Time for a complete 180 (360 if you're Jason Kidd): I'll bet you don't know anyone who believes the last entry's statement, and I wouldn't be surprised if practically everyone you know believes this one.  I remember many of my teachers claiming that we students went berserk during the full moon, to the point of saying "it must be a full moon" if we were acting up and getting preemptively irritable if they'd happened to look at a lunar calendar and see that it was the day of the full moon.

You can hardly blame them for thinking that the phase of the moon influences people's behavior.  The legend of the werewolf is ingrained in our culture, with lycanthropy playing a role in stories ranging from ancient myths all the way to Harry Potter.  The words lunacy and lunatic come from the name of the Roman moon goddess, Luna.  The purported effect of the full moon on behavior even has technical names such as the "lunar effect" or the "Transylvanian effect," though the latter sounds to me like people are mixing up their monster mythologies.

A few minutes of googling through woo-woo sites provides countless examples of things the full moon supposedly influences.  Stock prices, murder rates, baby births, car crashes, suicide rates, sleep cycles, and just about everything else under the sun (heh).  I didn't bother looking at the astrology sites, but I'm sure those guys just go bonkers over the full moon (ba dum, cha!).  Such a widespread and powerful belief must have some basis in reality, right?

Researching the Transylvanian Effect (Cue Thunder, Lightning, Horse Whinnying)

Wrong.  Over a hundred articles have been published examining the effect the full moon might have on all sorts of things, ranging from dog bites to binge drinking.  The majority of the studies show no effect, and many of the positive studies have been reviewed and refuted for a variety of reasons.  Some of the positive studies had statistical errors, others could not be replicated, still others championed 'increased effects' that were within the noise of their data, and one even seems to have divided by the wrong number of days in a lunar month.  I found a nice, recent, freely-available review of the literature in the Canadian Medical Association Journal called "Bad Moon Rising: The Persistent Belief in Lunar Connections to Madness."  Woo-debunking and some CCR, what more could you ask for?

Such a serious lack of evidence across the board was actually a bit surprising to me.  Since so many people believe that the full moon causes some kind of insanity, I'd think people would be more inclined to do wacky things on the night of a full moon.  A sort of "eh, why the heck not, it's a full moon" kind of attitude.  But, there's no need to invoke such a hypothesis, because the alleged lunar effect is nil.

Confirming the Bias

To heck with you and your studies, one might say, millions of schoolteachers and ER workers can't be wrong.  Well, yes they can.  Think back to my teachers, and how they responded to the class going bonkers or the knowledge that it was a full moon.  If the class was acting up, they would wonder if it was a full moon.  If it was indeed a full moon, they would use this positive evidence to support their full moon belief.  If it was not a full moon, they would shrug off this negative evidence and think nothing more of it.  Conversely, if it was a full moon, they started the day looking for misbehavior to ascribe to the full moon.  Anything the students did beyond perfect obedience would be used as positive evidence for the "lunar effect," regardless of how much the students normally acted up on any old day.

This is called confirmation bias.  People who believe in myths about the full moon are just counting the hits and ignoring the misses.  My teachers were only counting the positive evidence (misbehavior during full moons) and ignoring the negative evidence (normal behavior during full moons) and false positives (lots of misbehavior during non-full moons).  If you only pay attention to data that supports one hypothesis, then you're going to feel like that hypothesis just keeps accumulating more and more evidence.

Believe in the Lunar Effect?  That's Lunacy!

Not really, but I thought it sounded pretty good.  Believing that full moons cause people to act crazy isn't going to hurt anyone, but then again neither would believing that a completely undetectable dragon lives in your garage.  (Dragon analogy stolen egregiously from Carl Sagan.)  The only thing the full moon has an effect on is people's ability to accurately analyze their own observations.

The full moon myth is a great example of how pseudoscience thrives.  Take some long-held cultural traditions, add a dash of credulous reporting and storytelling, simmer until you achieve common knowledge status, and regularly baste with cognitive biases.  In no time, you'll have a pseudoscientific belief to rival the one about only using 10% of your brain.