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The term ”déjà vu” was first introduced around the 1890s in order to separate thephenomenon from other paramnesias, but a clear consensus on its definition was not reacheduntil mid 20th century. Since the middle of the 19th century, several dozens ofparapsychological, pseudoscientific and scientific theories have been proposed to explain thedéjà vu phenomenon, ranging from “messages from God” to “delayed neural transmissionspeed”. Most scientific theories can be divided into four categories: dual-processing,neurological, memory and attentional. This paper discusses and compares some of thesetheories. Memory and attentional theories are concluded to have most explanatory power andpotential to demystify the phenomenon through future research.

The present study set out to test some of the predictions made by the Threat Simulation Theory, which suggests an evolutionary source of dreaming (Revonsuo, 2000a). The qualitative content and frequency of threatening events in dreams were compared between traumatized Swedish subjects with experience of the tsunami-disaster in Southeast Asia in 2004 with Swedish subjects with no traumatic experiences. Only a few of the hypotheses were supported by the results. The results and unsupported hypotheses are discussed with focus on the Threat Simulation Theory, and alternative explanations are considered.