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.