Déjà vu was first described in scientific literature in the late 1800s, but it took almost another century for the phenomenon to be defined in a uniform fashion suitable for research purposes. The universal definition of déjà vu now in use was proposed by Dr. Vernon Neppe in 1979, who defined it as “any subjectively inappropriate impression of familiarity of the present experience with an undefined past.”
In medical circles, déjà vu is best understood as a symptom of temporal lobe epilepsy or schizophrenia. Both of these conditions are associated with this phenomenon—although déjà vu may be experienced differently in these patient subpopulations. For example, people with schizophrenia may experience déjà vu that lasts longer and involves more intense feelings of depersonalization.
However, déjà vu is also commonly experienced by people without psychiatric or medical illness. An estimated two out of three people claim to have experienced déjà vu at some point in their lives.
Here are 10 things to know about déjà vu:
- On average, people who report experiencing déjà vu experience it about once per year.
- Déjà vu most often occurs during periods of stress or extreme fatigue.
- Déjà vu is probably experienced equally by both men and women.
- The occurrence of déjà vu decreases with age.
- People who are more educated and of higher socioeconomic status are more likely to experience déjà vu.
- Déjà vu can be produced by electrical stimulation of the cortex and deeper brain structures.
- People who travel frequently experience déjà vu more than those who don’t travel, probably because travelers have more opportunities to encounter novel places.
- Some people who experience déjà vu say the experience resembles a prior dream.
- The opposite of déjà vu is jamais vu, or “never seen.” Jamais vu is a phenomenon wherein the banal may seem unfamiliar, and is reported much less frequently than déjà vu is.
- Various models have been proposed to explain déjà vu. For instance, split-perception refers to a phenomenon in which a person’s attention to an activity is divided into two separate perceptions by a distraction, which may be as fleeting as an eye blink. The first perception occurs at below-threshold when a person’s attention is degraded. For instance, if you were about to start your car, and you’re momentarily distracted by a fly in the car, by the time you return your attention to starting the car, your initial perception of putting the key into ignition may be perceived as having occurred further in the past.
Researchers still have much to learn about déjà vu. The limited number of studies done on the subject suffer from biases, including small sample sizes and issues with construct validity, as well as a general bias that lumps déjà vu in with paranormal phenomena such as out-of-body experiences, channeling, and psychokinesis.