Traders are on an eternal search for some “edge” that lets them beat the market. It turns out the answer was staring them right in the face.

The most reliable indicator has nothing to do with earnings or the Federal Reserve. Hang on to your hat: The returns of stock exchanges around the world are strongly affected by the phase of the moon.

In a nutshell, markets rise several annualized percentage points faster during the 14 or 15 calendar days around a new moon than they do in a similar period around the full moon.

That might seem pretty woo-woo. But when you think about it, there’s a logical basis. It’s well known that the moon raises the tides and increases the rate of homicides. Our bodies are mostly water. If the moon can move the oceans, why couldn’t it drive traders’ emotions?

Like anything that has to do with economics, there’s a healthy debate about the effect of this “lunar cycle.” But the evidence for stronger new-moon periods and weaker full-moon periods is growing:

• Ilia Dichev, currently at Emory University, and Troy Janes of Purdue measured the lunar effect in the stock markets of 25 different countries, including the United States. The researchers analyzed the Dow Jones Industrial Average DJIA, +0.51% back to 1896 and the S&P 500 SPX, +1.05% back to 1928, among other indexes. “The annualized difference between new moon and full moon returns is on the magnitude of 5 to 8 percent,” they write, although the daily variation can be huge. The lunar indicator was even stronger outside the U.S., on the order of 7% to 10% annualized. (See Dichev and Janes white paper.)

• Christos Floros and Yong Tan, both of the University of Portsmouth (U.K.), studied 59 international stock markets. They found significant effects from the new moon or the full moon in 14 of them. In other countries, the lunar cycle interacted positively with the previously known Monday effect and January effect. (See Floros and Tan.)

• Kathy Yuan, Lu Zheng, and Qiaoqiao Zhu, all formerly of the University of Michigan, analyzed the stock markets of 48 countries. They constructed an equal-weighted portfolio and a value-weighted portfolio. The return for the new-moon period over the full-moon period was 3% to 5% annualized. The researchers declare that the lunar effect is “independent of the January effect, the day-of-week effect, the calendar month effect, and the holiday effect.” (See Yuan, Zheng, and Zhu.)

The graph above, from the Dichev and Janes white paper, illustrates the findings. Annualized gains are notably stronger in the market days around the new moon (the red columns) than the full moon (the blue columns). All of the G-7 economies show the effect, to a greater or lesser degree. Go, Canada!

If the lunar indicator is statistically significant, the question becomes, “What do we do with it?”

Here’s where reality raises its ugly head. Just because an indicator is measurable doesn’t mean it’s tradable (i.e., you can effectively use it). And even if an indicator is tradable, that doesn’t mean it’s profitable (you can make gains after paying transaction costs).

• Let’s say you purchase stocks at the beginning of every two-week new-moon period and switch to cash at the end. Rinse and repeat. You’d make 26 switches a year, or a turnover rate of about 2,600%. The transaction costs could soak up a lot of your gains.

• On average, stock markets don’t go down during the full-moon periods, they just go up less. If you were in cash half the time, you’d miss a lot of gains and badly underperform the market.

• There’s nothing guaranteed about the so-called lunar cycle. Indicators work until they don’t. When an indicator stops working, it doesn’t hold up a little sign telling you how long it’s going to be gone or what indicator you should use instead.

If the lunar indicator mysteriously reverses or disappears entirely, who are you going to complain to, the man in the moon?

For most individual investors, it’s best to stick with tried-and-true long-term strategies: diversification across a broad set of asset classes, “the trend is your friend,” and boring old stuff like that.

The lunar indicator is an interesting quirk that most of us may never be able to take advantage of. But it serves to teach us just how much wild stock-market action is the result of fallible behaviors that are deep-seated in human nature.

If the market truly obeys lunar influences, that says a lot about the way inexplicable manias and sudden crashes are driven by lunacy.