It's Friday the 13th, and millions of people are on edge, fearing a calamity with personal or global repercussions-a broken leg, a stock market crash, or the trigger pulled for World War III.

Why all the anxiety? In short, because the fear is ingrained in Western culture, according to experts. (Get more Friday the 13th facts.)

"If nobody bothered to teach us about these negative taboo superstitions like Friday the 13th, we might in fact all be better off," said Stuart Vyse, a professor of psychology at Connecticut College in New London.

People who harbor a Friday the 13th superstition might have triskaidekaphobia, or fear of the number 13, and often pass on their belief to their children, he noted. Popular culture's obsession with the fear-think the Friday the 13th horror films and even this story-helps keep it alive, added Vyse, the author of Believing in Magic: The Psychology of Superstition.

Although superstitions can be arbitrary-a fear of ladders or black cats, for example-"once they are in the culture, we tend to honor them," said Thomas Gilovich, a professor of psychology at Cornell University in Ithaca, New York.

"You feel like if you are going to ignore it, you are tempting fate," he explained.

Origins Rooted in Religion

The trepidation surrounding Friday the 13th is rooted in religious beliefs surrounding the 13th guest at the Last Supper-Judas, the apostle said to have betrayed Jesus-and the crucifixion of Jesus on a Friday, which was known as hangman's day and was already a source of anxiety, Vyse said. (Read more: "Friday the 13th Superstitions Rooted in Bible and More.")

The two fears merged, resulting "in this sort of double whammy of 13 fallingon an already nervous day," he said.

The taboo against the number 13 spread with Christianity and into non-Christian areas, noted Phillips Stevens, Jr., an associate professor of anthropology at the University of Buffalo in New York."It became extremely widespread through the Euro-American world, embedded in culture, [and] extremely persistent," he said.

More interesting, he noted, is why people associate any Friday the 13th with bad luck. The answer, he said, has to do with what he calls principles of "magical thinking" found in cultures around the world.

One of these principles involves things or actions-if they "resemble other things in any way of resemblance-shape or sound or odor or color-people tend to think those things are related and in a causal way," he explained.

In this framework, there were 13 people present at the Last Supper, so anything connected to the number 13 from then on is bad luck.

Thomas Fernsler, an associate policy scientist in the Mathematics and Science Education Resource Center at the University of Delaware in Newark, said the number 13 suffers because of its position after 12.

According to Fernsler, numerologists consider 12 a "complete" number. There are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel, and 12 apostles of Jesus. (See "Lost Gospel Revealed; Says Jesus Asked Judas to Betray Him.")

Fernsler said 13's association with bad luck "has to do with just being a little beyond completeness. The number becomes restless or squirmy."

Then there's Friday. Not only was Christ crucified on that day, but some biblical scholars believe Eve tempted Adam with the forbidden fruit on a Friday. Perhaps most significant is a belief that Abel was slain by his brother Cain on Friday the 13th.

Crippling Impact

On Friday the 13th, some people are so crippled by fear that they lock themselves inside; others will have no choice but to grit their teeth and nervously muster through the day.

Nevertheless, many people will refuse to fly, buy a house, or act on a hot stock tip, inactions that noticeably slow economic activity, according to Donald Dossey, a folklore historian and founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina. (Read about animal phobias.)

"It's been estimated that [U.S.] $800 or $900 million is lost in business on this day because people will not fly or do business they normally would do," he said.

To overcome the fear, Vyse said, people should take small steps outside their comfort zone. Those who are afraid to leave the house could consider meeting a close friend at a cozy cafe, for example.