First things first, it's really hard to define déjà vu scientifically.
Most people probably understand what someone means when they say they've experienced déjà vu. In French, it literally means "already seen". But in order to study the phenomenon, scientists need to be more specific.
So we asked Dr Akira O'Connor, a researcher specialising in memory at the University of St Andrews. He told BuzzFeed Science over email that a common definition comes from psychologist Alan Brown and can be paraphrased like this:
Déjà vu is a subjective experience of familiarity, alongside an objective experience of unfamiliarity.
"That is, you know something shouldn't feel as familiar to you as it currently feels," says O'Connor.
Around 60% of people report experiencing déjà vu at some point.
But this percentage varies depending on who you ask and how. The number of déjà vu experiences people report goes down after the age of 25.
"There are a bunch of different ideas about what might cause déjà vu but the jury is very much still out on a definitive answer," says O'Connor.
He thinks the best theory is that it is caused by errors in your brain chemistry. That's because people with epilepsy often experience déjà vu just before they have a seizure, and tired people also seem especially prone it.
"The basic idea is that déjà vu results from a mini-seizure, like those experienced by people with epilepsy, but without it generalising to the rest of the brain and having catastrophic effects," says O'Connor. "It's like a tiny excitatory twitch in the memory-region of your brain that doesn't go on to affect anything else."
There's some evidence a brain chemical called dopamine could be involved.
Dopamine is a neurotransmitter that does lots of jobs in your brain.
The evidence for dopamine's involvement in déjà vu comes from a case, published in the Journal of Clinical Neuroscience, in which a man was taking two different flu medications and experiencing constant déjà vu. The flu medication is known to interfere with dopamine, and the man's déjà vu only stopped when he stopped taking the medication.
The medication could have been altering dopamine activity in the man's brain, which then prompted his déjà vu, says O'Connor. "For what it's worth, their patient completed the full course of medication because he enjoyed the experience so much."
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Kelly Oakes is science editor for BuzzFeed and is based in London.
Contact Kelly Oakes at kelly.oakes@buzzfeed.com.
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