Are there really more births on full moons?

Cognitive Bias and the Full Moon Birth Myth

Cognitive bias is one reason why the full moon birth myth has prevailed.
Cognitive bias is one reason why the full moon birth myth has prevailed.

The belief that the number of births increases on a full moon is a longstanding one, and one with cultural roots. Folkloric tales have, by definition, been around for long enough to seem like common sense. It can be counterintuitive to argue with a belief that has been passed down through countless generations.

It can also be hard to argue with experts like labor nurses and paramedics. Who would know better?

And then there's the media's love of the spooky old full moon. The press loves a good human interest story about the full moon filling up labor wards or causing a paramedic shortage; and Hollywood loves the iconic werewolf-howling-at-the-full-moon image.

Cognitive bias plays a strong role, as well. Cognitive bias is a psychological phenomenon in which people absorb all evidence that supports their belief and ignore all evidence to the contrary. In this case, that could mean a nurse noticing every time the ward is full on a full moon, but not noticing every time it's empty on a full moon.

And finally, there are some simple misconceptions regarding certain aspects of the moon. For instance, a full moon really only lasts an instant; a "full moon window" of three days, or even one day, is an artificial construct [source: Shulman]. Also, many people believe the moon's gravity is extremely powerful, since it affects the tides; in fact, it's a relatively weak gravitational force, and one unlikely to have an effect on the human body [source: Skeptic's Dictionary]. And the gravitational force that effects the tides is not even phase-related; it's a function of how far the moon is from Earth at any given time, which is different from the phase the moon is in [source: Skeptic's Dictionary].

But hey, all is not lost. We can still hang on to one of the best lunar-effect beliefs: There's no hard evidence that the moon doesn't affect werewolves.

For more information on the lunar effect and related topics, look over the links below.

Related HowStuffWorks Articles


  • Arliss JM, Kaplan EN, Galvin SL. "The effect of the lunar cycle on frequency of births and birth complications." Am J Obstet Gynecol. 2005 May; 192(5):1462-4. Pubmed.
  • Full Moon, Gravitational Pull and Childbirth! BirthSource.
  • Full Moon and Lunar Effects. The Skeptic's Dictionary. Feb. 23, 2009.
  • Gans, Joshua S. and Andrew Leigh. "Does the Lunar Cycle Affect Birth and Deaths?" The Australian National University Centre for Economic Policy Research. August 2006.
  • Joshi, Raksha, et al. "Labor ward workload waxes and wanes with the lunar cycle, myth or reality?" ScienceDirect. May 31, 2000.
  • Shulman, Holly B. "Investigating Lunar Cycles in Monthly Fertility Rates." Bureau of the Census. April 29, 1987.