May 06 2013

The Lunar Effect and Confirmation Bias

I gave a seminar recently to science teachers and the topic of whether or not there is a lunar effect came up. I was not surprised to find that 80% of them believed that emergency rooms and police stations are more busy during a full moon. I was also not surprised, but only because I have been there before, that they were highly resistant to my claim that the scientific evidence shows that there is no such effect.

Several questions emerge from the notion that the phases of the moon affect human behavior: what is the plausibility of such a claim, is there actually such an effect, and if not why do so many people believe that there is?


One of two justifications are commonly given for how the moon might influence human behavior. The moon basically has two physical effects on our environment – gravity and light. Astrological influences are not worth further discussion in this article, and I rarely hear that as a justification from the general public in any case.

Gravity is the far less plausible explanation of the two physical effects of the moon. The reasoning often goes something like this: the moon causes tides, which are powerful gravitational effects on water. Our bodies are mostly water, and in fact our brain are floating in water, therefore the moon’s tidal effect might affect our brain function.

This reasoning fails on many levels. First, the moon has a tidal effect regardless of phase. The only difference with lunar phase is the relationship between the lunar tide and the solar tide (yes, the sun has a tidal effect on the Earth as well). During a full moon the lunar and solar tides are lined up, and therefore the combined effect is additive (which is called a spring tide).

However, the same is true of the new moon, therefore if the full moon effect were due to tidal forces there should be an equal new moon effect.

The bigger problem with the gravity explanation is that tidal forces are dependent upon the difference in the distance of the near and far side of an object from another large object. So the ocean tides are caused by the different gravitational pull of the moon on the near side vs far side of the Earth. The tidal force of the moon between the near side and far side of your head is negligible. This is simply not a viable source of an effect on human behavior.

The more plausible mechanism for an alleged lunar effect is the light from the moon. It is reasonable to hypothesize that people are more willing to be outside and active during a full moon because there is more light. This effect, however, should therefore not be present when the sky is overcast or in large cities where artificial light trumps moonlight.

Is There a Lunar Effect?

This is one question that is very amenable to standard observational studies – is there a correlation between some kind of event and the lunar cycle? There have been hundreds of such studies over the last few decades, involving emergency room visits, births, accidents, crime, crisis center calls – just about any marker of human behavior you can think of. Systematic reviews of this research consistently demonstrate that there is simply no evidence for any such effect.

Many of these studies cover years or decades of data, and thousands or tens of thousands of data points. A recent German study, for example, found no lunar effect for births between 1920 and 1989. A review of studies looking at crisis center calls found:

12 studies are reviewed that have examined the relationships among crisis calls to police stations, poison centers, and crisis intervention centers and the synodic lunar cycle. 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.

An often cited 1985 review by Rotton, Kelly and Culver found no lunar effect for a long list of events, including suicide, homicide, crime, sleep walking, alcoholism, and many others. Several later studies of psychiatric hospital admission also found no effect.

The bottom line is this – this question has been asked and answered, there is no lunar effect. A massive amount of data simply shows no connection between lunar phases and human behavior.

Belief in the Lunar Effect

Why, then, does belief in this effect remain high? Surveys show that belief in this effect remains at about 40-45%, even among those highly educated. In fact, mental health workers have a greater belief in the lunar effect. The short answer as to why this is – confirmation bias.

Confirmation bias is a bias in human thinking that causes us to notice, accept, and remember information that confirms beliefs we already have, while ignoring, forgetting, or explaining away contradictory data. This is a powerful effect that can lead to the very compelling illusion that an effect is real when it isn’t.

Belief in a lunar effect, therefore, feeds on itself. When we encounter human behavior that seems out of the ordinary, or we are simply noticing the right side of the Bell curve of variation of crazy events, and we believe in the lunar effect, then we are likely to notice if there is a correlation.

For example, one night when I was working in the ER, and it was a busier than average night, a nurse commented, “Wow, it’s really crazy tonight. Is there a full moon?” The answer was no, the moon was in a completely different phase that night. When I informed her of this fact she simply shrugged and promptly forgot the whole thing. One can imagine that on other busy ER nights that happen to fall on or near the full moon that would resonate with her and confirm her belief in a lunar effect.

Confirmation bias can make it seem like there is strong evidence (anecdotal) for something that does not exist outside of our belief.

When I informed the teachers that the evidence shows there is no lunar effect, they simply did not believe it. Their personal experience spoke much more powerfully to them then abstract data.

Of course, the essence of scientific and critical thinking is setting aside personal anecdotes, experience, and belief, and embracing more rigorous data and analysis.


The data is clear, there is no lunar effect for any measured human behavior or medical events. This question has been sufficiently studied that this conclusion is very firm. We can never prove with finite data that an effect size is zero, but we can say there is no evidence for the effect, and if there is such an effect it must be extremely tiny – too small to have been detected even by hundreds of studies with tens of thousands of data points.

The real phenomenon here is one of human belief that persists in the absence of a real phenomenon. The real lunar effect is confirmation bias and understanding how the human brain is a belief machine.

27 responses so far

27 thoughts on “The Lunar Effect and Confirmation Bias”

  1. Kawarthajon says:

    I wonder if another effect is cultural, because we have for thousands of years believed that a full moon is significant and there is lots of media that continues this belief (i.e. news reports about full moon events, full moon parties, tv shows about werewolves, etc…). People now believe that there is something auspicious about the full moon, so they’re more willing to believe that there is something there. This would be similar to the impact of tv, books, etc., about intelligent aliens and now a certain percentage of people believe that there are aliens on the earth.

    I am interested as to whether someone who is smarter and more knowledgeable than me could actually calculate how much our head gets stretched because of tides. Just for interest, not because I believe that the tides have any impact on our behaviour.

  2. Jerry in Colorado says:

    A great blog Steven. While in college, I worked as an orderly in a large hospital on a psych ward. Times that called for extra staffing were full moon or the 4th of July. The 4th of July night noises and flashing lights excited veteran patients with war memories. Both events seem to have real effects, but the 4th of July event was obviously more than confirmation bias.

  3. BenevolentCow says:

    There is something I have noticed about the moon and myself. Intellectually I know the moon is up during the day like 50% of the time, but every time I notice it it gives me a far different impression than when it is the sky at night. At night it is just a normal part of the skyscape, nothing more amazing than the stars, but during the day it gives me a strong feeling that it is just there, an arms reach away, a place people have been and should still be going to.

    I guess it is a result of too many video games and movies that have an asteroid/meteor visible during the day being an imminent threat, but somehow the moon hanging in a blue sky gives me that feeling that it is a place rather than an object. The moon in the daytime is the only thing that gives me the sense of awe that religious people say they get from churches, synagogues, mosques and the like, and it is there half the month, just waiting for me to notice it.

    What I am saying is I can understand why the full moon myth can have such a convincing hold over people. It is hard to believe that such a dramatic change in the nature of the night sky has no effect, even when you know intellectually that there is no actual change in the moon itself.

  4. LarryCoon says:

    Kawarthajon was thinking something similar to what I was thinking — could there be a sociological effect? In other words, does the common belief in a lunar effect lead people to behave as though there is a lunar effect, and that behavior leads to…a tangible lunar effect (albeit with a more subtle causal relationship).

    I guess not, since Steve said studies have concluded that there’s no THERE there; i.e., no effect to explain.

  5. Sherrington says:

    As Scott Lilienfeld and his coauthors of “50 Great Myths of Popular Psychology” note, in regard to confirmation bias, many situations in life can be put into a 2 X 2 matrix. He actually uses the lunar effect as an example. The two rows are “Full moon” and “No full moon” and the two columns are “Psychiatric hospital admissions” and “No psychiatric hospital admissions.” People who believe in the lunar effect emphasize occurrences in the “Full moon”/”Psychiatric hospital admissions” cell to the exclusion of the other three cells. Of course, to see if there is a real correlation, we must pay equal attention to those cells.

  6. pdeboer says:

    moon Apogee (rm)= 362,570 km.
    moon mass (Mm) = 7.3477 × 10^22 kg
    mass of top half of the brain (Mb1) = 0.65kg
    mass of bottom half of the brain (Mb2) = 0.65kg
    distance between both halves of the brain (drb) = 0.1 m

    F1 = G*Mm*Mb1/rm^2 = 2.42E-05 N
    F2 = G*Mm*Mb2/(rm+drb)^2 = 2.42469E-05 N

    dF = 1.34E-14 N

    This is the spaghettification force difference applied to the brain.

    Whether it is a full moon doesn’t matter, “this stretching force” is constant, only the direction changes.

    For perspective, this stretching would be the equivalent of hanging The DNA from a single cell from the bottom of your brain.

    Unfortunately this force doesn’t exist at all because your brain is suspended, not anchored against the pull of the moon.

  7. ConspicuousCarl says:

    I am going to start spreading the rumor that -AHEM-

    I am going to start revealing the secret truth that a visible daytime moon can improve your golf game. The pros all know about it, which is why you see people like Tiger Woods and Phil Mickelson being so careful with their driver selection. What you do is take the angle of the moon from the horizon, subtract 23.5 degrees, and then choose a driver with an angle as close to that as possible. If you do it right, you will notice a slight period at the apex of the flight during which the ball seems to hang at the same altitude for a few seconds. It can be the edge you need to keep the ball on the green.

    Look for Conspicuous brand sextants at the pro shops this summer. Originally sold in Europe for $299, but I’m knocking it down to a nice patriotic $249 for my fellow Americans.

  8. BillyJoe7 says:


    “Unfortunately this force doesn’t exist at all because your brain is suspended, not anchored against the pull of the moon”

    Sorry, but I cannot make any sense of your closing statement.

    What do you mean by “your brain is suspended”?
    If you mean, floating in CSF, how does that make a difference?
    How does that cause the force to not exist?


  9. ksadrieh says:

    If there is no lunar effect, then how do you explain werewolves….

  10. HHC says:

    Is it that American mental health workers’ wishful-thinking bets on the planet Moon more than the Sun? Do they feel that their clientele are lunatics afterall?

  11. ccbowers says:

    BJ7 – I think you are right – that last line doesn’t make sense as written.

    When I read it the first time I thought he was saying that the pull of ‘hanging DNA’ could not exist since the brain (and the mentioned DNA of a single cell) is suspended in the CSF, which might make sense, but tidal forces can exist whether or not the brain is floating. Of course none of this matters because the magnitude of the force is so tiny, but floating will not protect you as you approach a black hole.

  12. gr8googlymoogly says:

    I’ve had similar experience discussing the ‘phone call premonition’ thing – where a person thinks of someone, and very shortly thereafter, the get a phone call from that person. Their argument is that it proves some kind of psychic phenomenon because of how improbable it is.

    My first questions to them are:

    How many times have you thought of a person and they didn’t call soon after?
    How many times did a person call without you thinking about them recently?
    How many times did no one call you and you thought of no one?

    Their immediate reply is almost always something like “but there is nothing special about those events”. To which I reply “Exactly”… not that they ever see my point.

  13. ccbowers says:

    Another factor that feeds into the confirmation bias of this lunar effect is the counting of “near hits.” Even though the moon is full approximately once a month, the moon will look pretty close to full for a few days before and after a full moon. A person looking to confirm a lunar effect will likely attribute unusual circumstances to the moon appearing full, even though it may be a 2-3 days before or after a full moon. Combine this with the dismissing of “misses” and you have a situation in which its pretty easy to self-deceive, especially for a person who doesn’t understand these mechanisms of self deception.

  14. eiskrystal says:

    I can’t help thinking that confirmation bias seems to take most of it’s effect from ignoring the misses, rather than seeing the hits.

  15. ccbowers says:

    “I can’t help thinking that confirmation bias seems to take most of it’s effect from ignoring the misses, rather than seeing the hits.”

    You really need both. Actually, I think its often the inclination to find ‘near hits’ compelling that I think drives the ignoring of the ‘misses.’ People are already primed for seeing an effect, so discomfirming examples can more easily be dismissed as exceptions. In other words, you can’t really separate the two

  16. mink says:

    As they say on the radio… first time caller, longtime listener.

    I can’t help but to think that even the fact that you’re compelled to talk about a Lunar Effect means that there is one. It clearly is not be a physical phenomenon, but simply the fact that, as you said, 40-45% of even highly educated people believe in one make it some plausible there could be an influence on behavior. In other words, sometimes belief in something may not make the thing being believed in real, but the effect of that belief is real.

    For example, if I believe in a Lunar effect and therefore avoid traveling during the evening on the full moon, but an equal number of people believe in a Lunar effect and therefore do some other activity that balances this out, then between the masses of people that don’t give it any consideration at all, these small fluctuations in behavior will be lost in the noise.

    In summary, maybe it’s best just to make my point in the form of a question. If even a small number of people believe in the Lunar effect and alter their behavior because of it, even if we can’t measure the impact at a population level, isn’t the effect still real because some number of people acted as if it were real?

  17. pdeboer says:

    “Unfortunately this force doesn’t exist at all because your brain is suspended, not anchored against the pull of the moon”

    You guys are correct, the force is there. I was noting that if the spaghettification force difference existed in any significant amount, then the force on your brain would pull harder than the force on your spine and so your brain would tug on your spinal cord and push against your skull.

    However, this isn’t the case. The force does exist however and is simply so insignificant that the fluid and movement and probably the spaghettification effect of the earth counter it.

    Which by the way is 2.00416E-07 N.

    Now, that is quite enough useless, insignificant and very rough calculations.

  18. BillyJoe7 says:


    If you’re saying that the belief in something that doesn’t exist has the effect of changing how the believer acts, I don’t think you’ll get any disagreement, because you’re stating something that is obviously true (I think we’ve all heard about September 11 for example).
    But this is not The Lunar Effect but the belief in The Lunar Effect effect.


    Thanks for acknowledging your error.

  19. ElTejon says:

    Tip to tail probably the best article I’ve ever read on Skepticism. I love the way the article was segmented, fundamental questions were asked and concepts like Confirmation Bias were explained and applied. Of course, it was written by Dr. Novella.

  20. daedalus2u says:

    There are other potential mechanisms. The length of the human menstrual cycle is very close to the length of the lunar cycle (for many women). There is a strong tendency for chaotic cycles (which includes virtually everything in physiology) of about the same period to become entrained with each other, either in phase or out of phase. It would not be unexpected for the 28 day lunar cycle and the 28 day menstrual cycle to become entrained.

    Before artificial light at night, the moon phase would have a pretty large effect on how much light was present at night. For humans living in the wild without artificial light, and having sexual interactions mostly at night, it would be expected that the lunar cycle and the menstrual cycle would become entrained.

    The new moon occurs when the moon is between the Earth and the Sun. This affects the Earth’s geomagnetic field because the moon blocks the solar wind. This shows up mostly as small scale fluctuations. Awake and moving humans are insensitive to small magnetic field variations because moving in the Earth’s magnetic field produces large variations through changed orientation. The only time such variations could be detected by humans would be during long motionless periods, during sleep.

    Artificial light and artificial magnetic field fluctuations (ELF from power line currents) might disrupt entrainment of humans to lunar cycles. Studies would need to be done on humans living where there is insufficient artificial light to affect behavior over the lunar cycle and where there is not ELF from power line currents. However then light from the moon would be sufficient to affect behavior through having increased light for activity rather than through a physiological effect.

  21. ccbowers says:

    “But this is not The Lunar Effect but the belief in The Lunar Effect effect.”

    If there is a ‘lunar effect effect’ this would have been picked up in the studies that have explored the ‘lunar effect.’ If there is such an effect it is likely neglible, and the only advantage that it has over the ‘lunar effect’ is some plausibility. But without first determining that such effect exists it seems pointless to try to think of a mechanism.

    Also, it appears to me that the confirmation bias we see with the lunar effect is more of a post hoc rationalization of external circumstances (e.g. asking if there is a full moon after a crazy ER shift) rather than thinking prospectively about one’s own bevaior. This makes the ‘lunar effect effect’ seem even less likely.


    Its refreshing to read someone explicitly acknowledge making an honest mistake. Too often people are unwilling to even acknowledge a mistake was made, or they ignore it and change subjects.

  22. Jacob V says:

    I wonder if large predator animals that tend to be nocturnal hunters are more successful or more active when it’s a full moon. And perhaps the notion that bad things happen when it’s a full moon goes way back to a Stone age family having their children getting eaten by a tiger or pack or wolves during a full moon got the stories going and the doors closed during full moons.

  23. BillyJoe7 says:


    BJ: “But this is not The Lunar Effect but the belief in The Lunar Effect effect.”
    ccbowers: “If there is a ‘lunar effect effect’ this would have been picked up in the studies that have explored the ‘lunar effect.’”

    I was responding to mink who was talking about a situation where the behaviours of a group of believers in the lunar effect is cancelled out by the alternate behaviours of an equal number of other believers in the lunar effect, so that an overall effect of belief in the lunar effect is not detected in studies of the lunar effect.

    Sorry – i know this is getting complicated – but if you can see your way through to your error I trust you will acknowledge it. 🙂

  24. daedalus2u says:

    Jacob, there is a correlation of hunter success with the moon phase, but they are most successful around the new moon, not the full moon.

  25. ccbowers says:

    BJ: Yeah, sorry. I used your quote because you gave a name to the terms, but I did not mean to direct my comments towards you specifically. Actually, I did not get one of mink’s points (from his second paragraph) fully until you pointed them out, but my comments still apply more generally, which was my intention.

    So, apparently he thinks that there could be a ‘belief in lunar effect effect’, but it can’t be detected in the general population because of the non specificity (or nondirectionality) of it’s effects, resulting in too much noise. That’s where my comments regarding the ‘post hoc explanation’ of external factors nature of this type of confirmation bias comes into play. I don’t see this as being a prospective effect influencing a given person’s behavior, but a post hoc attempt at explanation. I might be wrong, but there would need to be evidence for this, and there isn’t.

    It’s a bit of special pleading to see negative studies and conclude that there maybe is an effect, but it only applies to a subset of a population, without identifying that subset or why that subset is different in this regard. Of course I can imagine such a senario, but I can imagine a lot of things.

  26. BillyJoe7 says:

    Mmmm…..yes, like in the other thread where I used quotes from cognate’s post against sonic’s position and then was attacked by cognate thinking I was replying to him! It gets confusing at times.
    And a little annoying….when someone asks a question (mink here and sonic over there) and there’s no response from them when you go to the trouble of answering it. You expect some acknowledgment at least. Oh well…

  27. daedalus2u says:

    If you look at figure 4.

    There is a “full moon effect”. The incidence of fatal lion attacks is lowest during a full moon.

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