Why we knock on wood, and the truth about 7 other common superstitions

Do you knock on wood or avoid stepping on cracks to prevent something bad from happening? While the origins of some of these popular superstitions seem pretty silly, many people still do them today. We spoke with a few experts to find out where these beliefs come from and why we engage in this "magical thinking." Following is a transcript of the video.

- [Narrator] Have you ever knocked on wood for good luck? This superstition stems from an old Pagan belief that spirits live in trees, and if you knocked on the tree, you were seeking the help from the good spirits, or making sure the bad spirits couldn't hear you, and cause you harm. Seems kind of silly, right? So, why do people still do it today? The word superstition comes from the Latin words super and stare.

- It implies a superior attitude, and a standing point higher than yours. It means, I have a better explanation.

- [Narrator] Phillips Stevens is an expert on what some psychologists call magical thinking. Like when people were in a bad mood say, they got up on the wrong side of the bed this morning. Superstitions are examples of magical thinking.

- When we use the word superstition, it more often than not, has to do with the control of good or bad luck, whereas magical thinking, is not necessarily about luck, but just about, sort of, how something can cause something else, even when there's no, sort of, scientific evidence that those things would be causally connected.

- I've made my family disappear.

- [Paul] It's this sort of a belief that doesn't have an evidential base, even though no one can even understand. They say thirteen's an unlucky number. You know, my elevator in my building doesn't have a 13th floor. That's a totally crazy thing.

- [Narrator] While seemingly crazy, Stevens says most superstitions derive from three universal principles across all cultures. We all do them, to some extent.

- They represent... Cognitive features of humanity, somehow rooted in our evolutionary biology.

- [Narrator] The first is a belief in a higher power.

- [Stevens] A mystical, supernatural power, that is found in all things.

- [Narrator] This power is thought to be transferred through things like language, and symbols.

- [Man] Indeed.

- People everywhere believe in a coherent interconnected universe.

- [Narrator] For example, the cross in Christianity can be used as a personal amulet, which holds the power of God.

- Catholics do the sign of the cross on the body, right? With the right hand. But crossing the finger can be done with either hand, and it can be done out of sight. You're invoking the power of the cross.

- [Narrator] The second cognitive feature is the principle of similarity.

- Things that resemble other things have a causal relationship with those other things.

- [Narrator] Breaking a mirror, for example, is thought to bring seven years bad luck.

- The image and the reality are connected. If you break the image, you do some damage to the reality. It's the same sort of belief as the so-called voodoo doll.

- [Narrator] The seven years part of it comes from The Bible, and is not meant to be specific.

- All cultures have ideas of long time and short time, and in biblical thinking, seven years is a long time.

- [Narrator] The third and final principle is contact, or contagion.

- Things that have been in direct contact with other things, retain that contact after they are separated.

- For example, what do we avoid walking on cracks on the floor?

- The crack is damaged, and by stepping on the crack, you may transfer some of that damage into yourself.

- [Narrator] Or, why don't we open umbrellas indoors? An umbrella is exposed to a storm, a bad thing, which you could spread by way of the umbrella.

- Things of outside stay outside. They are not brought inside, because they might bring inside some of the unpredictability, the danger, the potential chaos of the outside.

- [Narrator] This is also why warriors would leave their helmets outside, to not bring the battle into their home, a place of peace. Early umbrellas were also made with a spring mechanism, that could have caused bodily harm to someone if it malfunctioned indoors.

- Some superstitions derive out of practical behaviors.

- [Narrator] People avoid walking under ladders, because it will supposedly bring bad luck. One popular theory behind this is that a ladder propped against a wall creates three sides of a triangle. The number three is holy, particularly in Christianity with the Holy Trinity. When you walk under a ladder, you're violating the Trinity, and therefore causing bad luck.

- The simplest explanation in this case, is the best one. It's potentially dangerous.

- [Narrator] Basically, if people are working on a ladder, there's a chance that something could fall on your head.

- It just makes more sense to walk around it.

- [Narrator] A number of superstitions can be traced back to early religious beliefs, which are intertwined with magical thinking, like the unlucky number 13.

- This derives from the Christian story of the last supper, that Seder meal in that upper room on a Thursday night.

- [Narrator] There were 13 people at the table, with a 13th person being considered either Judas, who betrayed Jesus, or Jesus himself. Jesus of course, died on the cross the next day, a Friday. Hence, the unlucky nature of Friday the 13th. When you pass by a cemetery, do you hold your breath? The origin of this superstition comes from the ancient belief that the breath and spirit are the same. In Hebrew, breath and spirit are the same word, Ruach. Spirits are thought to hang around cemeteries, and there's a chance you could breathe in their spirit, which would not be a good thing. As you can see, the origins of many of these common superstitions are a little absurd. Jane Risen says there are both motivational, and cognitive reasons why people are superstitious.

- The motivational side is that superstitions can help us manage the uncertainty, and the stress and tension that comes from not knowing what's gonna happen.

- [Narrator] On the cognitive side, there are two systems of thinking that help explain them. System one, and system two. System one refers to things like your intuition, which helps you make quick decisions about your surroundings.

- Some set of mental processes are super fast, and efficient, don't require and cognitive resources, don't require working memory, just kinda work like this, and they're basically, you know, most of life is in that system one space, and you couldn't function without it.

- [Narrator] But your intuitions can also encourage superstitious behavior. You've always been told to avoid cracks in the street, so you're going to do it just in case, since it doesn't take much effort.

- In the sort of, slower, deliberate system two, these are the mental processes that require more working memory, and more effort, and that's what you need to sort of, recognize that these beliefs don't necessarily make sense, but they still don't stop the intuitions from coming. Right, so even when you can, sort of, recognize that this doesn't make sense, it doesn't stop you from having the feeling that if I do this, blank will happen.

- [Paul] People are very reluctant to eat a piece of good chocolate shaped to look like a dog doo. Now, you could call that a superstition if you want, it's irrational. It isn't a dog doo, it's a piece of chocolate, and they know that. Their system two knows that, but system one says, it looks like a dog doo, it is a dog doo. Generally a good rule, if it looks like something, it's something. But in that sense, it becomes misapplied.

- [Narrator] So despite how crazy these superstitions may seem, you're not likely to stop knocking on wood any time soon, if your brain has anything to say about it. I refused to break a mirror in real life for this video, you know, just in case.