For centuries, the full moon has been associated with madness. The term “lunatic” was once used to refer to people who are considered mentally ill. It also was a label put on someone who was dangerous, foolish or unpredictable. “Lunacy” is now considered insulting and not used as a medical or legal term – though it is still used in jest.

The words are from lunaticus meaning “of the moon” or “moonstruck”. The term originally referred mainly to epilepsy and “madness” as these were diseases believed to be caused by the Moon.

By the fourth and fifth centuries astrologers began to commonly use the term to refer to neurological and psychiatric diseases Philosophers such as Aristotle and Pliny the Elder argued that the full Moon induced insanity in some individuals because the Full Moon provided light during nights which would otherwise have been dark. This extra light caused sleep deprivation.

Into the 17th century, it was also a common belief that the Moon influenced fevers, rheumatism, episodes of epilepsy and other diseases.

Scientific study has continually shown that Full Moons do not cause madness or an increase in suicides. You can find stories online about both of those beliefs and others. I have heard a number of times that there are more animals killed on roads in a Full Moon period.

Fauna fatalities peak along secondary roads through edge habitat (where two types of habitat meet). Add more deaths during late summer and early fall, when spring-born leave home to strike out on their own. And add more on new and full moons, when drivers seem more reckless and animals less reclusive.

The lunar theories continue. In 2005, Yuan, Zheng, and Zhu found “that stock returns are lower on the days around a full moon than on the days around a new moon. The magnitude of the return difference is 3% to 5% per annum based on analyses of two global portfolios: one equal-weighted and the other value-weighted.”  The return difference is not due to changes in stock market volatility or trading volumes. The lunar effect is not explained away by announcements of macroeconomic indicators, nor is it driven by major global shocks. Moreover, the lunar effect is independent of other calendar-related anomalies.

Is this truly a lunar effect? That remains to be seen.