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Chapters 1 (and 2) provide a common basis and a vocabulary for appreciating the contours of behavioral and psychiatric genetics. There are three dialogues between a behavioral geneticist and an appeals court judge who wishes to find out more about behavioral genetics for eventual use in her court cases. The distinction between classical quantitative genetics and modern molecular genetics is introduced. In the former, no specific genes are identified, though heritability estimates are provided. The contentious concept of "heritability" is analyzed in detail and its limitations delineated in the first dialogue. A simple mathematical argument is sketched to show how twin studies yield heritabilities and environmental influences for both traits and disorders, and the critical assumptions of such studies are summarized. The chapter closes with an analysis of the shared and nonshared environment issue and a sketch of what is known as an ACE diagram.

Modern psychiatry seeks to treat disorders of the brain, the most complex and least understood organ in the human body. This complexity poses a set of challenges that make progress in psychiatric research particularly difficult, despite the development of several promising novel avenues of research. New tools that explore the neural basis of behavior have accelerated the discovery in neuroscience, yet discovery into better psychiatric treatments has not kept pace. This chapter focuses on this disconnect between the challenges and promises of psychiatric neuroscience. It highlights the need for diagnostic nosology, biomarkers, and better treatments in psychiatry, and discusses three promising conceptual advances in psychiatric neuroscience. It holds that rigorous theory is needed to address the challenges faced by psychiatrists.

This chapter summarizes the book, which provided an overview of the recent history and methodology of behavioral genetics and psychiatric genetics. The perspective is primarily philosophical and addresses a wide range of issues, including genetic reductionism and determinism, "free will," and quantitative and molecular genetics. The latter covers newer genome-wide association studies (GWAS) that have produced a "paradigm shift" in the subject and generated the problem of "missing heritability." A novel account of biological knowledge emphasizing the importance of models, mechanisms, pathways, and networks is offered that clarifies how partial reductions provide explanations of traits and disorders. The book examined personality genetics and schizophrenia and its etiology, and concluded with some major worries about "free will," and arguments pro and con about why genes and DNA are so special. A pessimistic view of the current state of the field, but optimism for the future of the subject, was proposed.