In DSM-II, hyperkinetic reaction of childhood was defined in terms of extreme levels of motor activity, impulsivity, and attention deficits. The definition of DSM-III attention deficit disorder (ADD) deemphasized hyperactivity and allowed the diagnosis of subtypes with either maladaptive levels of inattention, impulsivity, and motor activity (ADD with hyperactivity) or attention deficits and impulsivity only (ADD without hyperactivity). DSM-III-R dropped these two subtypes, however, and defined a single category of ADHD much like DSM-II.

Many studies have indicated that the symptoms of ADHD are not unitary as assumed by the DSM-III-R definition of ADHD, but the three dimensions of DSM-III symptoms (inattention, impulsivity, and hyperactivity) have not been supported either (Lahey, Pelham, Schaughency, et al., 1988; Lahey, Carlson, Frick, 1997). Rather, two dimensions of symptoms underlie ADHD, one reflecting inattention and another comprising both hyperactivity and impulsivity (Lahey, Pelham, Schaughency, et al., 1988; Lahey, Carlson, Frick, 1997). Accordingly, DSM-IV distinguished three subtypes of youths who exhibit maladaptive levels of both dimensions (combined type), inattention only (inattentive type), and hyperactivity-impulsivity only (hyperactive-impulsive type) (Lahey, Applegate, McBurnett, et al., 1994). This two-dimensional structure of ADHD has since been supported by both confirmatory factor analyses (Burns, Walsh, Owen, et al., 1997; Burns, Walsh, Patterson, et al., 1997; DuPaul, Anastopoulos, McGoey, et al., 1997; DuPaul, Anastopoulos, Power, et al., 1998; Pillow, Pelham, Hoza, et al., in press) and discriminant validity studies.

Inattention and hyperactivity-impulsivity differ in their correlations with types of functional impairment (inattention is associated with academic deficits and peer unpopularity, whereas hyperactivity-impulsivity is associated with peer rejection and accidental injuries) (Lahey, Applegate, McBurnett, et al., 1994; Lahey, McBurnett, Applegate, et al., unpublished), and hyperactivity-impulsivity is more strongly associated with conduct problems than is inattention (Lahey, Carlson, Frick, 1997; Lahey, Applegate, McBurnett, et al., 1994; Lahey, McBurnett, Applegate, et al., unpublished). In addition, inattention and hyperactivity-impulsivity follow different developmental courses, with inattention declining less than hyperactivity-impulsivity from childhood through adolescence (DuPaul, Anastopoulos, McGoey, et al., 1997; DuPaul, Anastopoulos, Power, et al., 1998; Hart, Lahey, Loeber, et al., 1995).

Correspondence of DSM-IV ADHD to DSM-III, DSM-III-R ADHD, and ICD-10

When only changes in symptom criteria are considered, DSM-IV ADHD is somewhat more prevalent than DSM-III-R ADHD (Lahey, Applegate, McBurnett, et al., 1994; McBurnett, Pfiffner, Wilcutt, et al., unpublished), but revisions to the age of onset criterion and the new requirement of impairment in two or more settings in DSM-IV reduce the prevalence of ADHD. These revised criteria particularly affect the prevalence of the inattentive and hyperactive-impulsive subtypes because youths who meet criteria for these subtypes tend to be impaired only at home or at school, and the inattentive type tends to have a later onset (Lahey, Applegate, McBurnett, et al., 1994; Lahey, McBurnett, Applegate, et al., unpublished).

As a result, when full diagnostic criteria are used, the prevalence of DSM-IV ADHD is approximately the same as, or lower than, that of DSM-III-R ADHD, with a substantial degree of overlap (Lahey, McBurnett, Applegate, et al., unpublished; Biederman, Faraone, Weber, et al., 1997). However, a higher proportion of girls and children younger than 7 years of age are among those youths who meet criteria for DSM-IV ADHD but do not meet DSM-III-R criteria (Lahey, McBurnett, Applegate, et al., unpublished). When compared with DSM-III ADD, full DSM-IV diagnostic criteria identify essentially the same number of cases, with substantial correspondence among the subtypes of the two definitions (Lahey, McBurnett, Applegate, et al., unpublished). ICD-10 hyperkinesis uses the same list of symptoms as DSM-IV ADHD but identifies only the equivalent of the combined type.

In addition, unlike DSM-IV, ICD-10 requires that full diagnostic criteria be met independently according to both parent and teacher informants. As a result, the ICD-10 definition identifies half the number of children and adolescents as the DSM-IV definition and appears to under-identify impaired youths (Lahey, McBurnett, Applegate, et al., unpublished).

Validity of ADHD
Face Validity. Some view ADHD as a disorder with high face validity (Goldman, Genel, Bezman, et al., 1998), whereas others conceptualize ADHD as a valid syndrome of maladaptive behavior that warrants treatment but object to its being considered a mental disorder (British Psychological Society, 1996). Some researchers find the definition of ADHD to lack specificity (Prior, Sanson, 1986), and a few groups believe that ADHD simply describes the exuberant behavior of normal children and view efforts to treat ADHD as inappropriate “mind control” (Safer, Krager, 1992). For this reason, other forms of validity are of greater importance to an evaluation of ADHD.

Reliability, Concurrent Validity of DSM-IV ADHD, and Discriminant Validity of the Subtypes. The reliability of assessments of ADHD is quite high using both structured diagnostic interviews (Biederman, Faraone, Keenan, et al., 1992; Orvaschel, 1995; Schwab-Stone, Shaffer, Dulcan, et al., 1996; Shaffer, Fisher, Dulcan, et al., 1996) and parent and teacher rating scales (Conners, 1973; Quay, Peterson, 1983), and many, but not all, studies using mechanical measures have found that clinic-referred children who meet criteria for ADHD exhibit significantly higher levels of motor activity and less visual attending than comparison children (Paternite, Loney, Roberts, 1996; Porrino, Rapoport, Behar, et al., 1983; Teicher, Ito, Glod, et al., 1996). The concurrent validity of DSM-IV ADHD and the discriminant validity of its subtypes have been addressed in a number of ways (Goldman, Genel, Bezman, et al., 1998).

The three subtypes have different gender ratios, with the combined type having a higher male-to-female ratio than the inattentive type (Lahey, Applegate, McBurnett, et al., 1994; Lahey, McBurnett, Applegate, et al., unpublished). Controlling for demographic differences, the subtypes also differ on the number of concurrent conduct problems (the inattentive type exhibits the fewest, and the combined exhibits the most) and symptoms of depression (the combined and inattentive types display more) (Lahey, McBurnett, Applegate, et al., unpublished).

Two studies show that when differences in age, gender, intelligence, socioeconomic status, ethnicity, and concurrent psychopathology are controlled, the combined and hyperactive-impulsive types are rated as more globally impaired by parents and interviewers (Lahey, McBurnett, Applegate, et al., unpublished) and are more likely to have had unintentional injuries than control youths without ADHD (Lahey, McBurnett, Applegate, et al., unpublished; Lahey, Pelham, Stein, et al., in press).

The combined type has more homework problems (Lahey, McBurnett, Applegate, et al., unpublished), and the combined and inattentive types show lower academic achievement relative to intelligence than controls (Lahey, Pelham, Stein, et al., in press).

All three subtypes show greater deficits in peer social relations and are more likely to have used special education services than controls (Lahey, Pelham, Stein, et al., in press).

A third study of 6- to 12-year-old boys provides similar support for the validity of DSM-IV ADHD, but fewer confounds were controlled (Paternite, Loney, Roberts, 1996).

Thus, there is substantial support for the validity of ADHD and its subtypes. On the other hand, several studies suggest that both the DSM-IV age of onset criterion and the DSM-IV requirement of cross-situational impairment reduce the accurate identification of impaired cases (Applegate, Lahey, Hart, et al., 1997; Barkley, Biederman, 1997). Thus, although the concurrent validity of the current DSM-IV definition is substantial, it may be possible to improve it by reconsidering these criteria in the future. No data have been published on potential differences among the DSM-IV subtypes of ADHD in response to treatment, but one study found differences in response to methylphenidate between youths who met criteria for DSM-III ADD with and without hyperactivity (Barkley, DuPaul, McMurray, 1991).

Predictive Validity of ADHD in Childhood. Numerous longitudinal studies support the validity of childhood ADHD by demonstrating adverse adult outcomes (Lilienfeld, Waldman, 1990), but the diagnostic criteria used in all such studies predated DSM-III-R. In addition, there is evidence that the most commonly cited adverse adolescent and adult outcomes of childhood ADHD are actually attributable to comorbid childhood conduct problems (Lilienfeld, Waldman, 1990; Lahey, McBurnett, Loeber, in press). There is growing evidence that adverse outcomes in academic achievement, occupational attainment, and driving violations are independently associated with childhood ADHD after controlling for childhood conduct problems, but better controlled adult followups are needed. (Mannuzza, Klein, Bessler, et al., 1993; McGee, Partridge, Williams, et al., 1991; Nada-Raja, Langley, McGee, et al., 1997; Taylor, Chadwick, Heptinstall, et al., 1996).

ADHD in Adulthood

Children with ADHD are increasingly less likely to meet diagnostic criteria for ADHD as they grow older, but some children continue to meet criteria for ADHD and to be impaired into adulthood (Hill, Schoener, 1996). Thus, there is little doubt that ADHD is a valid diagnosis in adulthood for some individuals. A number of issues create concern about the use of this diagnosis with adults, however. First, there are concerns that adults without ADHD who are impaired because of other mental disorders seek out the diagnosis because they find it less stigmatizing than other diagnoses (Shaffer, 1994). If so, the suddenly popular term “adult ADHD” may cause many individuals not to receive optimal treatment for other mental disorders. Second, it is not clear that the retrospective assessment during adulthood of childhood ADHD symptoms is valid or that the impairment experienced by many adults with ADHD is not better accounted for by other mental disorders. Finally, much remains to be learned about the response of adults to pharmacologic and other forms of treatment for ADHD.

Is ADHD Better Conceptualized in Diagnostic or Dimensional Terms?

Some have suggested that ADHD is more appropriately viewed as a dimension of maladaptive behavior than a taxonomic category (Fergusson, Horwood, 1995; Levy, Hay, McStephen, et al., 1997). At present, there is strong evidence that two continuous dimensions of impairing ADHD behaviors can be identified, but there is no evidence of a natural threshold between ADHD and “normal” behavior. The distributions of numbers of inattention and hyperactivity-impulsivity symptoms in the general population are not bimodal, associations between numbers of ADHD symptoms and impairment are linear rather than curvilinear (Lahey, Applegate, McBurnett, et al., 1994), and a twin study of the heritability of ADHD found no evidence of a natural diagnostic threshold based on differential heritability (Levy, Hay, McStephen, et al., 1997).

This does not imply that ADHD cannot be treated as a diagnostic category, however. Even if ADHD is not naturally dichotomous, many individuals with higher numbers of ADHD behaviors present for treatment. This means that clinicians must make dichotomous decisions to treat or not treat each individual. Because all forms of treatment involve some iatrogenic risk, it seems more appropriate to adopt a well-considered diagnostic threshold than to require each clinician to make this decision individually, even if the threshold is viewed as more conventional than natural. This state of affairs is not unique to ADHD, as similar questions can be raised about many mental disorders. However, much remains to be learned about the taxonomic status of ADHD and other mental disorders.

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