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True or False: Full Moons Sway Tides -- and Minds

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mythbuster graphic The phases of the moon have traditionally been linked to human madness, and myths abound concerning the influence of the full moon on human behavior, psychology, and biological functioning. The word “lunacy” means insanity, and is derived from the Latin stem “luna,” meaning moon. For years, stories have been told about how incidences of violent crime, suicide, emergency room admissions, and cases of anxiety and depression increase during a full moon. Some people claim that the lunar cycle also affects birth rates, fertility, menstrual cycles, cancer recovery, seizures, surgical operations, and stock returns. Numerous studies have examined possible trends, and although a few have found potential links, larger, more comprehensive reviews have uncovered no cause-and-effect relationships.

Evidence for the Health Claim

Folklore from a diverse number of cultures provides the basis for most moon-related beliefs and rituals, some of which have carried over until today. For example, many religious ceremonies and holidays, such as Easter, are scheduled according to the lunar cycle.

Surveys in the US have found that about 50% of participants believe the lunar cycle affects individual behavior, and interestingly, psychiatric nurses, emergency room doctors, and police officers hold this belief more strongly than other types of professionals.

Too Much Light

One hypothesis supposes that the full moon may have had more of an effect before modern lighting was developed. Before the electric light was invented, full moons provided a significantly greater amount of light than other nights (there is one full moon approximately every 29-30 days). So, persons with bipolar and other mental disorders may have suffered sleep deprivation and other minor disturbances in behavior on these bright nights. Once electric light became available all night, every night, the lunar effect would have diminished, which could explain why so many stories persist from the past, but are difficult to confirm nowadays.

Similar Cycles

Various hypotheses have also been proposed linking human biological cycles to the phases of the moon. The most commonly cited of these is the 28-day menstrual cycle, and indeed some studies have found relationships between the phases of the moon and ovulation, fertility, birth rates, and babies’ genders. No precise explanation for how the moon might influence physiology has been confirmed by these studies, but ideas about the gravitational pull of the moon and other physical forces have been suggested.

Full Moon Over Wall Street

Extensive research has been done on the relationship between stock returns and the lunar cycle. These analyses, some of which have been quite extensive, have found that stock values are significantly lower during full moons as compared to new moons. Some researchers believe this is because the moon does affect mood, which in turn affects investor behavior and decision-making. For some mysterious reason investors may value financial assets less or be more pessimistic during a full moon.

Evidence Against the Health Claim

Although a few studies have found connections between the phases of the moon and human behavior, they have been few and far between, and have not demonstrated statistical significance—rendering them wholly unconvincing. The bulk of data indicates a lack of a lunar effect.

For example, research has shown that extreme outcomes like suicides and violent crimes are not affected by the full moon. Various studies have examined records from emergency room admissions, fatal traffic accidents, and crisis calls and found no trends that correspond to the lunar cycle.

There still exists the possibility that more subtle changes in human mood and behavior are connected to the lunar cycle, but even this remains open to debate.


The connection between the moon and madness has been the subject of much scrutiny over the years. While scientific studies have failed to demonstrate a link, many people and cultures still hold fast to myths and superstitions. Folklore, Hollywood, and misconceptions about the power of the moon all may contribute to such popular beliefs. And, of course, selective memory can result in a tendency to judge an otherwise innocent event as “strange” just because it occurred during a full moon.

So, notwithstanding evidence to the contrary, on days when wacky and wild occurrences abound, you will always hear people proclaim, “Must be a full moon!”

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  • Image Credit: Nucleus Communications, Inc.