Diagnosticians must consider not only the age-normed profile of symptoms but also the developmental trajectories of those symptoms (both age-normed and individual). For example, consider three 10-year-old children who exhibit aggressive behavior. As depicted in Figure 1-1, child A has displayed a relatively high rate of aggression historically, but the trajectory is downward. Child B has displayed a constant rate of aggressive displays, and child C’s aggressive displays have accelerated geometrically. Which child has a problematic profile? The diagnostician will undoubtedly want to consider not only current symptom counts (in relation to age norms) but also the developmental trajectory of these counts (and the age norm for the trajectory). Child C might be most problematic because of the age trend, unless this trend were also age normative (eg, some increase in delinquent behavior in adolescence is certainly normative). In contrast, child B’s constant pattern might be problematic if the age-normed trend were a declining slope.
Some DSM-IV disorders explicitly take into account the trajectory of an individual’s symptoms. For example, Rett’s disorder, childhood disintegrative disorder, and dementia of the Alzheimer’s type involve deviant trajectories. The diagnosis of other disorders may require trajectory information that is not yet available. This information must be based on longitudinal study of individuals and not cross-sectional data, because only longitudinal inquiry allows for the charting of growth curves within individuals over time. Population means at various ages indicate little about within-individual changes. Population-wide symptom counts might grow systematically across age even when individual trajectories are highly variable.