Superstition fills the gap when there's nothing you can do.
You don't have to believe in voodoo or sorcery to indulge the urge to knock on wood. Whether we consider them magical or not, simple physical movements may have an innately reassuring effect.
A paper in the Journal of Experimental Psychology: General suggests that exerting force away from our bodies may help us feel better about negative possibilities—as if we're physically repelling them. Coauthor Yan Zhang, a professor of marketing at the National University of Singapore, notes similarities between rituals across cultures, such as knocking on wood, throwing things (like salt), and spitting. "We think there's a common underlying mechanism," she says.
Zhang's team asked college students to "tempt fate" with presumptuous phrases ("Nobody I know would get into a bad car accident"). Students who then rapped their knuckles on a wooden tabletop tended to rate the odds of such an accident lower than students who didn't knock. Conversely, those who knocked up into the table—toward themselves—thought an accident was more likely than others did. A follow-up study showed that tossing a tennis ball, or even just pretending to toss it, had a similar effect, in contrast to holding or carrying it.
Our superstitions, it seems, reside in our bodies as much as in enchanted objects and suspicious omens. A 2012 paper found that students were less fazed by misfortune if they washed their hands—suggesting a sense that bad luck can be scrubbed away. "Superstition fills the gap when there's nothing you can do," says Stuart Vyse, author of Believing in Magic: The Psychology of Superstition. These seemingly innate feelings may help explain how quick and easy rituals serve that purpose.