As a more scientific approach to medicine was taken in the 19th century, the clinical symptoms of mental disorders were also systematised and categorised. In order to standardise the various approaches and thus promote international exchange, the World Health Organization (WHO) added a new chapter on mental illnesses to its 'International Classification of Diseases' (ICD) in 1948. In 1952, the American Psychiatric Association published the 'Diagnostic and Statistical Manual of Mental Disorders' (DSM). The two systems have since been revised and harmonised several times. They reflect trends in research as well as changes in social values and norms.