Meghan Bartels, Nautilus
Each year, from 1913 to 1917, the psychologist Edmund S. Conklin would hand out a questionnaire to his new psychology students. Conklin wanted to see which superstitious habits or beliefs were the most and least enduring. He found that just over a quarter of college students believed in lucky four-leaf clovers, making it the second most commonly cited superstition. Knocking on wood was first. (The top five are still with us today, though not the sixth. Sleeping on a wedding cake may be ill-advised, but bad luck?)