"For centuries, doctors have struggled to define mental illness--how do you diagnose it, how do you treat it, how do you even know what it is? In search of an answer, in the 1970s a Stanford psychologist named David Rosenhan and seven other people--sane, normal, well-adjusted members of society--went undercover into asylums around America to test the legitimacy of psychiatry's labels. Forced to remain inside until they'd "proven" themselves sane,...