Déjà vu is extremely difficult to study because it occurs briefly, unannounced, only in certain people, and has no witnesses or physical manifestations other than the person saying, "hey, déjà vu!" Because of this, there is little firm research and no definitive explanations. Déjà vu studies must depend on personal descriptions and recollection for data. For two centuries people have tried to come up with reasons we experience déjà vu. From philosophers, to psychologists, to paranormal experts, they've all had their theories.
Emile Boirac was a French psychic researcher who was the first to use the term déjà vu in his book, "L'Avenir des Sciences Psychiques." He did not research the phenomenon in depth, however. Sigmund Freud theorized that these experiences resulted from repressed desires or memories related to a stressful event that people could no longer access as regular memory. Scientists used this theory, called paramnesia, to explain déjà vu for a large part of the 20th century.
Over the years, many scientists ignored déjà vu completely due to its frequent association with past life experiences, ESP and alien abductions. These associations gave the study of déjà vu a bit of a stigma. Recently, researchers have set aside some of those associations and have begun putting brain imaging technology to work. Firmly placing déjà vu within the study of memory, they hope to discover more about how memories are formed, stored and retrieved.
They have since determined that the medial temporal lobe is involved in our conscious memory. Within the medial temporal lobe are the parahippocampal gyrus, the rhinal cortex and the amygdala. John D.E. Gabrieli at Stanford University found in 1997 that the hippocampus enables us to consciously recall events. He also found that the parahippocampal gyrus enables us to determine what's familiar and what isn't (and without actually retrieving a specific memory to do it).
While about 60 percent of people say they have experienced déjà vu, the rates are highest among people between the ages of 15 and 25. The upper age varies among researchers, but most agree that déjà vu experiences decrease with age. There have also been higher reported occurrences among those with higher incomes, those who tend to travel more and those with higher education levels. Active imaginations and the ability to recall dreams has also been a commonality among people who report déjà vu experiences.
Some researchers also report that the more tired or stressed you are, the more likely you are to experience déjà vu. Other researchers, however, have seen just the opposite. They report that the more refreshed and relaxed you are, the more likely you are to experience déjà vu. Obviously, the jury is still out about a lot of things related to déjà vu.
One reported finding is that the more open-minded or politically liberal a person is, the more likely they are to experience déjà vu. However, this may also mean that the more open-minded you are, the more likely you are to talk about something potentially seen as "weird," like déjà vu.