Triskaidekaphobia is the fear of 13, a number commonly associated with bad luck in Western culture. While fear of the number 13 can be traced back to medieval times, the word triskaidekaphobia itself is of recent vintage, having been first coined by Coriat (1911; Simpson and Weiner 1992). It seems to have first appeared in the general media in a Nov. 8, 1953 New York Times article covering discussions of a United Nations committee.
This superstition leads some people to fear or avoid anything involving the number 13. In particular, this leads to interesting practices such as the numbering of floors as 1, 2, ..., 11, 12, 14, 15, ... (OEIS A011760; the "elevator sequence"), omitting the number 13, in many high-rise American hotels, the numbering of streets avoiding 13th Avenue, and so on.
Apparently, 13 hasn't always been considered unlucky. In fact, it appears that in ancient times, 13 was either considered in a positive light or (more commonly) not at all (Adams). The association of bad luck with the number 13 has been attributed to the fact there were 13 people at the Last Supper of Jesus, although this association seems to have originated only in medieval times.
The association has also been linked to that fact that lunisolar calendars (such as the Hebrew and Chinese calendars) must have 13 months in some years in order to synchronize the solar and lunar cycles, while the solar Gregorian calendar in universal current usage always has 12 months in a year.
Triskaidekaphobia also may be related to Norse mythology, which tells how the god Odin invited eleven of his closest friends to a dinner party at his home in Valhalla, only to have his party crashed by Loki, the god of evil and turmoil, thus giving a total of 13 people. The legend further relates how Balder, one of the most beloved gods, tried to throw Loki out of the party, resulting in a scuffle and ultimately Balder's death with a mistletoe-tipped arrow.
Fear of the number 13 also leads to fear of Friday the thirteenth (a fear recently dubbed paraskevidekatriaphobia), despite the fact that Friday turns out to be the most common weekday on which the 13th of a month can occur in the Gregorian calendar. The association of bad luck with Friday appeared in Chaucer's Canterbury Tales in the late 14th century ("and on a Friday fell all this misfortune"), but references to Friday as a day associated with ill luck in general first appear around the middle of the 17th century (Mikkelson and Mikkelson). In particular, it appeared in numerous publications as a particularly unlucky day to start a new venture (beginning a journey, giving birth, getting married, moving, starting a new job, etc.) beginning around 1800 (Mikkelson and Mikkelson).
While 13 is associated with the number of people at the Last Supper, the Crucifixion took place on a Friday, thus leading to an association of back luck with the combination of this number and day. Note that the association of Friday the thirteenth with the arrest of Jacques de Molay, Grand Master of the Knights Templar, on Friday, October 13, 1307 by King Philippe IV of France--as repeated, for example, in D. Brown's novel The Da Vinci Code (Brown 2003, p. 163)--is a modern-day invention (Mikkelson and Mikkelson).
Paraskevidekatriaphobia is probably the most widespread superstition in the United States, possibly affecting tens of millions of Americans. Interestingly, a study by Scanlon et al. (1993) published in the prestigious British Medical Journal which analyzed the relation between health, behavior, and superstition surrounding Friday 13th in the United Kingdom by comparing the ratio of traffic volume to traffic accidents on Friday the 6th and Friday the 13th over a period of years found that, "The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended."
While there does not appear to be hard evidence to support the claim, it has been suggested that $800 to $900 million are lost each Friday the 13th (Roach 2004) as a result of people avoiding travel, wedding plans, moving, and so on.
