In the late 19th century, psychiatrists and artists in the western world became intrigued by the drawings and paintings made by mental patients. The idea arose that, by looking at those products, one might possibly gain a better understanding of how they felt. This in turn led to art therapy.
Studies regarding the survivors of concentration camps in WWII and the Vietnam war showed that, in many cases, verbal psychotherapy proved ineffective. For that reason, various forms of art therapy were developed and used successfully.
Initially, the artistic expressions of the clients/patients mainly served diagnostic purposes. Gradually, though, therapeutic elements were discovered in the various forms of expression. This led to art psychotherapy becoming a profession.