An important limitation is that the DSM-IV criteria for ADHD were developed with children in mind and offer only limited guidance regarding diagnosis among adulthood. Clinical studies make it clear that symptoms of ADHD are more heterogeneous and subtle in adults than children (32
), leading some clinical researchers to suggest that assessment of adult ADHD might require an increase in the variety of symptoms assessed (34
), a reduction in the severity threshold (35
), or a reduction in the DSM-IV six-of-nine symptom requirement (36
). To the extent that such changes would lead to a more valid assessment than in the current study, our prevalence estimate is conservative.
Three additional limitations are also noteworthy. First, adult ADHD was assessed comprehensively only in the clinical reappraisal sub-sample. Although the imputation equation was strong, the need to impute entire diagnoses made it impossible to carry out symptom-level investigations of such things as the notion that inattentive symptoms are more prominent than hyperactive/impulsive symptoms among adults than children.
Second, both the CIDI and clinical reappraisal interviews were based on self-reports. Childhood ADHD is diagnosed based on parent and teacher reports (37
). Informant assessment is much more difficult for adults, making it necessary to base assessment largely on self-report (38
). Methodological studies comparing adult self-reports versus informant reports of ADHD symptoms document the same general pattern of under-estimation in self-reports in adults as children (39
), suggesting that our prevalence estimates is probably conservative, although the only study of self versus informant assessment of adult ADHD in a non-clinical sample found fairly strong associations between the two reports (41
Third, even though the semi-structured interview used in the clinical reappraisal interviews, the ACDS, had been used in clinical studies of adult ADHD, no standard method of clinical validation of adult ADHD exists with the same level of acceptance as the SCID has for anxiety, mood, or substance disorders, limiting the interpretability of results.
Within the context of these limitations, the results reported document that adult ADHD is a commonly occurring and often seriously impairing disorder. The 4.4 % estimated prevalence is in the middle of previous estimates. This estimate is likely to be conservative for reasons described above. The findings that adult ADHD is associated with unemployment and being previously married are broadly consistent with studies that have documented adverse effects of adult ADHD (8
). The WHO-DAS analyses are also consistent with this broad pattern. However, the WHO-DAS might under-represent ADHD impairments because some WHO-DAS dimensions tap areas where ADHD is not highly impairing (e.g., people with ADHD are often very mobile and overwork) and because the WHO-DAS does not assess many dimensions where people with ADHD are thought to function least adequately (e.g., poor sleep and nutrition, high rates of accidents, high smoking). In addition, as noted in the last paragraph, people with ADHD might have poor insight into their impairments, leading to underestimation of WHO-DAS scores.
The finding of low prevalence among Hispanics and Non-Hispanic Blacks was unexpected. As the DSM-IV ADHD field trials found no effects of race-ethnicity (43
), the NCS-R result could reflect a race-ethnic difference either in adult persistence, in accuracy of adult self-report, in cultural perceptions of the acceptability of ADHD symptoms, or some combination. The finding that adult ADHD is significantly more prevalent among men than women, in comparison, is consistent with much previous research (44
). The 1.6 male:female OR is comparable to the OR’s found in studies of children and adolescents, suggesting that childhood-adolescent ADHD is no more likely to persist into adulthood among girls than boys (45
). This indirectly suggests that the high proportion of adult women in adult ADHD patient samples is due to help-seeking or recognition bias (46
). The finding that adult ADHD is highly comorbid is consistent with clinical evidence (42
). Methodological analysis shows that these comorbidities are not due to overlap of symptoms, imprecision of diagnostic criteria, or other methodological confounds (47
The average magnitude of OR’s between adult ADHD and other comorbid disorders is comparable to most NCS DSM-IV anxiety and mood disorders (48
). The absence of strong variation in comorbidity OR’s was surprising, as family studies would lead us prediction of high comorbidities with major depression (49
), bipolar disorder (50
), and conduct disorder (52
), and lower comorbidities with anxiety disorders (54
). One striking implication of the high overall comorbidity is that many people with adult ADHD are in treatment for other mental or substance disorders, but not ADHD. The 10% of cases who receive treatment for adult ADHD is much lower than for anxiety, mood, or substance disorders (55
). Direct-to-consumer outreach and physician education are needed to address this problem.
The comorbidity findings raise the question whether early successful treatment of childhood ADHD would influence secondary adult disorders. The fact that a diagnosis of adult ADHD requires at least some symptoms to begin before age 7, means that the vast majority of comorbid conditions are temporally secondary to adult ADHD. We know from the MTA study that successful treatment of childhood ADHD also reduces childhood symptoms of comorbid disorders (56
). Indirect evidence suggests that stimulant treatment of childhood ADHD might reduce subsequent risk of substance use disorders (57
), although this is not definitive because of possible sample selection bias. Long-term prospective research using quasi-experimental methods is needed to resolve this uncertainty.
A related question is whether adult treatment of ADHD would have any effects on severity or persistence of comorbid disorders. A question could also be raised whether ADHD explains part of the adverse effects found in studies of comorbid DSM disorders. A number of studies, for example, have documented high societal costs of anxiety (58
), mood (60
), and substance (62
) disorder, but these all ignored the role of comorbid ADHD. Reanalysis might find that comorbid ADHD accounts for part, possibly a substantial part, of the effects previously attributed to these other disorders.