An incubus is a demon in male form that seeks to have sexual intercourse with sleeping women; a succubus is the corresponding spirit in female form. Parallels exist in many cultures: in Germany it is known as the Alpdruck, in the Czech Republic the muera, and in France the cauchemar. The word is derived from the Latin incubare (“to lie upon”), and in recent times has been applied to nightmare spirits of either sex.[a]The “-mare” part of nightmare is derived from the Anglo-Saxon/Old Norse word mara or maere, meaning “crusher”, denoting a crushing weight on the chest.
From a 21st-century perspective, it seems likely that those reporting such occurrences have suffered a form of sleep paralysis, and have retrospectively tried to frame their experiences within an understandable context for their time: in the early modern period encounters with demons, more recently with aliens.
Theology and reproduction
Some early theologians, such as St Thomas Aquinas (1225–1274), taught that the incubus and succubus were two different corporeal forms adopted by the same shapeshifting demon, rather than two separate spirits. This explained how the incubus was able to fertilise a woman despite being unable to produce sperm, as it had only a virtual body. In the form of a succubus it could extract semen from a sleeping man, which it could then transfer to a woman in its form as an incubus.
In medieval Europe, union with an incubus was supposed by some to result in the birth of witches, demons, and deformed human offspring. Even though the sperm and the egg came from humans, the offspring of such unions were often thought to be supernatural, as in the case of King Arthur’s legendary magician, Merlin.
The 1495 edition of the Malleus Maleficarum opens with the words taken from Pope Innocent III’s papal bull of 1484, the Summis desiderantes affectibus Summis desiderantes affectibus, (Latin for "Desiring with supreme ardor"), sometimes abbreviated to Summis desiderantes was a papal bull regarding witchcraft issued by Pope Innocent VIII on 5 December 1484. , which is generally seen as having triggered the witch hunts that plagued Europe from the 15th to the 17th century:
It has indeed lately come to Our ears, not without afflicting Us with bitter sorrow … that many persons of both sexes, unmindful of their own salvation and straying from the Catholic faith, have abandoned themselves to devils, incubi, and succubi.
By the time of the 16th-century witch trials, the allegation that a woman had sex with the Devil in the form of an incubus was often the most damning charge she faced.
Sleep paralysis is a medical condition experienced by between 25 and 40 per cent of the population. It occurs during rapid eye movement (REM) sleep, when the brain may inhibit motor output to prevent the sleeper from acting out his or her dreams. One sleep paralysis episode, during which the sleeper feels a weight on their back or chest as if they are being crushed or choked, has been given the name Incubus. Some experiencing this episode, during which they are aware of their environment but are unable to move, describe the presence of an invisible entity that seems to be sitting on their chest, “shaking, strangling or prodding them”.
That people report such terrifying experiences is almost certainly because they are retrospectively interpreting them and creating false memories consistent with the times in which they live: in the early modern period of demons or witches, and in more recent times of aliens.