Two different sets of diagnostic criteria are currently used in the world literature. For most stud ies that emphasize biological markers, and for almost all of those conducted in the United States, the standard diagnostic criteria are from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV, American Psychiatric Association, 1994). However, international epidemiologic studies are likely to use the World Health Organization's International Classification of Diseases, Tenth Revision (WHO's ICD-10, World Health Organization, 1992). Differences in the choice of diagnostic criteria may affect the results of studies.

There are many similarities between the ICD and DSM, largely as a consequence of efforts by the ICD and DSM work groups to achieve as much concordance as possible. Both require 1 month of active symptoms and the presence of psychotic symptoms such as delusions or hallucinations. There are, however, important differences between the ICD and DSM. In most respects, the DSM provides a slightly narrower conceptualization of schizophrenia than does the ICD. For example, the ICD only requires 1 month of overall duration of symptoms, whereas the DSM requires 6 months. In addition, the ICD includes schizotypal disorder and simple schizophrenia within its nomenclature under the general heading of the diagnosis of schizophrenia. In the DSM, simple schizophrenia is excluded, and schizotypal disorder is placed among the personality disorders. Other less significant differences include a greater emphasis on first-rank symptoms in the ICD, as well as a much more specific and complex system list.

How important is the choice of diagnostic criteria for research on adolescents? It could be very important. Setting criteria boundaries more broadly or more narrowly will have a significant impact on the groups of adolescents chosen for study. Furthermore, although the developers of these criteria paid close attention to examining the reliability and, when possible, their validity, it was assumed almost without question that the criteria could and should be the same for children, adolescents, and adults. This decision was not based on any published empirical data but rather, primarily on “clinical impressions.”

A frequently expressed clinical impression among those who study schizophrenia or psy

chosis in children and adolescents, however, is that making a diagnosis in these younger age ranges is much more difficult than diagnosing individuals in their 20s. Multiple issues arise for a diagnosis in the adolescence age range. One important issue is comorbidity. Teenagers frequently may meet criteria for multiple diagnoses, such as conduct disorder or attention-deficit hyperactivity disorder (ADHD). Although the DSM tends to encourage the use of multiple diagnoses, this policy also has no empirical basis. An alternative approach that might be considered from research on adolescents is to try to identify a single “best” diagnosis that would summarize the child parsimoniously.

Many adolescents also abuse substances of many different kinds. This factor is important to consider in the diagnosis of adults who may have schizophrenia, but it poses even greater problems among adolescents. Abuse of substances such as amphetamines may potentially induce a psychotic picture that is very similar to schizophrenia. We do not know whether young people who continue to meet criteria for schizophrenia after discontinuing amphetamine use should be considered “typical schizophrenics” or whether they should in fact be given another diagnosis such as substance-induced psychotic disorder. However, because amphetamines have a significant effect on the dopamine system—a key neurotransmitter implicated in the neurochemical mechanisms of schizophrenia—it is at least plausible that amphetamines (and perhaps other substances as well) be considered triggers or inducers of schizophrenia. According to this view, substance abuse could be one of the many factors that rank among the nongenetic causes of schizophrenia. However, there is still no strong consensus on this issue.

In summary, there are many unanswered research questions under the heading of diagnosis. More studies are needed to explore how well existing diagnostic criteria actually work in children and adolescents. Specifically, studies of both the reliability and validity of these criteria are needed, as well as studies examining issues of comorbidity and longitudinal studies examining changes in both diagnosis and phenomenology in cohorts of adolescents and of adults.

The concept of phenomenology can be relatively broad, describing clinical symptoms, psychosocial functioning, cognitive functioning, and “neurological” measures such as soft signs. Here we will focus primarily on clinical symptoms and psychosocial functioning.

For the assessment of clinical symptoms in schizophrenia, choosing the appropriate informant is a key issue. Whatever the age, patients suffering from schizophrenia frequently have difficulty in reporting their symptoms and past history accurately. Optimally, one gets the best information from several informants, usually a parent plus the patient. In the case of adolescents, a friend may be a good additional informant. Another critical issue in phenomenology when assessing adolescents is to determine the distinction between “normal” adolescent behavior and psychopathology. Again, this can be difficult in assessing adults, but it is even more difficult in adolescents. It can be hard to draw the line between “teenage scruffiness” and disorganization, or a withdrawal to seek privacy versus avolition. As discussed above, drug use or abuse can also confound the picture. For example, when an adolescent known to be using marijuana regularly exhibits chronic apathy and avolition, is this due to marijuana use or is it a true negative symptom? At the moment, no data are available to help us address any of these issues pertaining to the assessment of clinical symptoms in adolescents versus those in adults. This is clearly an area in which more information is needed.

Another issue is the identification of appropriate developmental milestones and needs that are appropriate to the adolescent age range for the assessment of psychosocial functioning. For example, when we assess peer relationships in young or older adults with schizophrenia, we are evaluating the extent to which they have a circle of friends with whom they get together socially. In the case of adolescents, peer relationships are far more important and are more intensely driven by a need to establish independence from the family setting and to bond with others from the same age range. Likewise, the assessment of family relationships among adolescents is