Psychiatry and clinical psychology have long been divided about the roles of nature and nurture in the pathways to psychopathology. Some clinicians offer treatment almost entirely based on neuroscience. Some psychologists offer psychotherapies almost entirely based on the impact of environmental stressors. Paris argues for a balanced middle ground between nature and nurture in human development. This book reviews and integrates research showing that the key to understanding the development of mental disorders lies in interactions between genes and environment. It explores why personality is a key determinant of how people respond to stress, functioning as a kind of psychological immune system. This model represents a shift from overly simple and reductionistic constructs, based primarily on biological risks or on psychosocial risks in development. Instead, it offers a complex and multivariate approach that encourages a broader approach to treatment.