Last updated: Aug 08, 2016

You know that feeling you get when you step inside a new house or walk around a foreign city—places you know you’ve never been before—and you can’t help but think, I’ve done this already? It's déjà vu, and if you've never had it before, take it from us: It’s kind of creepy.

Déjà vu is French for “already seen,” and about two out of three people have experienced the phenomenon at one time or another, according to a 2003 review in the journal Psychological Bulletin. Despite being fairly common, “it’s not a widely studied subject,” says Alice Medalia, PhD, a professor of medical psychology at Columbia University Medical Center. And because déjà vu is a subjective experience—in other words, it’s difficult to induce in research subjects—testing the theories behind it can be tricky.

That said, researchers have a few guesses about why we experience déjà vu (and no, it's probably not flashbacks to a previous life):

You've been somewhere similar before

Some researchers believe déjà vu is triggered when you enter an environment similar to one you've experienced in the past. For example, you could experience it when you enter a hotel lobby where the furniture is configured the same way as your childhood home's living room. 

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Researchers tested that theory in a 2009 study published in the journal Psychonomic Bulletin & Review. They showed volunteers images that had nothing to do with one another—a fenced-in courtyard, and then later, a locker room—and the volunteers felt déjà vu because the images were composed in a very similar way. The researchers concluded that there was probably a connection between déjà vu and the feelings of “familiarity.”

You travel a lot 

People who travel and people who can recall their dreams are more likely to experience déjà vu than those who stay at home or don’t remember their dreams, according to the 2003 review. These people can draw on a wider range of sources (either from, say, their adventures Europe, or just their own imagination), so it makes sense that they should think other environments feel familiar, too.

Something's up with your brain

Some people who have temporal lobe epilepsy (a type of epilepsy that occurs in the part of your brain that handles short-term memory) experience déjà vu right before they have a seizure—another sign that the phenomenon may be connected with the way memories are activated. Plus, it’s why some experts think déjà vu is triggered by a kind of disruption in the firing of neurons in the brain, says Dr. Medalia.

It could also be the result of your brain struggling to process multiple pieces of information, but for some reason, can’t align them correctly, she says. That lack of “synchrony,” in med-speak, might be responsible for that déjà vu feeling.

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The bottom line?

Regardless of what’s happening (or what’s causing it), for the vast majority of people, déjà vu is pretty harmless. Unless you’re experiencing an epileptic seizure—and in that case, there are plenty of other signs to watch out for—it’s a relatively normal experience.

And you never know—maybe that castle in London looks so familiar because, in your past life, you were Kate Middleton’s great-great-great-great grandmother-in-law. Hey, we can dream, right?