Researchers estimate that at least 10 percent of the U.S. population has a fear of the number 13, and each year the even more specific fear of Friday the 13, known as paraskevidekatriaphobia, results in financial losses in excess of $800 million annually, as people avoid marrying, traveling or in the most severe cases, even working. But what’s so unlucky about the number 13, and how did this numerical superstition get started?

An early myth surrounding the origin of the fear involved one of the world’s oldest legal documents, the Code of Hammurabi, which reportedly omitted a 13th law from its list of legal rules. In reality, the omission was no more than a clerical error made by one of the document’s earliest translators who failed to include a line of text—in fact, the code doesn’t numerically list its laws at all.

Mathematicians and scientists, meanwhile, point to preeminence of the number 12, often considered a “perfect” number, in the ancient world. The ancient Sumerians developed numeral system based on the use of 12 that is still used for measuring time today; most calendars have 12 months; a single day is comprised of two 12-hour half days, etc. Following so closely on the heels of a “perfect” number, some argue, the poor 13 was sure to be found lacking and unusual. This fear of the unknown would seem to play into two other popular theories for the number’s unlucky connotation, both of which revolve around the appearance of a 13th guest at two ancient events: In the Bible, Judas Iscariot, the 13th guest to arrive at the Last Supper, is the person who betrays Jesus. Meanwhile ancient Norse lore holds that evil and turmoil were first introduced in the world by the appearance of the treacherous and mischievous god Loki at a dinner party in Valhalla. He was the 13th guest, upsetting the balance of the 12 gods already in attendance.

It also seems as if unexplained fears surrounding the number 13 are a primarily Western construct. Some cultures, including the Ancient Egyptians, actually considered the number lucky, while others have simply swapped numbers as the base of their phobias—4 is avoided in much of Asia, for example. According to the Stress Management Center and Phobia Institute in Asheville, North Carolina, more than 80 percent of hi-rise buildings in the United States do not have a 13th floor, and the vast majority of hotels, hospitals and airports avoid using the number for rooms and gates as well. But in much of East and Southeast Asia, where tetraphobia is the norm, you’d be hard-pressed to find much use of the number 4 in private or public life, thanks to similar sounds for the Chinese language (and Chinese-influenced linguistic sub-groups) words for “four” and “death.”