The year's most famous full moon—the harvest moon, which occurs on September 10—recalls how deeply the moon is intertwined with our culture. For millennia, people have looked to the moon not only to mark the passage of time but to celebrate the cycle of life, and even to assign blame for the vagaries of human biology and behavior. Many beliefs about the moon's influence on human affairs remain embedded in myths. But do any stand up to scientific scrutiny?
Popular literature often associates the full moon with mental illness. When psychologist Ivan W. Kelly of the University of Saskatchewan and two colleagues reviewed 100 studies of putative moon-influenced behaviors, however, they could find no correlation. A study of 4,190 suicides over 58 years in Sacramento County, California, uncovered no ties with the moon, full or otherwise. Several studies likewise revealed no link between lunar phases and psychiatric-hospital admissions, while one claiming to reveal such a connection showed that mental crises seem to be lowest during a full moon.
Crime is also commonly linked to the phases of the moon. Arnold Lieber, a psychologist at the University of Miami, analyzed 14 years of homicides in Dade County, Florida, and reported evidence of a lunar pattern. The claim, published in 1972 and updated by Lieber in a popular book, is widely cited on the Internet. Subsequent analyses of the Dade County data, including one by the late astronomer George Abell of the University of California at Los Angeles, failed to support Lieber's conclusions. Crime rates increased during hot weather and on weekends, but they had no association with the moon.
Many physicians still agree with the folk wisdom that human births march to a lunar drumbeat. A 1959 study of 500,000 births in New York City, carried out by father-and-son doctors Walter and Abraham Menaker, seemed to show a 1 percent increase in births around the full moon. Once again, follow-up research could not duplicate the finding. Kelly, in collaboration with Belgian chemist Ronnie Martens and psychologist Donald H. Saklofske of the University of Saskatchewan, evaluated 21 moon/birth studies. More recently, astronomer Daniel Caton of Appalachian State University sorted through 70 million birth records from the National Center for Health Statistics. Both studies reached the same conclusion: No connection exists.
The moon brightens abruptly as it moves into position opposite the sun in the sky, where shadows vanish and sunlight bounces straight back from the lunar soil. In the 2 1/2 days leading up to the full moon, it doubles in brilliance, which is one reason that phase seems so dramatic.
Menstrual cycles have been linked with the lunar cycle for thousands of years. The word menstruation is derived from the same root as moon, and recent research shows that the average woman's menstrual period is in fact very close in duration to the 29 1/2-day length of the lunar month. On the other hand, only about 30 percent of all women have periods that are within two days of the average, and many are irregular. Furthermore, the only other creature with an estrous cycle of similar duration is the opossum. One would have to believe that humans and opossums, among all mammals, were specially chosen by nature to be linked to the moon.
There are many ways in which the moon unambiguously affects life on Earth. Ocean tides, which average five feet high worldwide, rule the activity cycles of marine organisms and their predators, especially within the intertidal zone. Atmospheric tides change barometric pressure and may cause a slight statistical increase in cloudiness and rainfall around the full moon. Solid tides raise the ground eight inches. These gravitational effects steadily slow Earth's rotation, so our day is many hours longer than it would be without the moon.
The evidence linking the moon with everyday human life, in contrast, seems little more than hearsay. But only a lunatic would consider the case closed.