Robert Sapolsky in Nautilus:
We start with the case of a woman who experienced unbearable tragedy. In 1899, this Parisian bride, Madame M., had her first child. Shockingly, the child was abducted and substituted with a different infant, who soon died. She then had twin girls. One grew into healthy adulthood, while the other, again, was abducted, once more replaced with a different, dying infant. She then had twin boys. One was abducted, while the other was fatally poisoned.
Madame M. searched for her abducted babies; apparently, she was not the only victim of this nightmarish trauma, as she often heard the cries of large groups of abducted children rising from the cellars of Paris.
As if all this pain was not enough, Madame M.’s sole surviving child was abducted and replaced with an imposter of identical appearance. And soon the same fate befell Madame M.’s husband. The poor woman spent days searching for her abducted loved ones, attempting to free groups of other abducted children from hiding places, and starting the paperwork to divorce the man who had replaced her husband.
In 1918, Madame M. summoned the police to aid her in rescuing a group of children locked in her basement. Soon she was speaking with a psychiatrist. She told him she was the direct descendant of Louis XVIII, the queen of the Indies, and of the Duke of Salandra. She had a fortune of somewhere between 200 million and 125 billion francs, and had been substituted as a toddler in a conspiracy to deny her this money. She was constantly under surveillance, and most, if not all, of the people she encountered were substituted doubles, or even doubles of the doubles.
The psychiatrist, Joseph Capgras, listened patiently. It’s delusional psychosis—disordered thought, grandiosity, paranoia—he thought. Pretty standard fare. But then again, no one had ever described the particular delusion of a loved one being replaced by an identical double. What could that be about?