You may be surprised to learn that behind closed doors, lots of people are knocking on wood, holding their breath when driving past graveyards, or avoiding crossing paths with black cats. In one YouGov poll, only 13% of respondents claimed to be superstitious—but almost three times that many believed that finding a penny brings good luck. Superstition inveigles itself on even the most logical minds: According to research published in Nature, many scientists watch their experiments obsessively in the hope of encouraging positive results.

It’s a tough habit to kick because it appeals in those moments when the stakes are highest. We need the help of this horseshoe, or that four-leaf clover, or this rabbit’s foot because so often we feel helpless without them. At crucial moments, blow...