First paragraph of the paper titled: "Friday the 13th: The Empirics of Bad Luck":
"Superstitions, or beliefs that one event or occurrence can cause another without the two being linked by any observable natural process, are very common phenomena, and some have been around for hundreds if not thousands of years. Examples include believing that bad luck can be brought about by walking under a ladder, seeing a black cat crossing one’s path, opening an umbrella indoors, spilling salt or breaking a mirror. On the other hand, finding a horseshoe or a four-leaf clover or carrying a specific talisman or token in one’s wallet or purse can supposedly bring luck and ward off evil. Associating some events with luck, or the absence thereof, may have rational or practical origins. However, believing in lucky and unlucky numbers or days, despite being very wide-spread, seems devoid of any rational basis. For example, the number 13 is considered almost universally unlucky in the European culture and Friday the 13th, is seen as a particularly inauspicious day."