The classification of affective disorders in DSM-III-R evolved from Kraepelin's biological nosology. The discovery of pharmacological treatments for specific types of illness revived interest in this model, which stands in contrast to the psychosocial orientation of Adolf Meyer that formed the basis of psychiatric thought in the United States for many years. Current issues in this field include the differentiation of bipolar (mania and depression) from unipolar (depression only) disorders, the delineation of subclasses within each group, and the clinical separation of depressions into endogenous or reactive, neurotic or psychotic, and primary or secondary. The DSM-III-R category "Dysthymia" is not equivalent to neurotic depression. Classical