What superstitious beliefs have a scientific basis?

Superstitious belief, by definition, have no scientific basis. Science requires that ideas are tested in a way that is repeatable and falsifiable. Falsifiable means that there must be a way that the test shows that a stated belief is not true, while most superstitions rely on metaphysical entities or powers that are so vague they can not be dis-proven.

Here are some examples of superstitions, and where they may have come from.

• Breaking a mirror is seen as seven years of bad luck from a historical root.
  • Back in the medieval times mirrors were very expensive. And if you broke one, it was reguarly someone important, such as the lord of the kingdom of the king or a high-ranking nobleman. And if you broke it, it was common that if they were un-forgiving, they would put you in jail, possibly for seven years.
• Walking under a ladder is seen as bad luck
  • Typically a ladder means someone at the top of the ladder and that person can easily drop things - onto your head. So it really is a bad idea to walk under one.

• Never sweep the floor at night or you'll sweep sorrow into your life.
  • You may not be able to see where you're sweeping and fall and get injured.
• Chase away any owls outside your window; they are a harbinger of death.
  • They may erode your windowsill by pecking on it, and when you lean on it, it may break and you may fall.
• Never start or buy anything on a Friday.
  • Since Friday is the last business day, you or your order will probably not be active on the weekend.
• Cut your hair on a full moon and it will grow back faster.
  • Well, if you cut your hair at a barber's shop, and you can see the moon, the barber will probably bee in a hurry to leave and so will give you a shorter haircut.
• Crickets in your home are good luck (not in my home and definitely not for the nasty crickets!)
  • They tell the temperature.
• Killing a spider is bad luck.
  • In addition to reducing local disease-carrying insects, spiders provide humans with other medical benefits. Spider venom is used in neurological research and may prevent permanent brain damage in stroke victims. The silk produced by spiders is used in many optical devices including laboratory instruments.
• Ivy growing on a house protects the inhabitants from witchcraft and evil.
  • Evil may mistake it for poison ivy and stay away.

• Friday the 13th is unlucky
  • The Templars were all arrested (and most were tortured and executed) one Friday the 13th!
• Actors believe that using real money as a prop is bad luck.
  • This is probably rooted in the fact that leaving real money on stage or in your costume means that there is a good chance that the prop will disappear before the next performance.
• Opening an umbrella in the house is bad luck.
  • Like walking under a ladder, this is just a hazard; you could hit someone or break something.
• Bad luck comes in threes.
  • This is sometimes expressed in notable deaths occurring in threes. In fact, this is a well-known psychological bias known as "confirmation effect"; when two events occur, people naturally anticipate a third (two things don't form a pattern, but three do). When the expectation is satisfied, it "proves" the adage. Of course, if it is not, then the pattern is not recognized.