1. What is déjà vu?
Déjà vu (French for “already seen”) is an experience that many people have (roughly 67% of the adult population say they have had such experiences) in which they have the startling impression that reliving an event or revisiting a place while at the same time knowing this is not possible. Since it would seem that there are actually a number of different experiences that are all called "déjà vu" (and they occur to congenitally blind people), it would be better to use terms that more accurately describe what persons say has occurred. In this way, we can speak of "déjà experiences" as the general term (as proposed by Prof. Vernon N. Neppe) and use déjà vécu ("already lived through"), déjà visité ("already visited") and other such terms which better reflect the actual experiences.
2. Why do déjà experiences occur?
At this point there is no good answer. Many explanations have been put forward (see the several chapters in Prof. Alan Brown's 2004 book "the déjà vu experience") but none of them have been proven to be true (or false). Attempts at reproducing déjà experiences reliably in the laboratory have not met with great success. Déjà experiences tend to be sporatic and, like dreams, are very subjective. Just Like when studying dreams, we have to take the person's word for what they say they have experienced and proceed from there. There is the additional problem that there are many experiences that are all called "déjà vu" and each may well have its own cause or explanation. Actually we know far too little about what people are experiencing and until we know a lot more, we are unable to make any clear classification of them. All this has to be done before we can begin to speculate intelligently about what may be the various causes.
3. What effects do déjà experiences have on one’s mind?
Most are baffled and confused by the paradoxical situation that they are living through something again or visiting a location once more while simultaneously knowing with certainty that this cannot be true. In addition, some people are frightened, scared that they may be going crazy. Others are intrigued, full of awe and wonderment that such things are possible and can happen. Some feel reassured, convinced that they are on the right path. It is very varied.
4. Why do déjà experiences affect people in different ways?
Because people are such unique individuals, each with her or his own background, education, culture, personality and so on.
5. Who is most likely to experience déjà experiences?
They tend to happen more frequently and are more intense among young people, say between the ages of 15 to 25. According to surveys, the incidence among college students (ca. 79%) is higher than among the general population (ca. 65%). Why this is no one knows. There does not seem to be any dependence on gender or nationality.
6. Why do some people get déjà experiences more often than others?
At this point of our knowledge, this is not known. There seems to be some connection with stressful life situations and may also result from experimentation with recreational or prescription drugs.
7. Is there any way to stimulate a déjà experience?
There have been several attempts (using hypnosis and psychological strategies) but it seems none have succeeded in producing the specific déjà experience wherein one is confused and baffled by the seeming contradiction between what one is experiencing and the utter certainty that what is happening is impossible.
8. Why do people with temporal lobe epilepsy (TLE) sometimes recall having a déjà experience before having a seizure?
The temporal lobe and the limbic system in the brain seem to be vital in the formation, storage and recall of memories. That those afflicted with TLE sometimes have déjà feelings before or during their seizure is thus not very surprising. It should be noted that the incidence of deja vu among TLE patients is lower than that in the general population (see https://deja-experience-research.org/surveys). There is a problem in that people can have déjà experiences (65% do) and also have TLE but whether the TLE actually causes the déjà experiences is not clear.
9. Is there any correlation between stress/anxiety and déjà experiences?
There seems to be, based on questions asked in emails and also those on Quora (https://www.quora.com). This needs to be investigated.
10. Is there a possibility that déjà experiences could cause harm or have long term effects?
Some sensitive souls might become so frightened by having such experiences that they become morbid about them, believing they are crazy. Some are reluctant to speak about what they are experiencing for fear that others will judge them as being unbalanced. Fortunately, thanks to the Internet, younger people today appear to be less inhibited and are able to speak more readily and openly about their experiences.
11. Why do people decide to study/research déjà experiences?
Because they are intrigued by such amazing but poorly understood happenings. Having had such experiences make the mystery even stronger and beg for study. This is especially true when precognition in parapsychological forms of déjà experiences occurs.
12. Any additional comments?
It is important that we distinguish between true déjà experiences and what may be termed "anomalous familiarity". A true déjà experience is accompanied by the strong sense of bafflement, surprise and confusion as mentioned above.
Many are convinced their déjà experiences arise from precognitive dreams that are not remembered until the situation or location experienced in the dream comes true. This is not the explanation for all déjà experiences but it does seem to explain some of them.
Déjà experiences tend to be connected with very banal events or places. People never, or very rarely, say their déjà experiences happened in connection with some important or momentous event in their lives. Why this is so is not known.