First, let's get the historical context out of the way. You probably already know that the word "lunatic" comes from the Roman moon goddess "Luna". In ancient Greece and Rome, philosophers posited that the water in the brain might be subject to the same kind of tidal motions as the sea, creating a wave of strange behavior whenever the moon was particularly full or large in the sky. Some version or another of this belief has survived through hundreds of years and countless cultural shifts. Even today, some psychiatrists continue to hold up this watery brain theory. But while humans are indeed made of mostly water, this theory doesn't hold any. From Scientific American Mind:
As the late astronomer George Abell of the University of California, Los Angeles, noted, a mosquito sitting on our arm exerts a more powerful gravitational pull on us than the moon does. Yet to the best of our knowledge, there have been no reports of a “mosquito lunacy effect.” Second, the moon’s gravitational force affects only open bodies of water, such as oceans and lakes, but not contained sources of water, such as the human brain. Third, the gravitational effect of the moon is just as potent during new moons — when the moon is invisible to us — as it is during full moons.
But even without a plausible physical cause, some still point to anecdotal evidence of higher rates of crime, emergency room admissions and surgical mistakes, just to name a few loony examples. One-off studies have occasionally shown one or another of these things to be "true," but remember: A single study means nothing. No one has ever been able to show consistently, with multiple studies, that the full moon has any effect on behavior.
In fact, when researchers Ivan Kelly, James Rotton, and Roger Culver reviewed over 100 of these studies back in 1991, they found none of them to show a significant relationship between the moon and human behavior. Much of the research had been poorly conducted, they found, or ignored obvious variables (one study concluded that full moons increase the incidence of car accidents, ignoring the fact that nearly all the full moon nights used for data had occurred on the weekend – when car accidents are more likely no matter what).
Kelly, Rotton and Culver suggested that many of the study participants were swayed by certain cognitive biases. They might recall the full moon nights when strange things happened and forget all the gloriously mundane full moon nights they'd seen, or even be more likely to see an occurrence as being spooky or unusual because they knew a full moon was hanging in the sky at the time.
"It is important to note that there are two hurdles to overcome before any findings on lunar variables and human behavior are deserving of public attention," the men concluded. "The first hurdle is that reliable (i.e., replicable) findings need to be reported by independent investigators. The second hurdle is that the relationship should not be a trivial one. The lunar hypothesis fails on both counts."
But full moons might make certain behaviors more likely even without any sort of freaky mind control. One of the more convincing studies on lunar madness showed that pets are more likely to be admitted to the vet with injuries after a full moon.
Let's think about that one for a second. If someone told you that the consumption of ice cream puts you at a greater risk of drowning, would you believe it? Good, because it doesn't. But the consumption of ice cream does go up and down in the same patterns as death by drowning does, because both of those things happen when it's warm. When the moon is full, pet owners might be more likely to take their animals out at night. The bright light of an inviting moon could also explain upticks in crime (though there's no strong evidence that this actually occurs). Just as warm weather puts more people outside, interacting with others and sometimes getting up to no good, a well-lit night might help some folks end up in the wrong place at the wrong time.
The brightness of the full moon brings us to a possible explanation for all of the historical work on lunar lunacy. Some researchers believe that the full moon did create a kind of madness at one time – back before humans controlled their light exposure with indoor electricity and street lights. Those living outside or in shelters that did little to block the sky might have been kept awake by the full moon's intense light, making them act strangely or exacerbating any mental illnesses they may have suffered from.
So the next time your teacher tries to blame the rambunctiousness of the classroom on an upcoming full moon, feel free to correct them: It's not the moon. You guys are just a handful.