Wikipedia site states that David Ronsehan conducted this experiment in 1973, where he sent volunteers into Psychiatric hospitals , to prove that the diagnostic and treatment regime was no good. it says this
Rosenhan himself and eight mentally healthy associates, called “pseudopatients”, attempted to gain admission to psychiatric hospitals by calling for an appointment and feigning auditory hallucinations. The hospital staffs were not informed of the experiment. The pseudopatients were a psychology graduate student in his twenties, three psychologists, a pediatrician, a psychiatrist, a painter and a housewife. None had a history of mental illness. Pseudopatients used pseudonyms, and those who worked in the mental health field were given false jobs in a different sector to avoid invoking any special treatment or scrutiny. Apart from giving false names and employment details, further biographical details were truthfully reported.
During their initial psychiatric assessment, they claimed to be hearing voices of the same sex as the patient which were often unclear, but which seemed to pronounce the words “empty”, “hollow”, “thud” and nothing else. These words were chosen as they vaguely suggest some sort of existential crisis and for the lack of any published literature referencing them aspsychotic symptoms. No other psychiatric symptoms were claimed. If admitted, the pseudopatients were instructed to “act normally,” report that they felt fine and no longer heard voices. Hospital records obtained after the experiment indicate that all pseudopatients were characterized as friendly and cooperative by staff.
All were admitted, to 12 different psychiatric hospitals across the United States, including rundown and underfunded public hospitals in rural areas, urban university-run hospitals with excellent reputations, and one expensive private hospital. Though presented with identical symptoms, 11 were diagnosed with schizophrenia at public hospitals, and one with manic-depressive psychosis, a more optimistic diagnosis with better clinical outcomes, at the private hospital. Their stays ranged from 7 to 52 days, and the average was 19 days. All were discharged with a diagnosis of schizophrenia “in remission,” which Rosenhan takes as evidence that mental illness is perceived as an irreversible condition creating a lifelong stigma rather than a curable illness.
Then in a twist when one hospital claimed that there is no way they would so such a thing, he did this:
The non-existent impostor experiment
For this experiment, Rosenhan used a well-known research and teaching hospital, whose staff had heard of the results of the initial study but claimed that similar errors could not be made at their institution. Rosenhan arranged with them that during a three month period, one or more pseudopatients would attempt to gain admission and the staff would rate every incoming patient as to the likelihood they were an impostor. 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 genuine patients. This led to a conclusion that “any diagnostic process that lends itself too readily to massive errors of this sort cannot be a very reliable one”. Studies by others found similarly problematic diagnostic results.
He published the results in SCIENCE.
Here is an excerpt from the BBC Documentary THE TRAP, about it.