The biggest study I can find debunking the myth is a 2004 paper from Iran. It analysed just under 55,000 trauma-related admissions to three EDs over 13 months. There was no difference between full-moon days and other days in the number of attendances, nor in the type or severity of patients treated.
Other well-conducted studies include this 1992 Canadian analysis of the relationship between crisis calls to police stations and poison centres, and the lunar cycle (there was no correlation). And this review, also from 1992, which found no relationship between full moons and increased suicide attempts.
A less technical summary from Scientific American explains the ancient origins of the belief: the moon was thought to cause intermittent insanity. That’s why the latin word for moon, luna, forms the base of the out-dated term lunatic.
Perhaps more interesting in the Scientific America summary is that the myth persists within groups of professionals who deal with unpredictable patients, such as mental health professionals and emergency service personnel.
Two important effects are at work here. The first is confirmation bias. This is the innate tendency of people to remember and pay attention to facts that confirm an already-held belief, and ignore or downplay facts that tend not to support it.
There are many, many busy and stressful nights in ED and some of them inevitably fall on nights when the moon is full. These will be the ones that ED staff remember, and they will tend to forget the ones which are not on full moon nights.
The second likely cause of such a persistent myth is that those who work in fields that are inherently unpredictable – where stakes are high and conditions demanding – are more likely to be prone to superstitious or magical thinking. It is a form of the illusion of control cognitive bias.
Part of feeling able to cope with the randomness of life is to develop associations which can be given a meaning, even if the belief seems absurd. The psychologist Michael Shermer has coined the term “patternicity” to describe this tendency in the context of evolutionary psychology.
So despite a pedigree dating back to Aristotle, the belief that the full moon affects behaviour in any way only persists because of the very human responses of our front-line health personnel. When they’re feeling under the pump, they begin to instinctively look for patterns in random events.