Are more crimes committed during a full moon?

Chris Smith put this to Cambridge University's Kyle Treiber...

Kyle - Well, I really like this question because it immediately made me start to think about why this relationship could actually exist. Is there anything about the full moon that could actually cause someone to commit an actual crime? Sadly, I have to say my very quick review of the literature suggest there isn’t a very strong statistical evidence that there’s a consistent effect of the moon on criminal behaviour.

However, we can still think about what those effects might be and there’s some interesting new research. We seem to have ruled out any gravitational effects. The moon does not have a gravitation pull at individual levels, apparently. But there’s some new interesting research about the fact that there may be some lunar based elements of our circadian rhythms (our natural rhythms) that we have throughout the day. And that could relate back to mechanisms that had to do with the tide and the importance of the tide for food, for aspects of mating or bringing people together, being able to move from one environment to another. So that’s interesting to think there might be mechanisms and it does affect aspects like cortisol, so hormones which we know are related to stress responses which, therefore, can be related to crime.

I think though, arguably, these effects are going to be probably as small, or much smaller, than things like what you ate for dinner or how much sleep you got the night before and, more likely, if there is an effect is going to be more environmental. So maybe that it’s lighter outside and people spend more time being active in contexts where they can get in trouble. And maybe the police are more alert because they had the feeling that people are going to be more troublemaking and so they’re more likely to catch the offenders. So maybe the moon helps them apprehend more offenders. So all of these could be feeding into any relationship.

Chris - There was a study in Current Biology - I remember this about three or four years ago. They actually had some data collected, more than a decade ago - people coming to a lab and doing a sleep study and someone said: “well look, actually we’ve got all of the data and we know exactly who turn up and how long the slept for, and how well they slept.” And someone said, “why don’t we just compare that with what the moon was doing at the same time.” They found that people attending for the study at the time when there was a full moon had much worse sleep performance initially in the study than individuals who came at other phases of the moon’s cycle. So they were suggesting that there does appear to be some kind of monthly cycle built into our brain, perhaps dictated by the moon, or perhaps that’s just an indirect marker, perhaps it’s light or something. But there did appear to be an effect.

Kyle - Yeah. I think from what I know and listening and it’s obviously not my area of expertise but I think that yes, there is evidence that there is such an effect. But it was something like 20 minutes difference in sleep a night, and we know about how many other factors can influence our sleep in 20 minutes of sleep at night. So, again, it’s very interesting to think about that. I think it was robust actually, whether or not whether they experienced any changes in light as well, so it’s ruling out that aspect of which I think is a very important aspect of that kind of research. But, ultimately again, I think the effects are very small.