In 1923, French psychiatrist Joseph Capgras and his colleague Jean Reboul-Lachaux described a 53-year-old woman convinced that her husband, her children, and even local shopkeepers had been murdered and replaced by identical doubles. She sought a divorce from the man who looked exactly like her husband. They called it the illusion of doubles, and for decades psychiatry treated the condition through psychoanalysis as a female disorder born of repressed desires. In the 1980s doctors began finding organic brain lesions in many patients, and the delusion was reframed as a hardware failure tied to brain injury, dementia with Lewy bodies, Alzheimer’s, and psychosis.

The breakthrough came from comparing it with face blindness. In 1984, Bauer showed that prosopagnosia patients who consciously failed to recognize family photos still produced skin conductance spikes. In 1990 Hadyn Ellis and Andy Young proposed Capgras as the mirror image: recognition intact, emotional response gone. V.S. Ramachandran located the break between the fusiform gyrus and the amygdala, William Hirstein added his hybrid representation theory in 2010, and Ellis’s two factor account added a failure of frontal lobe reality checking.

• The delusion appears in up to 50 percent of patients with schizophreniform psychosis
• It occurs roughly 50 percent more often in women than in men
• A 2019 review found seven cases linked to cannabis, and a 2022 case involved a 28-year-old with no psychiatric history
• Ketamine has temporarily induced the delusion in a healthy subject
• Reduplicative paramnesia, the belief that a place has been duplicated, is tied to frontal lobe lesions