By Marissa Kantor, published on January 1, 2005 - last reviewed on June 9, 2016
Q: Does the full moon influence human behavior?
A: Everything from increases in violent crime and psychotic behavior to stock market fluctuations has been blamed on the effects of the fully illuminated moon. In 19th-century England, lawyers used the "guilty by reason of the full moon" defense to claim that their "lunatic" clients could not be held accountable for acting under the moon's influence.
In his 1978 best seller, How the Moon Affects You, psychiatrist Arnold Lieber argued rather unscientifically that the moon has an effect on the human body (which is 65 percent water) that is similar to its pull on the ocean's tides.
Despite these many assertions, scientists who have investigated the matter have come up empty-handed. University of Sydney researchers found no link to the moon's cycle in two separate studies, one of violent or aggressive behavior, the other of dog bites that required human hospitalization.
And in an analysis that ought to put to rest any lingering doubts, Ivan Kelly, a psychologist at the University of Saskatchewan, found in a review of over 100 studies of lunar cycles and behavior -- including emergency room admissions and suicide attempts -- nothing to suggest that humans are affected by Earth's satellite.
So why do 81 percent of mental health professionals, according to a University of New Orleans study, believe that lunar cycles affect human behavior? Part of the reason is historical: The illuminated moon played a more prominent role for our ancestors as both a calendar and a night-light. Before electric lighting became ubiquitous, a bright moon was more likely to disrupt sleep, producing widespread grouchiness.
Kelly also cites what psychologists call confirmation bias, selective thinking whereby we seek out information that confirms our beliefs and ignore evidence that challenges them. Says Kelly, "Some beliefs are just exciting to hold, whatever the evidence."