Ever wonder why there's no gate 13 at Washington's Dulles Airport, no floor 13 between 12 and 14 at the Westin Hotel in Detroit's Renaissance Center, , no 13th Street among the numbered thoroughfares in Santa Monica, Calif.? There's a word for it . . .

Triskaidekaphobia. Fear of the number 13.

Oh sure, there are plenty of scoffers. Take British writer Matthew Arnold. He pooh-poohed the notion it was unlucky for him to be the 13th guest at a dinner party for the painter Sir John Everett Millais, or so the story goes; six months later, he was dead. In France, now, "quatorziemes" or "fourteens" can be hired at short notice by skittish hosts.

Friday, witches' day, the day time ran out for not-so-merry-olde-Englanders and Scots awaiting the gallows -- inspires a few potent myths of its own.

Despite official denials, popular lore has it the U.S. Navy doesn't sail on Friday and cruise ships prefer to leave port and dock on other days of the week. ("I've never heard anything like that," says Cunard Line spokeswoman Susan Alpert.) Talk to fishermen in San Pedro, Calif., and they'll tell you they wait till after midnight Friday to cast off.

If you admit you're superstitious, say the folklorists, don't look for a job on Friday, cut your hair or nails, or start something you can't finish. Buy a car on Friday in Ohio, and you'll wreck it.

Put Friday and 13 together and some folks go positively bonkers: patients who refuse surgery, investors who shy from trading, travelers who decide they rather like where they are, clients who ask for court postponements.

This, against all logic: "If something bad happens to you on Friday the 13th, it's not because it's Friday the 13th but because you're nervous," says Larry Alferink, psychology department chair at Illinois State University in Normal, Ill. "It's a self-fulfilling prophecy."

Was it superstition, then, that caused two massive bank failures, one in 1869 and the other in 1873, on Friday the 13th?

Certainly by that time, the date's fearsome legend had been around for centuries. Judas was the 13th guest at the Last Supper. In Norse mythology, the mischievous god Loki was associated with the number. In the legend of Sleeping Beauty, 12 good fairies gave their blessings and the 13th, uninvited, bestowed a curse.

But laughing at such cultural lore doesn't mean you're superstition-free. Perhaps the most prevalent superstitions, says Alferink, are highly personal ones "where we act as if our behavior is responsible for something even though it is not."

In other words, much like B.F. Skinner's pigeons. In a classic experiment, the behavioral psychologist once fed pigeons every 15 seconds independent of their actions. "Each pigeon," says Alferink, "developed idiosyncratic behavior as if its behavior pattern caused the feeding , not unlike the way baseball players touch their caps or move the ball around in particular ways before they pitch it. Because their behavior is followed, accidently, by a particular consequence, it tends to strengthen their doing it again."

"Rituals," says phobia therapist Jerilyn Ross, "always provide comfort for people." No one knows that better than a performer or an athlete. Tallulah Bankhead required that visitors to her dressing room enter with their right feet first or leave and reenter properly; Al Jolson made a point of wearing old clothes on opening nights.

The sporting world, says Bruce Ogilvie, sports psychologist and adviser to the Olympic sports medicine committee for several years, "is fraught with all sorts of ritualistic behavior. We talk about containment of anxiety, containment of tension, blocking the intrusion of negative thoughts . . . A batter can only use a particular bat to maintain his batting average. In track and field, the athlete will run on spikes so badly worn they're no longer efficient, but they're the ones worn when he won an event."

Winning Villanova basketball coach Rollie Massamino wore the same sweater every day of the NCAA championship games last season. Mickey Mantle used to ritually tag second base on the way to or from the outfield.

How widespread is such behavior? Dr. George Serban, associate professor of psychiatry at NYU medical school and author of The Tyranny of Magical Thinking, cites a Harvard study: "Thirty-five percent of the people are thinking logical all the time, 50 percent of people are most of the time logical and the remainder, never."