Book excerpt: New discoveries about the genetic underpinnings of many kinds of human experience are now continually being made. This book explores the impact of these discoveries on the ways in which the common mental disorders are best conceptualized and treated. Most people think of research in genetics as the search for genes. This is only one focus of effort, and even with the reliable identification of susceptibility genes, the clinical applications of their discovery, such as gene therapies and new drug development, are a long way off. For the present, the impact of genetic research on our understanding of mental illness is tied to our ability to estimate the effect of all genes by means of family, twin, and adoption studies. The results of these studies challenge some deeply cherished ideas and theories, and support others. Of course, the effect of genes is only half the equation. The role of experience, environment, and living conditions accounts for as much, often considerably more, of the variability in psychopathology. In this book, Kerry Jang attempts not to answer questions about what is "genetic" and what is not, but about what a knowledge of the relative influence of genes versus environment means at a psychological level of analysis--to show how it changes common assumptions about classification, etiology, diagnosis, and intervention. He first offers an overview of contemporary behavioral genetics, dispels common misconceptions, responds to the criticisms that have been leveled at this new field, and describes in basic terms how genetic and environmental effects are estimated and how susceptibility genes are pinpointed. He then points to new directions in which standard nosological systems are likely to evolve as new information about vulnerabilities and covariances emerges. Finally, he synthesizes and evaluates the consistency of the last decade's findings for the most common categories of psychopathology that have been studied by behavior geneticists: mood, personality, and anxiety disorders, substance abuse; and schizophrenia and the psychotic disorders. Clinicians and researchers alike need to understand the genetic influences on the feelings and behaviors they are seeking to change or study if they are to be effective in their work. The Behavioral Genetics of Psychopathology: A Clinical Guide empowers them with this understanding.