ADHD and juvenile onset bipolar disorder (Moreno, Laje, et al., 2007). It is certainly the case that more young people are being given these diagnoses.

INCIDENCE OF MENTAL, EMOTIONAL, AND BEHAVIORAL DISORDERS

To estimate the incidence, or number of new cases, in a given period of time, it is necessary to make repeated estimates in the same representative population sample, excluding those who had the disorder at the previous assessment. The same lack of basic data from repeated, representative sampling hampers the ability to answer questions about incidence. However, in this case, some of the small community-based longitudinal studies can provide data about incidence of the more common disorders. For example, data on 1,420 youth ages 9-21, over a 14-year period, from the Great Smoky Mountains Study (GSMS), a community study from the southeastern United States, shows a mean annual incidence rate of any disorder of around 3.5 percent in this age group. Of the 55 percent of youth in this community sample who had MEB disorders in one or more years of assessment, more than half (57.2 percent) had a diagnosis at two or more assessments, indicating that, in the majority of cases, the disorder was not confined to a single episode (Costello, Angold, et al., 1996).

A related issue relevant to prevention is the age at onset of child and adolescent emotional or behavioral disorders. In the NCS and NCS-R studies of adults, which ask people with a lifetime history of mental illness to remember their age at the first episode, half of all adults report onset in childhood or adolescence; the NCS-R found that in a population sample ages 18 and older, “half of all lifetime cases start by age 14 years and three fourths by age 24 years” (p. 593). Similarly, as noted earlier, in the GSMS, 55 percent of participants had been diagnosed with at least one MEB disorder by age 21 (see also Kim-Cohen, Caspi, et al., 2003).

Age at Onset

Figure 2-3 shows the age at onset of the first symptom in youth from the GSMS sample who would eventually receive a diagnosis by age 21, as well as the age at onset of the full-blown disorder. Disruptive behavioral disorders and ADHD had the earliest onset, followed by emotional disorders (anxiety and depressive disorders). Although many adolescents began using alcohol and other illicit drugs in their early teens, they tended not to meet criteria for abuse or dependence until their late teens.

Epidemiological findings like these raise questions of the utmost importance for prevention. If at least half of those who will have an MEB disorder during their lives have onset in childhood, then prevention resources need



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