This strange condition makes you believe that people around you are impostors
Imagine waking up every day and not being sure whether the people you said goodnight to the evening before were the same ones you'd greet at the breakfast table that morning. Imagine tiptoeing around your own home, uncertain of who or what else could be inhabiting the rooms you thought belonged to your loved ones. Imagine, just for a second, looking at your housemates or your family or even your partner, and feeling - somewhere deep down in your gut - that they weren't the person they claimed to be, and instead were some impostor sent to infiltrate your life.
Though this may sound like some kind of Truman Show-ish sci-fi concept, it's actually a genuine medical condition known as Capgras Syndrome: a form of delusion which makes the sufferer believe that one or more people around them have been replaced by identical replicas.
And, horrifyingly, anyone can develop it.
Capgras Syndrome was first described in 1923 in a study by two French doctors: Joseph Capgras (after whom the disorder is named) and Jean Reboul-Lachaux. Their research was mainly based on one woman, known as "Madame M", who sincerely believed that everybody in her life - her friends, her family, her neighbors - had all been replaced by exact lookalikes. She even thought that her husband was an impostor, and claimed that he had been switched out for a doppelganger more than 80 times.
But Madame M was far from alone in her predicament. In fact, though the disorder is still extremely rare, it can actually affect anyone at any time (but it most frequently occurs in adult women), and it is believed that between 1.3 to 4.1 per cent of all psychiatric patients suffer from Capgras either as a standalone delusional disorder or as part of another illness.
So, how much exactly do we understand about this mysterious phenomenon?
Since its discovery, Capgras Syndrome has been investigated and reinvestigated time and time again, but nobody is quite sure what causes it to develop in some people.
Psychoanalysts originally believed it was a repressed Oedipus or Electra complex (which, if you're familiar with Freud, you'll recognize as being the names for men and women's supposed desires to sleep with their parents), but that theory was thrown out - along with basically all of Freud's other crazy, incestuous beliefs - decades ago. After that, people began to believe the disorder could be caused by repressed emotions; but more recent research suggests it may be a physical defect in the brain.
As it stands, it's most likely that the symptoms are caused by a combination of mental and physical factors.
No matter what causes it, though, the illness has proven to be deadly on a number of occasions, not necessarily for the sufferers, but for those whom Capras patients assume to be imposters.
In 2014, 32-year-old Jeremiah Wright from Louisiana was found to be insane after decapitating his seven-year-old son. The man claimed that he had done so because he was convinced his child had been replaced by a CPR dummy, and that he believed murdering the boy would be the only way to tell if he was correct.
Even after admitting to his actions, Wright was not found guilty of murder due to the fact that he always insisted (and, as far as anyone knows, continues to say) that the murder victim was not his real son. Instead, the 32-year-old was sent to a psychiatric facility to get help with his disorder.
Other similar reports in the past have noted that one man killed his father because he was convinced that he had been replaced by a robot, and another woman on a psychiatric ward killed a fellow patient because she believed that she (the other patient) was intent on getting rid of her daughter's impostor.
Most often, people who suffer from the disorder believe that it is a spouse or a close family member that has been replaced - but some individuals have the condition so severely that they believe buildings, countries, or even they themselves, have been replaced by an identical double.
As a symptom of the disorder is a complete lack of emotional connection for the accused imposter, those close to people with Capgras Syndrome can often be impacted just as harshly - if not worse - than the patient themselves.
Unfortunately, the only viable treatment right now is therapy, and that has varying success rates.