A doppelgänger is popularly known as a “double” of oneself, with the exact same appearance but usually with a completely different personality.
It comes from German folklore (the word is German for “double walker”), where it is believed that seeing a doppelgänger is an omen of bad luck, possibly death. It is also said that meeting one’s own doppelgänger will result in one of the two dying.
However, in modern times this concept of doppelgängers have been largely replaced by the first meaning, that of an alter ego who is only identical physically, and completely different persona-wise. This can sometimes overlap with the concept of an “evil twin”, as a doppelgänger is sometimes described to have an inverse personality, values and motives.
Although in most cases a doppelgänger is just a look-alike, it could be the result of the brain projecting an identical image of oneself in another space.
It has been discovered that electrical stimulation of an area of the brain called the left temporoparietal junction causes the sensation that someone is next to you. This “someone” has the same shape and body posture as the subject, and is sometimes described as having a completely different personality. It is reported that this sensation is extremely unpleasant.
The cause of this phenomenon is that the area of the brain is associated with self-image, such as body posture, location and proprioception (sense of what oneself appears as in space). Therefore, a disruption leads to the self-image being projected elsewhere, which the brain then interprets as a doppelgänger.
There are similar mental disorders such as the Capgras delusion (belief that someone close has been replaced by a doppelgänger), Fregoli delusion (belief that many people are the same person in disguise) and subjective doubles (belief that one’s own doppelgänger is acting on its own to interfere with one’s life).
Doppelgängers are common plot devices in many works, especially fantasy and science fiction where the aforementioned “evil twin” concept is used. However, the doppelgänger may also be described as benevolent or simply a clone.
They are notably used in How I Met Your Mother, where each character has a doppelgänger completely different to their persona (e.g. Stripper Lily, Fertility Expert Barney).