This month the American Psychiatric Association (APA) will publish the fifth edition of its guidebook for clinicians, the Diagnostic and Statistical Manual of Mental Disorders , or DSM-5 . Researchers around the world have eagerly anticipated the new manual, which, in typical fashion, took around 14 years to revise. The DSM describes the symptoms of more than 300 officially recognized mental illnesses--depression, bipolar disorder, schizophrenia and others--helping counselors, psychiatrists and general care practitioners diagnose their patients. Yet it has a fundamental flaw: it says nothing about the biological underpinnings of mental disorders. In the past, that shortcoming reflected the science. For most of the DSM 's history, investigators have not had a detailed understanding of what causes mental illness.

That excuse is no longer valid. Neuroscientists now understand some of the ways that brain circuits for memory, emotion and attention malfunction in various mental disorders. Since 2009 clinical psychologist Bruce Cuthbert and his team at the National Institute of Mental Health have been constructing a classification system based on recent research, which is revealing how the structure and activity of a mentally ill brain differs from that of a healthy one. The new framework will not replace the DSM , which is too important to discard, Cuthbert says. Rather he and his colleagues hope that future versions of the guide will incorporate information about the biology of mental illness to better distinguish one disorder from another.