Happy Halloween! I decided to revise and repost this piece from November 1, 2010, on dog bites, full moons, and confirmation bias. Click the archives icon to see the original post.

Our story begins in March 2000, when Dr. Simon Chapman and colleagues from the University of Sydney published a paper in which they assessed the effectiveness of an educational intervention for the prevention of dog bites in primary school children. Per the researchers, "the programme aims to instill precautionary behaviour around dogs, assuming that this might reduce the incidence of attacks. A randomised controlled trial of the efficacy of the intervention was conducted in Australian children aged 7-8 years who were presented with an unsupervised opportunity to approach a strange dog."

Shortly after the publication of this paper, Chapman received a short note from a local farmer: "Have you university types ever looked at whether dog bites happen more around the full moon? It's a well known fact that they do."

It is also a well known fact that farmers are the unwitting victims of confirmation bias, recall bias, and a menagerie of other cognitive biases. The nature of the human mind, combined with the rich oral folkloric traditions passed down from earlier generations of farmers to later generations, has allowed for the persistence of these myths. Being university types of the highest order and respectability, Chapman and a colleague, Stephen Morrell had no choice but to respond, so they "leashed their skepticism" and investigated.

They note: "Although more women have been documented to menstruate around the full moon," (though see this, by Scicurious) "research has generally failed to confirm any association between the full moon and the manifestation of psychiatric disorders or violence in psychiatric settings, consultations for anxiety or depression in general practice, suicide and self poisoning, agitation among nursing home residents, car accidents, major trauma, or emergency department admissions."

But perhaps man's best friend is more susceptible to the tidal effects of the moon than is man himself. Chapman and Morrell decided that they had to determine if dogs bite humans more often when it is a full moon, or if Rover's bark is truly worse than his bite.

The researchers accessed twelve months' worth of public health records, and extracted the data on daily admissions for dog bites from all accident and emergency departments in Australian hospitals. They selected a year (June 13, 1997 through June 12, 1998) that had sufficient full moons and dog bites so that any significant correlation could be ...sniffed out. The ended up with a total of 1671 dog bites, an average of 4.58 bites per day. Peak days were identified, which contained higher than ten admissions per day. Full moons coincided with exactly zero peak days. (Click image to enlarge)

The average number of hospital admissions for each day was analyzed in comparison to the occurrence of a full moon, divided by day of the week. So the average number of dog bites that occurred on all full moon Fridays was compared with the average number of dog bites that occurred on all other Fridays, for example.

The most dog bites occurred on weekends (average 5.5 for Saturday, 5.6 for Sunday), which makes sense because more people spend more time interacting with their pets on weekends. The average number of bites on weekdays ranged from 4.1 to 4.9, forming an inverted "U" with the lowest number on Wednesday, and higher numbers approaching the weekends.

There were more dog bites on full moon days for Sunday, Tuesday, and Thursday, and significantly fewer dog bites on full moon days for Monday, Wednesday, and Saturday. There was no significant difference for Friday. This haphazard, random association of full moons with dog bites would be expected if dog bites occurred, well, randomly with respect to full moons.

Chapman S, & Morrell S (2000). Barking mad? another lunatic hypothesis bites the dust. BMJ (Clinical research ed.), 321 (7276), 1561-3 PMID: 11124174

Top illustration by the awesome Glendon Mellow of the Symbiartic blog here on Scientific American. Painted in ArtRage 3.5.