Many people believe that the full moon causes people to act crazy, or influences human behavior in some significant way. And it's not just the credulous crowd of New Age, crystal-loving, astrology aficionados who believe this; many police officers, hospital employees, and even some scientists believe this.
At first glance, this belief actually seems plausible, because there is evidence that some animal behavior is attuned to the lunar cycle. Some coral and fish spawning, for example, occurs during specific periods of the lunar cycle. Since there's such a consistent pattern of lunar phases, and because the moon is so bright and easily detectable in the night sky, it's no surprise that certain animals have evolved to have their behavior overlap with lunar phases, because this allows all of the animals to do the same thing at the same times of the year.
So let's start by asking: could there be an evolutionary reason for human behavior corresponding to the lunar cycle? Well, evolution is all about passing on your genes, and, as we've already seen, some animals do have reproductive cycles that overlap with the phases of the moon. So maybe human reproduction is somehow affected by the phases of the moon? What about the menstrual cycle of women? Well, it actually is just about as long as the lunar cycle is. And, furthermore, women are known to be emotionally erratic during their period, and their moodiness could perhaps also influence the temperament of the men around them.
So maybe this could provide us both with an evolutionary explanation as well as an explanation for the allegedly crazy behavior that people exhibit during a full moon? If criminal activity actually is higher during a full moon, maybe men are more prone to go around raping women when they're fertile? Maybe humans, in general, have evolved to have a greater desire to reproduce during a full moon? The only way that this explanation would work, however, is if all women had menstrual cycles that weren't just the length of the lunar cycle, but that actually all coincided with the specific phases of the moon. And this clearly is not the case. All women don't hit the same stage of their menstrual cycle during the full moon. Different women have different stages at different times of the month. So much for that explanation.
Let's set aside the evolutionary aspect of this, and just ask: what is the evidence used to support this belief? Not surprisingly, a lot of it is anecdotal evidence. Everyone has a story about some spooky experience they had during a full moon, and I'm equally unimpressed by all of them. Anecdotes are not the way that we establish truths. For every absurd claim that there is, you can find personal experience stories that allegedly substantiate them. There are people who will tell you about their alien abduction experience, about their encounter with shapeshifting reptilians posing as politicians, about their bigfoot or ghost sighting.
Personal experience is not enough to substantiate outlandish claims for several reasons. First off, people can lie, perhaps for attention, or maybe to make money selling books or giving lectures, or maybe just because they think it's funny. Also, people can be deceived: they can hallucinate; they can mistake an ordinary experience for an extraordinary one; they can concoct false memories; and they can exaggerate and embellish a story either to impress others or just as a consequence of their memory of the initial event fading. In fact, there has actually been research done on this subject, and it shows that the greater the period of time that has elapsed since the initial event, the less accurately people will describe the event, and the more exaggerated their account of it will be. (See: "The Rise and Fall of the Indian Rope Trick" (1996) by Peter Lamont and Richard Wiseman.)
So telling me a spooky story about something that happened to you during a full moon is not enough to convince me. What these people need to do is first actually demonstrate that behavior is altered during a full moon, and then present a causal mechanism for this effect. To their credit, this actually has been attempted. A number of studies have allegedly demonstrated that behavior is influenced by the full moon. Some of the things that were measured in these studies were crime data, car accidents, and violent crime. Although you can find individual studies that point to a full-moon effect, meta-analyses do not detect this trend. A meta-analysis is performed when the results of multiple studies are pooled together and looked at, giving you more data to work with, providing a stronger basis to draw conclusions than individual studies provide.
One of these ("Much ado about the full moon: A meta-analysis of lunar-lunacy research" (1985) by James Rotton and I. W. Kelly) looked at 37 studies, some of which detected no full-moon effect, while others did. The researchers also took a look at the individual studies, and found a number of different problems with some of them. For example, one of the studies claimed to demonstrate that car accidents occurred more frequently during full moons. But the full moons in their data occurred more often on weekends than other days. And more accidents, in general, occur on weekends, regardless of the moon phase. Some of the other studies contained statistical mistakes. When these were corrected for, the detected full-moon effect in some of the studies vanished. So the scientific data largely does not support this belief.
What about the proposed mechanisms? We've already looked at reproduction and the menstrual cycle. What about gravity? Some argue that just as the moon causes tides here on earth, maybe it also pulls on the water within our cells and causes some sort of internal tide that influences our behavior. There is a monumental problem with this argument: the distance between the earth and the moon doesn't coincide with the phases of the moon. The moon's orbit around the earth isn't perfectly circular; its distance from earth varies between 226,000 and 252,000 kilometers. But we don't necessarily see a full moon when it's closest to earth and a new moon when it's farthest from earth; these things just do not overlap with one another 100% of the time. The phases of the moon are just different amounts of reflected sunlight being visible on the moon's surface from our position here on earth; so there's no necessary connection between the amount of light that we see and the gravitational force that the moon exerts on earth. Occasionally the two things do overlap: sometimes we see the full moon when it's closest to earth, in what's called a supermoon. Other times, however, we see a full moon when it's farthest from earth, in what's called a micromoon. So the gravity argument is just nonsensical.
So why does anybody believe that the full moon influences human behavior? One obvious reason is that this could be a self-fulfilling prophecy: If a sizeable percentage of the population believed that the full moon affects behavior, many of these people might have their behavior affected by this belief. And this could be another explanation for why some studies show a full-moon effect.
Another reason people have this belief is confirmation bias: during a full moon, people who have this belief are going to be more on the lookout for confirmatory evidence. Their mind is already primed with the expectation that things are more likely to go wrong during a full moon. So if something strange happens—if they drive by a car accident, or hear a story on the news about somebody getting killed that night—they're going to count this as evidence in favor of their belief. But they don't remember all of the times that the moon was full and nothing interesting happened. They never stop the next day and think: "Huh...maybe I should count this uninteresting night as evidence against my belief?" No, they just won't think anything of it, or maybe they'll even make room for this uneventful night within their belief system by telling themself something like: "Wow, I really dodged a bullet last night. It was a full moon and nothing bad happened to me! I must've just got lucky!"
Imagine, by analogy, that I genuinely believed that eating toast made people act crazy. If I had some toast to go with my breakfast, I would, of course, be more on the lookout for signs of insanity in myself. So if I had some morbid thoughts during breakfast—let's say I thought of somebody I don't like and said to myself "Ahh, I can't wait 'till that sack of shit kicks the bucket" (a pretty standard breakfast for me, actually)—if I had some twisted thoughts like this after eating toast, I would immediately ascribe those thoughts to the toast if I believed that toast influenced behavior. If I said something rude to another person, I would catch myself and say "Please, forgive me, that's just the toast talking." And if I saw other people eat toast and do something a bit crazy, I would also chalk their behavior up to the toast. But it's important take into consideration all the times that I or other people ate toast and didn't do anything crazy.
And the belief itself could actually influence my behavior in the predicted way. The placebo effect is well documented, where pills without the active ingredient are still reported as being effective by some percentage of the users. Our beliefs and expectations can influence our experience. So if I had some toast, it's quite possible that I would start acting a bit crazy because of my expectation that eating toast would cause me to act that way. And then the confirmation bias would kick in, and I would use that behavior as evidence in favor of the belief that caused me to act that way in the first place. It is a vicious cycle of sloppy thinking and self-delusion mutually reinforcing one another. Only in that sense does the full moon influence human behavior.