Mental illness is a term rooted in history that refers collectively to all of the diagnosable mental disorders. Mental disorders are characterized by abnormalities in cognition, emotion or mood, or the highest integrative aspects of behavior, such as social interactions or planning of future activities. These mental functions are all mediated by the brain. It is, in fact, a core tenet of modern science that behavior and our subjective mental lives reflect the overall workings of the brain. Thus, symptoms related to behavior or our mental lives clearly reflect variations or abnormalities in brain function. On the more difficult side of the ledger are the terms disorder, disease, or illness. There can be no doubt that an individual with schizophrenia is seriously ill, but for other mental disorders such as depression or attention-deficit/hyperactivity disorder, the signs and symptoms exist on a continuum and there is no bright line separating health from illness, distress from disease. Moreover, the manifestations of mental disorders vary with age, gender, race, and culture. The thresholds of mental illness or disorder have, indeed, been set by convention, but the fact is that this gray zone is no different from any other area of medicine. Ten years ago a serum cholesterol of 200 was considered normal. Today, this same number alarms some physicians and may lead to treatment. Perhaps every adult in the United States has some atherosclerosis, but at what point does this move along a continuum from normal into the realm of illness? Ultimately, the dividing line has to do with severity of symptoms, duration, and functional impairment.

Despite the existence of a gray zone between health and illness, science can study the mechanisms by which illness occurs. Indeed, understanding mood regulation and its abnormalities, for example, proceeds independently from any set of diagnostic clinical criteria. Family studies, molecular genetics strategies, epidemiology, and the tools of clinical investigation tailored to specific populations are being used to investigate the mechanisms of mental illness. Specific manifestations of mental illness will be covered in succeeding pages.

This overview of mental illness focuses on those features of the disease process that are most common and characteristic of these disorders. The chapters that follow will present specific details about major categories of mental disorders that occur across the life span. The purpose here is to provide a framework upon which subsequent discussions of specific disorders can rest. The section leads with a descriptive overview of the cardinal manifestations, signs, and symptoms of mental disorders. It then describes how mental disorders are diagnosed and classified and provides an overview of the epidemiology and societal burden of mental disorders.

Persons suffering from any of the severe mental disorders present with a variety of symptoms that may include inappropriate anxiety, disturbances of thought and perception, dysregulation of mood, and cognitive dysfunction. Many of these symptoms may be relatively specific to a particular diagnosis or cultural influence. For example, disturbances of thought and perception (psychosis) are most commonly associated with schizophrenia. Similarly, severe disturbances in expression of affect and regulation of mood are most commonly seen in depression and bipolar disorder. However, it is not uncommon to see psychotic symptoms in patients diagnosed with mood disorders or to see mood-related symptoms in patients diagnosed with schizophrenia. Symptoms associated with mood, anxiety, thought process, or cognition may occur in any patient at some point during his or her illness.