The approximately fortnightly tidal cycle has large effects on intertidal organisms. Hence their biological rhythms tend to occur in rough multiples of this period. Many other animals such as the vertebrates, display similar rhythms. Examples include gestation and egg hatching. In humans, the menstrual cycle lasts roughly a lunar month, an even multiple of the tidal period. Such parallels at least hint at the common descent of all animals from a marine ancestor.
full moon and lunar effects
25 Nov, 2011
The full moon has been linked to crime, suicide, mental illness, disasters, accidents, birthrates, fertility, and werewolves, among other things. Some people even buy and sell stocks according to phases of the moon, a method probably as successful as many others. Numerous studies have tried to find lunar effects. So far, the studies have failed to establish much of interest. Lunar effects that have been found have little or nothing to do with human behavior, e.g., the discovery of a slight effect of the moon on global temperature,* which in turn might have an effect on the growth of plants. Of course, there have been single studies here and there that have found correlations between various phases of the moon and this or that phenomenon, but nothing significant has been replicated sufficiently to warrant claiming a probable causal relationship.
Ivan Kelly, James Rotton and Roger Culver (1996) examined over 100 studies on lunar effects and concluded that the studies have failed to show a reliable and significant correlation (i.e., one not likely due to chance) between the full moon, or any other phase of the moon, and each of the following:
-the homicide rate
-crisis calls to police or fire stations
-births of babies
-casino payout rates
-aggression by professional hockey players
-violence in prisons
-psychiatric admissions [one study found admissions were lowest during a full moon]
-agitated behavior by nursing home residents
-emergency room admissions [but see]
-behavioral outbursts of psychologically challenged rural adults
If so many studies have failed to prove a significant correlation between the full moon and anything, why do so many people believe in these lunar myths? Kelly, Rotton, and Culver suspect four factors: media effects, folklore and tradition, misconceptions, and cognitive biases. A fifth factor should be considered, as well: communal reinforcement.
the media perpetuate lunar myths
Lunar myths are frequently presented in films and works of fiction. “With the constant media repetition of an association between the full moon and human behavior it is not surprising that such beliefs are widespread in the general public” (Kelly et al. 1996). Reporters also “favor those who claim that the full moon influences behavior.” It wouldn’t be much of a story if the moon was full and nothing happened, they note. Anecdotal evidence for lunar effects is not hard to find and reporters know that one good anecdote trumps ten scientific studies when it comes to reader interest, even though such evidence is unreliable for establishing significant correlations. Relying on personal experience ignores the possibility of self-deception and confirmation bias. Such evidence may be unreliable, but it is nonetheless persuasive.
folklore and tradition
Many lunar myths are rooted in folklore. For example, an ancient Assyrian/Babylonian fragment stated that “A woman is fertile according to the moon.” Such notions have been turned into widespread misconceptions about fertility and birthrates. For example, Eugen Jonas, a Slovakian psychiatrist, was inspired by this bit of folklore to create a method of birth control and fertility largely rooted in astrological superstitions. The belief that there are more births during a full moon persists today among many educated people. Scientific studies, however, have failed to find any significant correlation between the full moon and number of births (Kelly and Martens 1994; Martens et al.1988 ). In 1991, Benski and Gerin reported that they had analyzed birthdays of 4,256 babies born in a clinic in France and “found them equally distributed throughout the synodic (phase) lunar cycle” (Kelly, et al. 1996: 19). In 1994, Italian researchers Periti and Biagiotti reported on their study of 7,842 spontaneous deliveries over a 5-year period at a clinic in Florence. They found “no relationship between moon phase and number of spontaneous deliveries” (ibid.).
Despite the fact that there is no evidence of a significant correlation between phases of the moon, the menstrual cycle, and fertility, some people not only maintain that there is, they have a “scientific” explanation for the non-existent correlation.* Some think the light of the moon affects fertility in women, the way it does in corals. The light of the moon is a very minor source of light in most women’s lives, and is no more likely than the moon’s gravitational force to have a significant effect on a woman’s ovulation. Furthermore, the average menstrual cycle is 28 days but varies from woman to woman and month to month, while the length of the lunar month is a consistent 29.53 days.* Some of us have noticed that these cycles are not identical. Furthermore, it would seem odd that natural selection would favor a method of reproduction for a species like ours that depended on the weather. Clouds are bound to be irregularly and frequently blocking moonlight, which would seem to hinder rather than enhance our species’ chance for survival.
Some mythmakers believe that long ago women all bled in sync with the moon, but civilization and indoor electric lighting (or even the discovery of fire by primitive humans) have disturbed their rhythmic cycle. This theory may seem plausible until one remembers that there are quite a few other mammals on the planet that have not been affected by firelight or civilization’s indoor lighting and whose cycles aren’t in harmony with the moon. In short, given the large number of types of mammals on our planet, one would expect that by chance some species’ estrus and menstrual cycles would harmonize with lunar cycles (e.g., the lemur). It is doubtful that there is anything of metaphysical significance in this.
What we do know is that there has been very little research on hormonal or neurochemical changes during lunar phases. James Rotton’s search of the literature “failed to uncover any studies linking lunar cycles to substances that have been implicated as possible correlates of stress and aggression (e.g., serotonin, melatonin, epinephrine, norepinephrine, testosterone, cortisol, vasopressin [directly relevant to fluid content], growth hormone, pH, 17-OHCS, adrenocrotropic hormone [? adrenocorticotropic hormone?])” (Rotton 1997). One would think that this area would be well-studied, since hormones and neurochemicals are known to affect menstruation and behavior.
Misconceptions about such things as the moon’s effect on tides have contributed to lunar mythology. Many people seem to think that since the moon affects the ocean’s tides, it must be so powerful that it affects the human body as well. The lunar force is actually a very weak tidal force. A mother holding her child “will exert 12 million times as much tidal force on her child as the moon” (Kelly et al., 1996: 25). Astronomer George O. Abell claims that a mosquito would exert more gravitational pull on your arm than the moon would (Abell 1979). Despite these physical facts, there is still widespread belief that the moon can cause earthquakes.* It doesn’t; nor does the sun, which exerts much less tidal force on the earth than the moon.
The fact that the human body is mostly water largely contributes to the notion that the moon should have a powerful effect on the human body and therefore an effect on behavior. It is claimed by many that the earth and the human body both are 80% water. This is false. Eighty percent of the surface of the earth is water. Furthermore, the moon only affects unbounded bodies of water, while the water in the human body is bounded.
Also, the tidal force of the moon on the earth depends on its distance from earth, not its phase. Whereas the synodic period is 29.53 days, it takes 27.5 days for the moon to move in its elliptical orbit from perigee to perigee (or apogee to apogee). Perigee (when the moon is closest to earth) “can occur at any phase of the synodic cycle” (Kelly et al. 1990: 989). Higher tides do occur at new and full moons, but not because the moon’s gravitational pull is stronger at those times. Rather, the tides are higher then because “the sun, earth, and moon are in a line and the tidal force of the sun joins that of the moon at those times to produce higher tides” (ibid.: 989).
Many of the misconceptions about the moon’s gravitational effect on the tides, as well as several other lunar misconceptions, seem to have been generated by Arnold Lieber in The Lunar Effect (1978), republished in 1996 as How the Moon Affects You. In The Lunar Effect, Lieber incorrectly predicted a catastrophic earthquake would hit California in 1982 due to the coincidental alignment of the moon and planets. Undeterred by the fact that no such earthquake had occurred, Lieber did not admit his error in the later book. In fact, he repeated his belief about the dangers of planet alignments and wrote that they “may trigger another great California earthquake.” This time he didn’t predict when.
cognitive biases and communal reinforcement
Many believe in lunar myths because they have heard them repeated many times by members of the mass media, by police officers, nurses, doctors, social workers, and other people with influence. Once many people believe something and enjoy a significant amount of communal reinforcement, they get very selective about the type of data they pay attention to in the future. If one believes that during a full moon there is an increase in accidents, one will notice when accidents occur during a full moon, but be inattentive to the moon when accidents occur at other times. If something strange happens and there is a full moon at the time, a causal connection will be assumed. If something strange happens and there is no full moon, no connection is made, but the event is not seen as counterevidence to the belief in full moon causality. Memories get selective, and perhaps even distorted, to favor a full moon hypothesis. A tendency to do this over time strengthens one’s belief in the relationship between the full moon and a host of unrelated effects.
the moon, madness and suicide
Probably the most widely believed myth about the full moon is that it is associated with madness. However, in examining over 100 studies, Kelly et al. found that “phases of the moon accounted for no more than 3/100 of 1 percent of the variability in activities usually termed lunacy” (1996: 18). According to James Rotton, “such a small percentage is too close to zero to be of any theoretical, practical, or statistical interest or significance” (Rotton 1997).
Finally, the notion that there is a lunar influence on suicide is also unsubstantiated. Martin et al. (1992) reviewed numerous studies done over nearly three decades and found no significant association between phases of the moon and suicide deaths, attempted suicides, or suicide threats. In 1997, Gutiérrez-García and Tusell studied 897 suicide deaths in Madrid and found “no significant relationship between the synodic cycle and the suicide rate” (p. 248). These studies, like others which have failed to find anything interesting happening during the full moon, have gone largely unreported in the press.
postscript: There are likely to be many studies in the future that find a positive correlation between some lunar phase and some human behavior (or process affected by human behavior, such as the stock market). Remember to consider a few caveats: correlation doesn’t establish causation; studies that are well designed still need to be replicated before they are accepted as not being flukes; some studies with positive results will suffer from design flaws or methodological errors. For example, in 2005 Yuan, Zheng, and Zhu found “that stock returns are lower on the days around a full moon than on the days around a new moon. The magnitude of the return difference is 3% to 5% per annum based on analyses of two global portfolios: one equal-weighted and the other value-weighted.” Whether this is a lunar effect remains to be seen. The study needs to be replicated with a significant number of data points.
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