Evil twins make for great soap opera drama. But this idea, used to drum up ratings and appeal to viewers' fears and desires, actually has a basis in paranormal folklore and history in the concept of "doppelganger." The term, German for "double walker," can refer to a number of different phenomena, including the appearance of a ghostly double or apparition, or a vision of an identical twin in the flesh. Doppelgangers often have distinctive physical characteristics, including no shadow.

The ancient belief of doppelgangers could represent a danger-free curiosity, with the fascinating idea of a having your own non-related identical twin somewhere out in the world. In some stories, doppelgangers exist as a result of magic or temporal shifts. But more and more throughout history, the idea of a doppelganger indicates a dangerous or malevolent force. Doppelgangers will act like an evil twin, causing mischief by confusing friends and family and giving bad advice. Alternatively, doppelgangers could be hands-off but be a silent sign of extreme bad luck or even death. The poet John Donne claimed to see his wife's doppelganger in the 1600s, shortly before their child was stillborn. Percy Bysshe Shelley told friends of seeing his doppelganger not long before his death by drowning, and Abraham Lincoln also saw his twin before his assassination.

In 2006, researchers found that electromagnetic stimulation of a certain area of the brain made patients see "shadow people," figures that at times resembled themselves and other times appeared as strangers. This development could be a fascinating insight into the long history and folklore of doppelgangers.