A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do things get crazy when the moon is full?

March 13, 1987

Dear Cecil:

For years I've heard cops and emergency room nurses say things really get crazy out there when the moon is full. Is there anything to this? I mean, I've never bought into astrology, because it seems like the stars are too far away to have any effect on us. But the moon is another story — look at the tides. What do you say, Cecil?

Cecil replies:

You're not the first person to wonder about this. There have been lots of studies over the years, some of which have purported to show that there really is such a thing as a "lunar effect." For example, one study claimed that an unusual number of traffic accidents occurred during the evenings right around the full and new moons (Templer, Veleber, and Brooner, 1982). But later researchers showed that during the time period studied, a disproportionate number of full and new moons fell on weekends, when traffic accidents are always higher.

That's pretty much been the story with all lunar-effect claims — when you look at them closely, they fall apart. Another study of homicides in Dade County, Florida (Lieber and Sherin, 1972) claimed to have found there was an upsurge in killings in the 24 hours before and after the full moon. Other researchers, however, found that the Dade County researchers had used dubious statistical methods. When the figures were reevaluated using proper methods, the alleged pattern disappeared.

But many people remain convinced that the moon must do something. After all, they say, the earth's surface is 80 percent water and we all know about the tides; the human body is 80 percent water, so why shouldn't there be a human tidal effect? But this reasoning doesn't take the question of scale into account. The tides are only noticeable in the oceans, where the vast distances act as a multiplier. Even so, tidal variation in most coastal areas seldom exceeds ten feet. In smaller bodies of water, such as lakes and presumably the human body, tides are negligible.

Besides, when it comes to exerting any influence on humankind, the moon has a lot of competition. Researchers have calculated that a mother holding her baby exerts 12 million times the tide-raising force on the child that the moon does, simply by virtue of being closer.

Another thing to remember is that the tides don't occur just once or twice a month; they occur once or twice a day. What happens at full and new moon is that the earth, moon, and sun are lined up, resulting in higher tides than usual. (At full moon the earth is between the moon and the sun; at new moon the moon is between the sun and the earth.) So when we talk about the influence of the full moon, we're really talking about the additional influence of the sun. But small though the moon's pull on the earth is, the sun's is only half as much.

Just to make sure about all this, a pair of admittedly skeptical scientists (Rotton and Kelly, 1985) did what they called a "meta-analysis" of 37 studies of the moon's effect on things like psychiatric admissions, suicides, crime, etc. They found that the moon accounted for no more than 3/100 of 1 percent of the monthly variation.

A new twist was recently given to lunar-effect theorizing by the discovery that positive and negative ions in the atmosphere have an effect on behavior (negative ions usually favorable, positives the opposite). It turns out that positive ions are more abundant when the moon is full. However, the effect is slight compared to major sources of positive ions like air conditioning and air pollution.

So how do we explain all those cops and emergency room nurses who believe in the lunar effect? Easy. Nobody notices when there's a full moon and nothing happens — you only notice when something does happen. In other words, heads I win, tails don't count. Case closed.

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