In 1973 psychologist David Rosenhan and eight other mentally healthy people reported one-time auditory hallucinations in order to gain admission to mental Hospitals. Once inside they acted normally and reported no further issues. All were diagnosed with serious mental disorders, the most common being schizophrenia. All were released as people ‘in remission’ and not as ‘sane’ people. [WIKI]
“During the first three hospitalizations, when accurate counts were kept, 35 of a total of 118 patients on the admissions ward voiced their suspicions, some vigorously. “You’re not crazy. You’re a journalist, or a professor (referring to the continual note-taking). You’re checking up on the hospital.” While most of the patients were reassured by the pseudopatient’s insistence that he had been sick before he came in but was fine now, some continued to believe that the pseudopatient was sane throughout his hospitalization . The fact that the patients often recognized normality when staff did not raises important questions.”
You can Rosenhan’s paper on his experiment ‘On Being Sane in Insane Places’ here.
As a follow up, Rosenhan challenged a teaching hospital who knew of the first experiment to see if they could detect impostor patients themselves.
“Out of 193 patients, 41 were considered to be impostors and a further 42 were considered suspect. In reality, Rosenhan had sent no pseudopatients and all patients suspected as impostors by the hospital staff were ordinary patients.”