Obviously, a full moon isn't some supernatural portent of doom in the real world. Movies just use it to symbolize the fact that bad stuff is about to go down because ... well, just because.
But in Reality ...
Wait a minute, why are people so freaked out by the moon in the first place? We're talking about the most powerful natural source of light at night here -- you'd think the cultural standard would be to give the moon a nod and say, "Thanks, moon. You're all right in our book." And it's not just movies: Folklore has connected the full moon to almost everything confusing and scary throughout human history, from werewolves to gods to women's menstrual cycles. Even the word "lunacy" comes from the Latin word for "moon." Why is that?
Well, it all comes down to lions.
A bunch of scientists studying lion attacks in Africa discovered a weird correlation: In the days immediately following a full moon, attacks on humans spiked. They discovered that the most dangerous time for people to be out is "the active hours after sunset, particularly the day after the full moon."
Why? Because a full moon means more light, and more light means the lions' prey can see them coming and more easily avoid an attack. So the lions get really friggin' hungry when the moon is full. But after that, the moon starts to rise later, meaning more darkness in the early evening -- which is right about the time people are most likely to be cruising around, their supple butt cheeks quivering in the darkness like a pair of steaks all marinated up for a tasty lion barbecue.
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"No, no, girl, I'm telling you. You have to chase for a bit before you kill them. The adrenaline makes the meat so tender."
Much of the human timeline for the hundreds of thousands of years since we decided to quit dragging our knuckles on the ground has seen us living alongside nocturnal predators that wanted nothing more than to gnaw on our tastybits -- not just lions, but jaguars, tigers, leopards, big-ass spiders ... you name it. So when you look at it that way, it seems pretty reasonable that we would have developed a cultural fear of the full moon, considering that it historically meant your hunting/gathering party was about to become a re-enactment of The Ghost and the Darkness, minus the protection of a rifle or Val Kilmer's "Irish" accent.