Branch of psychology. It is concerned with mental and emotional disorders (e.g., neurosis, psychosis, mental deficiency) and with certain incompletely understood normal phenomena (such as dreams and hypnosis). The chief tool used in classifying psychological disorders is the American Psychiatric Assn.'s Diagnostic and Statistical Manual of Mental Disorders 4th edition (DSM-IV).
Learn more about abnormal psychology with a free trial on Britannica.com.