The concept of medicalization was devised by sociologists to explain how medical knowledge is applied to behaviors which are not self-evidently medical or biological. The term "medicalization" entered the sociology literature in the 1970s in the works of Irving Zola, Peter Conrad and Thomas Szasz, among others. According to Dr. Cassell's book, "The Nature of Suffering and the Goals of Medicine (2004)," the expansion of medical social control is being justified as a means of explaining deviance. These sociologists viewed medicalization as a form of social control in which medical authority expanded into domains of everyday existence, and they rejected medicalization in the name of liberation. This critique was embodied in works such as Conrad's "The discovery of hyperkinesis: notes on medicalization of deviance", published in 1973 (hyperkinesis was the term then used to describe what we might now call ADHD). Nevertheless, opium was used to pacify children in ancient Egypt before 2000 BC.