What does it mean to be sane? How reliably can even medical professionals distinguish the sane from the insane?

Psychologist Dr. David Rosenhan of Stanford University had long been interested in these age-old questions and, in 1969, devised a unique experiment to put them to the test.

Rosenhan and seven other perfectly sane subjects went undercover inside various psychiatric hospitals from 1969-1972 and acted insanely in order to see if the doctors there could tell that they were faking. The doctors could not.