hood, this is a stark gap in our epidemiological knowledge relevant to the prevention of mental disorders.
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.
Epidemiological research on children and adults should gather and retain data on a wide range of signs and symptoms, as well as disorders, to help ensure that maturational changes and changes in the diagnostic classification system do not interfere with the study of the development of psychopathology over time.
Prospective epidemiological studies that estimate incidence of specific risk factors and disorders in childhood, adolescence, and during the transition to adulthood, from age 15 to 25, are greatly needed for prevention research. Such studies could help clarify the mechanisms linking risk factors to the first occurrence of disorders.
For a particular disorder, a review of what is known about prodrome, age of onset, diagnostic criteria, course, co-morbidity, incidence and prevalence, effectiveness of treatment, and costs to society can help the investigator determine if the knowledge base is sufficient to consider designing a preventive intervention. For example, the incidence of a disorder will help determine the necessary size of the sample so that statistical analyses are meaningful (Muñoz and Ying, 1993); 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. Such a review could in turn help policymakers set priorities for preventive intervention programs.
Many mental disorders, including conduct disorder, depressive disorders, alcohol abuse and dependence, schizophrenia, and Alzheimer's disease, are thought to be a cluster of several different illnesses or to have subtypes. Identification of these groups and clearer delineation of their etiologies may clarify which individuals may be most amenable to preventive interventions.
Risk factors that have a role in the etiology of a mental disorder may be differentially malleable at different phases of the life course. More research regarding the sensitive periods of risk factors, that is, when they contribute most to etiology, could lead to more strategic