A simple but powerful developmental concept that has affected psychiatric nosology is that of age norms. Rather than evaluating a set of behaviors or symptoms according to a theoretical, absolute, or population-wide distribution, diagnosticians increasingly use age norms to evaluate psychiatric problems. Consider the evaluation of temper tantrums. In a 2-year-old child, tantrums are normative, whereas in an adult, angry outbursts could indicate an intermittent explosive disorder or antisocial personality. More subtle examples have begun to affect the diagnosis of many disorders in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV), such as attention-deficit/hyperactivity disorder, mental retardation, and conduct disorder. With regard to major depressive episodes and dysthymic disorder, age norming has resulted in consideration of different symptoms at different ages in order to diagnose the same disorder (eg, irritability and somatization are common in prepubescent depression, whereas delusions are more common in adulthood). DSM-IV explicitly requires consideration of age, gender, and culture features in all disorders, suggesting the importance of evaluating symptoms within the context of their expression.
The importance of age norming suggests the need for empirical studies of symptoms in large epidemiologic samples and the linking of research on normative development to psychopathology. In this way, developmental psychopathology is similar to psychiatric epidemiology. Despite the increased emphasis on age norming, ambiguity pervades current practice. DSM-IV defines disorders in terms of symptoms that are quantified as “often,""recurrent," and “persistent” without operational definition. Some clinicians intuitively contextualize their use of the term “often” relative to a child’s agemates (so that “often displays temper tantrums” might mean hourly for a 2-year-old child and weekly for a teenager), whereas other clinicians do not (so that “often” has the same literal meaning across all ages). The specific meaning of these terms is not clear in the context of some DSM-IV disorders. Complete age norming might imply the removal of all age differences in prevalence rates (reducing disorder merely to the statistical extremes of a distribution at an age level), whereas complete neglect of age norms implies that at certain ages a disorder is ubiquitous. To resolve these problems, developmental researchers need to learn which patterns of symptoms ought to be examined epidemiologically, and psychopathologists need to compare their observations to empirical norms.