Nearly half of Americans have suffered from mental illness at some point in their lives. One quarter has been mentally ill during the previous year. These figures come from the finest survey research available, funded by the National Institute of Mental Health. Is madness, then, rampant in America?The answer depends on what we mean by “madness.”The term brings to mind schizophrenia—a psychotic disorder involving a break from reality and marked by delusions and hallucinations. Yet what counts as mental illness spans far more than psychosis. Many more people suffer from anxiety disorders, depression, and substance abuse than from schizophrenia.Nevertheless, the sheer numbers of Americans reported to suffer from mental illness still seem implausibly high. Which implies that our system of diagnosing people may often mistake the normal emotional problems of everyday life as mental illness.This system, embodied in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders, known as DSM-IV, provides the criteria for over 360 different forms of mental illness. Immensely influential, the DSM-IV determines eligibility for reimbursable treatment, provides the foundation for clinical research, and sets the boundary between normality and abnormality in American society.The DSM is the flashpoint of growing critical attention—especially because the American Psychiatric Association (APA) is currently debating proposals for revising it. The next edition is due to appear in 2013.In What Is Mental Illness?, I provide an accessible, but nuanced, guide to the current controversies regarding how we conceptualize mental illness. The book covers cutting-edge theory and research from evolutionary psychology, genomics, psychology, history, and cross-cultural psychiatry—to weigh the merits of proposed solutions of how best to draw the boundary between mental distress and mental disorder.