“Fool moon” (Full moon effects)


Some folks assert that the moon can impact behavior, and there is evidence for this. For instance:

  • Its light and gravity can affect the growth rate of plants. But for lunar effect believers, this is mighty boring, unless it leads to a gargantuan Venus Flytrap.
  • California grunions mate and spawn on four consecutive nights, beginning with new and full moons. But this is because tides are high, making it a successful reproduction strategy that is only indirectly related to lunar phases.
  • Deep sleep decreased by 30 percent during a full moon in a Swiss study.  There could be a connection, but this was a small sample size and no attempt has been made at test replication.

However, an insomniac feeding azaleas to his fish isn’t what people envision when they consider the lunar effect. Rather, they connect the full moon with crime (preferably committed in a London fog), along with suicide, mental illness, natural disasters, accidents, fertility, and werewolves. Hey, have you ever seen someone succumb to lycanthropy under a quarter-moon?

These beliefs trace to at least ancient Assyrian and Babylonian times. They permeate most cultures and have been a regular feature in Hollywood. Nineteenth Century Englishman Charles Hyde got away with a murder committed during a full moon by convincing jurors that the satellite had sapped his sanity.

However, meta-analysis of more than 100 studies reveals no correlation between any phenomenon commonly attributed to a full moon.

Lunar phases are also sometimes erroneously associated with women’s monthly cycles. This is likely due to three factors. One, most moon deities, such as Diana, are female. Also, the moon was central to agrarian cycles, and one can draw a corollary between this and a woman bringing forth a child. Finally, the length of moon and menstrual cycles are similar. However, a lunar month is always 29.5 days, whereas women’s cycle is about 28 days, so this notion is as mistaken as the others.

Despite all this, British police inspector Andy Parr told the BBC, “From my experience of 19 years, undoubtedly on full moons we get people with strange behavior. They’re more fractious and argumentative.”

For every study that refutes this, there are 100 anecdotes from cops, nurses, and friends, who report sinister urges springing forth when the moon waxes. Indeed, there are factors at play here, but they are confirmation bias and communal reinforcement. With confirmation bias, if something malevolent happens during a full moon, it gets noticed. If something doesn’t happen, it goes unnoticed. It something fortunate happens, it likewise is forgotten. Closely related is communal reinforcement, where believers swap tales of moon madness, strengthening the conviction.

Skeptic leader Dr. Steven Novella related this experience: “I was working in the emergency room during a busier than average night. A nurse commented, ‘Wow, it’s really crazy tonight. Is there a full moon?’ When I informed her no, she shrugged and forgot the whole thing. But other busy ER nights that fall on a full moon would resonate with her and confirm her belief.”

Some prefer the moon’s power be veiled in mysterious cloak, while others try and inject a more scientific-sounding spin. One idea is that since the moon affects the ocean’s tides, it must affect the mostly-water human body as well. But astronomer George Abell pointed out that if observing a mosquito on one’s arm under a full moon, the annoying insect is exerting more gravitational pull than the light source. Besides, the moon’s tidal force depends on its distance from Earth, not its phase.

Then we have the notion that positive ions greatly increase during a full moon and that this impacts human behavior. This likewise is without merit. This may be selective memory similar to what believers in the lunar effect experience, but it seems to me that “ions” is one of those words that people who don’t understand science throw out there to make it sound like they know what they’re talking about.

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