It's better light, not worse behaviour, that explains crimes on a full moon

It’s a full Moon on September 25.

If past months have been anything to go by, this will be accompanied by a round of public chat about how this affects human behaviour – claims of more hospital admissions and arrests, to crazy antics in children.

Beliefs in the Moon’s behavioural effects are not new and date back to ancient times. But what evidence is there that the Moon has an impact on behaviour?

As a criminologist, I look at evidence related to arrests and behaviour linked with criminal activity.

The only explanation I can see that links criminology with Moon phases is just about the practicalities of being a criminal: when it’s a full Moon, there’s more light.

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While somewhat dated, one of the most significant studies looking at Moon phases and linking this with behaviour is a 1985 meta-analysis – a study of the findings of 37 published and unpublished studies. The paper concludes it is not sound to infer that people behave any more – or less – strangely between Moon phases. The authors write:

Alleged relations between phases of the moon and behavior can be traced to inappropriate analyses […] and a willingness to accept any departure from chance as evidence of a lunar effect.

Two more recent studies have looked at links between criminal activity and phases of the Moon.

A study published in 2009 looked at more than 23,000 cases of aggravated assaults that took place in Germany between 1999 and 2005. The authors found no correlation between battery and the various lunar phases.

A study reported in 2016 was careful to make a distinction between indoor and outdoor crime committed in 13 US states and the District of Columbia in 2014.

The authors found no link between lunar phases and total crime or indoor crime.

But they did find the intensity of moonlight to have a substantive positive effect on outdoor criminal activity. As moon illumination increased, they saw an escalation in criminal activity.

One explanation for this finding is what is referred to as the “illumination hypothesis” – suggesting that criminals like enough light to ply their trade, but not so much as to increase their chance of apprehension.

It may also be that there is greater movement of people during lighter nights, thus providing a bigger pool of victims.

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Why do some people still cling to the belief that the Moon causes criminal or other antisocial behaviour? The answer most likely lies in human cognition and our tendency to focus on that which we expect or predict to be true.

During an expected lunar event – such as a full or super Moon – we expect that there will be a change in behaviour so we pay more attention when we see it. In the area of cognitive psychology this is known as confirmation bias.

But other questions remain, including why any behavioural effects must be inherently negative? Even if there was a direct effect, explanations as to why acts of kindness and altruism do not increase or decrease during Moon phases are conspicuously absent.

It is likely that we just assume the folklore is true, and believe that we become the werewolf and not the sheep.