Mental illnesses are all, at root, disorders of thought or emotion, stemming from physical problems. Over the past decade, evidence is accumulating concerning the genetic basis or contribution to these physical aspects of mental illness--the chemistry at nerve cell junctions, the metabolism in different regions of the brain, and anatomical anomalies. For example, nerve cell development may become impaired during fetal development. In other cases, chemical malfunctions, at least partially attributable to genetic malfunction, may impair normal nerve cell communication.

Most mental disorders fall into three broad categories: schizophrenia and related disorders, mood disorders (major depression, bipolar disorder) and anxiety disorders (such as compulsive disorder and panic disorder).

The genetic contributions to schizophrenia are subtle. Brain scans of identical twins have shown that the brain of the affected individual is differs slightly in the smaller size of certain cavities and in the...