The Rosenhan experiment (also known as the the 'Thud' experiment, was a famous investigation into the validity of psychiatric diagnosis conducted by psychologist David Rosenhan in 1973. It was published in the journal Science under the title "On being sane in insane places."

Rosenhan's study was conducted in two parts. The first part involved the use of healthy associates or "pseudopatients" who briefly simulated auditory hallucinations in an attempt to gain admission to 12 different psychiatric hospitals in five different states in various locations in the United States. All were admitted and diagnosed with psychiatric disorders. After admission, the pseudopatients acted normally and told staff that they felt fine and had not experienced any more hallucinations. Hospital staff failed to detect a single pseudopatient, and instead believed that all of the pseudopatients exhibited symptoms of ongoing mental illness. Several were confined for months.

Despite constantly and openly taking extensive notes on the behavior of the staff and other patients, none of the pseudopatients were identified as impostors by the hospital staff, although many of the other psychiatric patients seemed to be able to correctly identify them as impostors. In the first three hospitalizations, 35 of the total of 118 patients expressed a suspicion that the pseudopatients were sane, with some suggesting that the patients were researchers or journalists investigating the hospital.

All were forced to admit to having a mental illness and agree to take antipsychotic drugs as a condition of their release.

The second part involved asking staff at a psychiatric hospital to detect fake patients that Rosenhan agreed to send. After a month, the staff identified large numbers of ordinary patients as impostors. Rosenhan then revealed that he had not sent any fake patients.