Friggatriskaidekaphobia gotcha? No reason to fear Friday the 13th.

“Friggatriskaidekaphobia.” You almost certainly know what it is, even if you’ve never heard the tongue-tying, nine-syllable official term for it. It is the fear of Friday the 13ths, one of the most widespread and enduring superstitions in the world. And whether or not you think it’s silly, the phobia does actually affect us, dragging down economic productivity at least once each year.

Fears of Friday the 13th usually revolve around personal injury, but studies have shown no reliable relation between Friday the 13ths and rates of car accidents or other serious damages. However, a 1987 study by two finance professors at the University of Miami showed that you are more likely to lose in the stock market on a Friday the 13th, as opposed to a “normal” Friday.

Robert Kolb and Ricardo Rodriguez studied stock market returns for 39 Friday the 13ths from 1962 to 1985 and compared them with the other 1,141 Fridays on which the stock markets were open in the same period. Their result: “Returns for Friday the Thirteenth have been significantly lower than the returns for all other Fridays over [the] long period,” Kolb and Rodriguez wrote.

Though those who truly believe Friday the 13th is unlucky might see Kolb and Rodriguez’s study as proof of the day’s built-in misery, but the professors noted that the effect was the impact of the superstition on behavior, not the some result of the day itself.

Edmund Kern, a professor of history at Lawrence University in Appleton, Wis., says “confirmation bias” often leads superstitious people to attribute misfortune to a ritual or a defense wrongly performed, rather than to their own actions.

“Superstitions have a psychological origin, to some degree, but they also have a definite real effect,” says Mr. Kern, who studies the history of superstition. “Thinking certain things about the world, or doing certain things, actually changes the world around you.”

In other words, pay too much attention to broken mirrors and black cats, and you risk neglecting to look both ways before you cross the street.

Of course, this still begs the question of how Friday the 13th, of all the possible day-number combinations, became unlucky in the first place. The historical record is inconclusive, says Kern.

“The first thing I say when I talk about superstition is that anyone who says they can tell you the origin of one is lying,” Kern says, laughing. “We can only try to find historical precedence and then engage in informed speculation.”

Kern believes that separate traditions of superstition about the number 13 and Fridays merged in the 19th century, when the first known reference to ill fortune on Friday the 13ths was recorded.

“There is a long tradition, dating back to the Middle Ages, of 13 being mentioned as an unlucky number,” Kern says. Thirteen is still considered an unlucky number by many, but fewer recall that Friday was once a day of ill repute.

The day has been considered unlucky in history for reasons ranging from the day of Jesus’ crucifixion in the Christian religion to a reference in Geoffrey Chaucer’s "The Canterbury Tales."

For reasons unknown, the two phobias combined, and endured.

“For the most part, we live relatively free from magical and superstitious thinking today,” Kern says. “But a lot of superstitions take on meaning within a cultural or familial context. You have to be raised with superstitions for them to really take hold.”

Given the heightened interest every time Friday the 13th rolls around – not to mention the eponymous horror movie series – it seems that friggatriskaidekaphobia can be hard to shake.