"The Mask of Sanity" is a book written by psychiatrist Dr. Hervey Cleckley, first published in 1941. It explores the characteristics of psychopathy, a personality disorder marked by a lack of empathy, guilt, and emotional depth. Cleckley describes how individuals with this disorder can appear charming and normal, effectively wearing a "mask" that conceals their true nature. The book is significant in the field of psychology as it provides detailed case studies and insights into the behavior of psychopaths. Cleckley's work has influenced both clinical practice and popular culture, contributing to the understanding of related topics such as criminal behavior and mental health treatment.