Back in the 1940s, the behavioral psychologist B. F. Skinner did a famous experiment with pigeons that he thought had some relevance to human superstitions. He put birds in the kind of cages used for training animals by reinforcement -- peck a bar, get some birdseed, that sort of thing. Except in this new experiment, the feed was provided at regular intervals regardless of what the pigeons did.
And guess what? The pigeons fell into certain behaviors all by themselves -- nodding or turning or pecking for food -- although their behaviors had nothing to do with the reward being offered. Skinner wrote: "A few accidental connections between a ritual and favorable consequences suffice to set up and maintain the behavior in spite of many unreinforced instances...The experiment might be said to demonstrate a sort of superstition. The bird behaves as if there were a causal relation between its behavior and the presentation of food, although such a relation is lacking. There are many analogies to human behavior," said Skinner.
One could go on at length about the implications of this experiment, and I have done so elsewhere. It is a common human propensity to mistake coincidence for causality, which is why science puts such store in controlled reproducible experiment. Some results of superstition are fraught with consequence (appeasing the gods by human sacrifice, Ronald Reagan consulting an astrologer), other are merely silly (Friday the 13th is unlucky).
Anyway, here we are, on the first of three Friday the 13ths this year. Maybe you'd better stay in bed.
And while you are there, think about Friday the 13th in April 2029. That's when asteroid 99942 Apophis will come zipping by the Earth, a chunk of rock the size of three football fields that could wreak havoc if it hits. Apparently, the best current calculations show a near miss -- closer than the orbits of geosynchronous satellites -- but you wouldn't want a bullet that close to your head. I doubt if I'll still be around twenty years from now. As for the rest of you: Good luck.