The full moon does not cause craziness, but confirmation bias is a bitch


The Earth's moon
Photo credit: Peter Freiman. CC 3.0.

The moon will be full (99.6%, waxing gibbous) tonight, which often conjures expectations of increased high jinks.  However, these expectations of lunar lunacy are just wrong.  Many studies confirm that the moon does not affect the actions of us earthlings.  And the plausibility of the moon controlling peoples’ behavior is fairly ridiculous.  After all, the moon is always orbiting us no matter what amount of sunlight bounces off of it.

Background on the lunar lunacy belief:

Lunar lunacy is a supposition and superstition that’s based largely on legend (wow, that’s altogether alliterative).  It is the belief that the phases of the moon affect the behavior of people or animals.

The belief that the moon can induce madness is in no way new.  Way back in the first century AD, Pliny the Elder suspected that people were driven crazy during the full moon.  He hypothesized that this was due to increased dew, which caused brains to become “unnaturally moist” (Raison et al., 1999).  However, old Pliny never explained how the full moon created excessive dew, or how that dew got into peoples’ brains.  So, that wasn’t really a very useful, or correct as it turns out, hypothesis.

Now let’s fast forward to the 1970’s.  A relatively more sophisticated idea was proposed that involved gravity.  The basic idea was that, since the moon’s gravity affects the oceans (causing tides), it should also affect the water in our bodies.  Studies even revealed some evidence for this (Lieber and Sherin, 1972).  However, subsequent investigations could not reproduce those results.  Also, it was not ever made clear what exact mechanism in this gravitational scenario caused madness.

To this day, many people still equate the full moon with full crazy.  One examination shows that up to 81% of mental health professionals believe that the moon manipulates behavior (Vance, 1995).

What scientific investigation reveals:

This subject is veritably a part of pop culture and data on it is relatively easy to acquire.  Thus, there are many studies into the occurrence of moon madness.  However, many of these studies are of questionable quality and others provide results that are not reproducible.  So to get a better idea of what’s going on, meta-analyses (studies of previous studies) are a good option.

One such meta-analysis examined 37 studies (Rotton and Kelly, 1985).  This study examined potential moon-influence on sex, publication, population and lunacy.  The analysis found no significant correlation.  The authors concluded that “inappropriate analyses” played a role in correlations found by other studies.

Another meta-analysis by Byrnes and Kelly (1992) investigated correlations between crisis calls and the moon phases.  The authors summarize their conclusions by saying that:

“On the basis of the studies considered it is concluded that no good foundation exists for the belief that lunar phase is related to the frequency of crisis calls. In addition, there is no evidence whatsoever for the contention that calls of a more emotional or “out-of-control” nature occur more often at the full moon.”

Is lunar lunacy even plausible?

Many researchers explore potential associations between the moon and madness.  However, this is really a type of worm-ridden, low-hanging fruit.  This is due to the fact that not many researchers offer any explanations of how the moon could affect us.

Could the moon’s gravity affect the water in our bodies?  Well… no.  Not really.  The Astronomer George Abell summed this up best.  His explanation: the gravitational pull of the moon is less than the gravitational pull of a mosquito on your arm.  However, despite the relative strength of mosquito gravity, Abell quips that there are no hypotheses of the “mosquito lunacy effect.”  More importantly, it’s never made clear how the change in gravitational pull would affect the hypothesized changes at all.

And as for brain moistness.  That’s just ridiculous.  Come on Pliny.  If strange full-moon dew could moisten your brain enough to cause insanity, what would rain or swimming do?  The Irish would all have been well mad—wait a minute…  (Just kidding; love ya Ireland.)

Here is an even more important point: the moon doesn’t get smaller when less light reflects off of it.  The new moon has just as much gravitational pull as the full one.  So why in the world would we think that the full moon affects our physiology?

Really, after giving it some thought, it’s sort of sad that scientific research was ever necessary in this case.

Why lunar lunacy beliefs persist (confirmation bias):

This, and many other unfounded beliefs, are perpetuated by confirmation bias.  This is a bias that all of us struggle with.  It is an insidious problem that affects the way we process available information.  Confirmation bias causes us to notice and remember information that supports our preconceived beliefs.

For example: I often think that I try to plug in my USB connections upside-down at least 90% of the time.  I’ve not investigated this, but it’s almost certainly not true.  However, the thought is conceived because I only ever remember the times that I try to plug the damn thing in the wrong way.  Why would I remember the times that it works perfectly?

Similarly, when police officers or ER nurses have some horribly crazy night of work, they notice and remember the other variables from their shift.  Sometimes these variables include the full moon just by chance.  But no one notices all the times when the moon was full but their day was good!  Why would they?


A lot—a metric lot, even—of people believe that the full moon affects behavior.  My use of the word “believe” instead of the word “think” is intentional.  This is because, when armed with a small dose of facts and some basic knowledge of our own biases, the lunar lunacy phenomenon disappears faster than duck liver in a bottle of homeopathic medicine.  The myth of moon-driven madness persists due to confirmation bias, which we all need to work hard to control.


Byrnes, G., & Kelly, I. W. (1992). Crisis calls and lunar cycles: a twenty-year review. Psychological reports, 71(3), 779-785. 10.2466/PR0.71.7.779-785

Lieber, A. L., & Sherin, C. R. (1972). Homicides and the lunar cycle: Toward a theory of lunar influence on human emotional disturbance. American Journal of Psychiatry, 129(1), 69-74.

Raison, C. L., Klein, H. M., & Steckler, M. (1999). The moon and madness reconsidered. Journal of affective disorders, 53(1), 99-106.

Rotton, J., & Kelly, I. W. (1985). Much ado about the full moon: A meta-analysis of lunar-lunacy research. Psychological Bulletin, 97(2), 286.

Vance, D. E. (1995). Belief in lunar effects on human behavior. Psychological reports, 76(1), 32-34.

Jared Peters

Jared Peters

Jared Peters, PhD, is a geoscientist who specialises in marine sedimentology, marine palaeoglaciology and climate change.
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Jared Peters, PhD, is a geoscientist who specialises in marine sedimentology, marine palaeoglaciology and climate change.