We see what we expect to see

The power of expectations was dramatically illustrated in a bold experiment conducted years ago by the psychologist David L. Rosenhan. In that study each of eight "pseudopatients" made an appointment at one of a variety of hospitals and then showed up at the admissions office complaining that they were hearing strange voices. The pseudopatients were a varied group: three psychologists, a psychiatrist, a pediatrician, a student, a painter, and a housewife. Other than alleging that single symptom and reporting false names and vocations, they all described their lives with complete honesty. Confident in the clockwork operation of our mental health system, some of the subjects later reported having feared that their obvious sanity would be immediately sniffed out, causing great embarrassment on their part. They needn't have worried. All but one were admitted to the hospital with a diagnosis of schizophrenia. The remaining patient was admitted with a diagnosis of manic-depressive psychosis. 

From The Drunkard's Walk: How Randomness Rules Our Lives by Leonard Mlodinow