“The full moon causes erratic behavior.”

The enduring belief that the full moon triggers erratic behavior, often termed "lunacy," traces its roots back to antiquity. Ancient Greek physicians like Hippocrates and Galen theorized that the moon, much like its influence on tides, could disturb the balance of bodily fluids in the brain, leading to madness in susceptible individuals. This idea was further cemented by the Romans, whose word for moon, "luna," became the origin of "lunatic," specifically referring to those whose mental state seemed to worsen with lunar phases. This historical connection persisted through medieval Europe and into later centuries, shaping both medical understanding and legal definitions of insanity.

Despite this long-standing cultural conviction, scientific investigations have consistently debunked any direct link between the full moon and human behavior. Numerous large-scale studies, analyzing everything from psychiatric hospital admissions and crime rates to suicides and instances of aggression, have found no significant or consistent correlation with lunar phases. While some theories have suggested the moon's gravitational pull might affect the water in our bodies, akin to ocean tides, scientists point out that this force is far too weak to influence the contained fluids within a human body.

The persistence of this myth largely stems from psychological phenomena like confirmation bias and illusory correlation. People are more likely to notice and remember unusual or chaotic events that happen during a full moon, while overlooking similar occurrences during other lunar phases. This selective recall reinforces the preconceived notion, creating a perceived pattern where none truly exists. Anecdotal reports from professions like healthcare and law enforcement, often shared and reinforced over time, also contribute to the myth's longevity, despite scientific evidence to the contrary.