Apr 02 2015

Lunar Cycle Effects Busted

When I was an intern doing a rotation in the emergency department, on one particularly busy shift a nurse commented (to no one in particular) that it must be a full moon. I habitually look at the moon and generally know what phase it is in (right now it is a waxing gibbous, almost full), and so I knew at the time that in fact there was a crescent moon in the sky. I informed her of this. She gave a disappointed look and then went on with her work without any apparent further thought on the matter.

The episode struck me at the time. It seemed to me that I just witnessed a clear example of confirmation bias – what if it had been near a full moon? That would have confirmed her prior belief in a lunar effect, while this negative correlation was brushed aside and likely did not have any negative effect on her belief. (Although, my interpretation and memory of this event can itself be an example of confirmation bias regarding confirmation bias.)

Belief in the so-called lunar effect, that the phases of the moon exert an influence on human behavior with the most common element being a full-moon inducing extreme behavior, is very common. In my experience it is one of the most common pseudoscientific beliefs I encounter in the general public. One survey indicates that 43% of adults believe in the lunar effect, especially mental health professionals, including nurses.

When someone expresses such a belief to me I often use it as an opening to discuss skeptical principles. While belief in the lunar effect is widespread, it is usually not part of any emotionally held religious or ideological belief. It is therefore an excellent teaching opportunity. One question I like to ask is, “how do you think that works?” The most common answer I receive is probably the least plausible – that the tidal effects of the moon influence the brain because the brain is sitting in water (spinal fluid).

The tidal effect answer is incredibly implausible for a number of reasons. Tidal effects depend upon distance, and the difference between one side of the skull and the other is insignificant when compared to the distance to the moon (very much unlike the difference between one side of the earth and the other). Further, the phases of the moon do not directly relate to its tidal effects. They are related to the relationship between the lunar tidal effects and the solar tidal effects (because the sun has a tidal effect on the earth also), but that is an even more subtle effect. Further, if it is the alignment of lunar and solar tidal effects that are important, then new moon would have at least as much of an effect as full moon.

There is some plausibility to the notion that the light from the near full moon is what is important. Perhaps the persistence of light at night has a hormonal effect on the human brain, or simply the availability of light influences our willingness to go outside and cause havoc. Modern technology, however, has destroyed any such effect if it existed. We spend our days inside and our evenings with artificial light. Any natural daily light cycle has been obliterated (which may cause its own problems, but that’s another article).

In any case, suffice it to say that the plausibility of a lunar effect on human behavior is extremely low, but I will grant it is non-zero. What about the actual evidence? Every alleged lunar effect – hospital admissions, crime, birth, suicide, etc., has been shown to be false. No correlation holds up to scientific scrutiny.

A recent study now gets added to the list – No Evidence of Purported Lunar Effect on Hospital Admission Rates or Birth Rates.┬áThe study is a reanalysis of data from a 2004 study purporting to show a lunar effect for hospital admissions, however the authors point to statistical errors and their analysis shows no effect. They also review the literature on a lunar effect for birth rates, finding a consensus that there is no effect, and they discuss the persistence of a lunar cycle belief among nurses.

This is an old myth that has been debunked numerous times, but that is also part of the point. Despite this, belief in the myth remains high. It is therefore necessary to revisit it from time-to-time, especially when there is new data available.

Of course, the more interesting question is always, why to people believe in something so implausible and false? Assuming that the scientific data is accurate and therefore there is no lunar effect, people form a strong belief in the effect despite its absence. It seems that all that is necessary for such beliefs to form is a pre-existing cultural belief (and maybe not even that) and the mischievous influence of confirmation bias.

In fact, I would rank a thorough understanding and appreciation for confirmation bias on my short list for the most important principles of critical thinking. It is a all-purpose, powerful, and pervasive cognitive bias. It is why (as Gilovich pointed out) husbands arrive at the firm belief that they always put the toilet seat down, while wives arrive at the firm belief that their husbands never put the toilet seat down, even though both parties must be operating off the exact same set of data.

Another way to state this is that there can often be a profound disconnect between belief and reality. It is curious that our brains evolved such a powerful and seemingly counterproductive tendency. I suspect it is an epiphenomenon only. There is also an available fix – logic and critical thinking. We need to foster a healthy skepticism towards our own beliefs, and realize how profoundly wrong our memories and perceptions can be. The skeptical position is the humble position. Despite the fact that your memory suggests a strong lunar effect, it may not actually exist and may solely be due to flawed perception and memory.

I know I am beating a familiar drum, but it bears frequent repeating. This is the core principle of critical thinking that I wish were more widespread.

The lunar effect is a great way to demonstrate confirmation bias – belief is very common, the concepts are easily understood, and yet emotional investment is usually quite low. It is critical thinking education gold.

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