
http://www.dailymail.co.uk/femail/article-1385380/Why-throw-salt-shoulder-superstitions-answered.html
http://www.angelfire.com/biz6/Psyteric/Kitmyths.html
http://www.webmd.com/mental-health/features/psychology-of-superstition?page=2
Superstitions are based on the belief of the irrational. People choose to belief something and they decide to firmly stand by it making it their own reality. In their mind it’s true. It can become a fear or an obsession.
The fear of Friday the 13th is a phobia called paraskavedekatriaphobia. The foundation for this psychological problem dates back to ancient times. All through out history people have believed that 13 is an unlucky number and a lot of historical and biblical tragedies have taken place on a Friday. The combination of the so-called bad luck number and day has created the notion that Friday the 13th is a day to be feared.
http://www.corsinet.com/trivia/scary.html#friday
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It has been passed down through generations that the number 13 is unlucky. The fear of the number 13 is called triskaidekaphobia and is apparently everywhere: office buildings, parking spots, airport gates, and office numbers. The number 13th is thought of as to be taboo, and extremely unlucky. However, is it really that unlucky?
First off, no matter what you call it, the number after 12 is still 13. You can pretend that it is the 14th floor, or 14th parking spot, but in reality it is still the 13th. Even William Shakespeare stated in his play Romeo and Juliet, “a rose by any other name would smell as sweet.” In this situation, the number 14 would become just as unlucky as the number 13. This, however, has not happened.
Where did this fear of the number 13 come from?
On the other hand, the ancient Egyptians felt the number 13 was very lucky. The 13th rung of their ladder to immortality lead to paradise. Also, the number 1 +3=4 which is the number of security.
The number 13 is not necessarily unlucky, and those who believe it suffer from triskaidekaphobia- the fear of the number 13.
Source: http://ezinearticles.com/?Superstitions,-Numerology-and-the-Number-13---Is-it-Really-Bad-Luck?&id=2327815