meaningful; the demographics of a disorder will help determine who is at highest risk and what population groups should be targeted; and if a specific treatment is known to be effective, it could be considered for use before onset of the disorder.

In Chapter 5 the discussion of this knowledge is organized around five major mental disorders: conduct disorder, depressive disorders, alcohol abuse and dependence, schizophrenia, and Alzheimer's disease. These five disorders were chosen as illustrations for use throughout the report because they are all serious disorders that have enormous emotional and financial costs associated with them. They represent the great diversity of mental illness, have their onset at varying stages in the life cycle, and reflect a spectrum of causation, arising from primarily psychosocial factors in conduct disorder to clear biological contributions in Alzheimer's disease. The choice of these five disorders is by no means meant to imply that these are the only disorders that should be targeted for preventive intervention research programs. Anxiety disorders, post-traumatic stress disorder, obsessive-compulsive disorder, and other adult and childhood mental disorders may also be appropriate targets. These five disorders are simply illustrative of the range of factors and approaches that must be considered in designing preventive intervention research programs, and the brief descriptions given in Chapter 5 are examples of how the available information should be reviewed.

A disorder may be preventable up to the point of onset of first episode. Although onset can rarely be accurately pinpointed, the time at which an individual meets full criteria for diagnosis can be used as an approximation. As more becomes known about precursors and prodromes, the age of onset will become more accurately known. The prodrome is the period prior to onset of a disorder, when some early signs or symptoms are nevertheless present. But individuals with early signs and symptoms of disorder often do not go on to develop the full criteria for diagnosis. In this situation the signs and symptoms are not prodromal, in the strict sense of the word. Therefore, for a particular individual a prodrome can be known only in retrospect, after he or she has developed the disorder. If he or she never develops it, the early signs were not part of a prodrome. Signs and symptoms from a diagnostic cluster that precede disorder, but do not predict the onset of disorder with certainty, are referred to here as precursor signs and symptoms. Currently, there are few or no signs and symptoms that predict onset with certainty. Nevertheless, prospective epidemiological studies could identify precursor signs and symptoms, as well as the age of the first occurrence of these precursors. Thus it may be possible to identify individuals at heightened risk for developing the full-blown disorder,