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ADHD Combined Type and ADHD Predominantly Inattentive Type Are Distinct and Unrelated Disorders Richard Milich, Amy C. Balentine, and Donald R. Lynam University of Kentucky We comprehensively reviewed research assessing differences in attention-deficit hyperactivity disorder (ADHD) subtypes to examine the possibility that ADHD/ combined type (ADHD/C) and ADHD/predominantly inattentive type (ADHD/I) are distinct and unrelated disorders. Differences among subtypes were examined along dimensions identified as being important in documenting the distinctiveness of two disorders. These include essential and associated features, demographics, measures of cognitive and neuropsychological functioning, family history, treatment response, and prognosis. Important differences among subtypes were found in several areas of study, supporting the conclusion that ADHD/C and ADHD/I may best be characterized as distinct disorders. We identify major limitations of the available research and present future directions for research. Key words: attention defict hyperactivity disorder, ADHD combined, ADHD inattentive, ADHD subtypes, ADHD classification. [Clin Psychol Sci Prac 8:463–488, 2001] The history of the classification of attention-deficit hyperactivity disorder (ADHD) is rife with controversies. Early concerns were raised over whether ADHD and conduct disorders (CD) were distinct disorders or whether the classification of these disorders could be handled more parsimoniously by combining them into a broader, externalizing category. Although this controversy has been resolved in favor of distinct externalizing categories (Hinshaw, 1987), other diagnostic debates have come to the Address correspondence to Richard Milich, Ph.D., Department of Psychology, 115 Kastle Hall, Lexington, KY 40506-0044. E-mail: [email protected]. 2001 AMERICAN PSYCHOLOGICAL ASSOCIATION D12 fore. For example, the changing DSM classification schemas have emphasized different primary symptoms underlying the disorder, starting with hyperactivity in DSM-II, moving to attention problems in DSM-III, and currently emphasizing both attention problems and hyperactive/ impulsive behaviors in DSM-IV. The identification of the primary symptom of ADHD is a debate that continues to the present, with some proposing deficient inhibitory processes (e.g., Barkley, 1998) and others adhering to the earlier emphasis on attention problems as the core difficulty (e.g., Brown, 1999). Although these classification controversies have come and gone as the zeitgeist and research findings have changed, one diagnostic debate has persisted for the past 20 years. This concerns the validity of the inattentive subtype. Before the introduction of this disorder with the publication of DSM-III in 1980, the literature had not raised the possibility that children may have significant attentional problems without the concomitant overactivity and impulsivity. Since 1980, when the inattentive subtype was introduced, a large number of studies have appeared examining the validity of this subtype of ADHD, attempting to identify commonalities and differences between the two major subtypes of ADHD: those with concomitant hyperactivity and impulsivity and those with attention problems alone. As the literature addressing this issue grew, a number of reviews appeared arguing for the validity of the inattentive subtype (Carlson, 1986; Carlson & Mann, 2000; Carlson, Shin, & Booth, 1999; Goodyear & Hynd, 1992; Lahey, Carlson, & Frick, 1997; Lahey & Willcutt, 2000; Wheeler & Carlson, 1994). This conclusion appears to hold regardless of whether the review involves studies of the DSM-III subtypes, attention deficit disorder with hyperactivity (ADD/H) and attention deficit disorder without hyperactivity (ADD/WO) (Edelbrock, Costello, & Kessler, 1984; 463 Lahey et al., 1997) or studies of DSM-IV subtypes, ADHD/C (combined type) and ADHD/I (predominantly inattentive type) (Gaub & Carlson, 1997; Morgan, Hynd, Riccio, & Hall, 1996). Despite all the current research on the validity of the inattentive subtype, few investigators seem to be addressing the most important, albeit controversial, issue concerning this disorder: Is the inattentive subtype actually a subtype of ADHD or a completely distinct diagnostic disorder? This is the question we address in this review. The answer to this question has important implications for future investigations in this area, especially in regard to the inclusion and exclusion criteria for studies of ADHD. If the inattentive subtype is not a subtype of ADHD, as some investigators have suggested (see, e.g., Barkley, DuPaul, & McMurray, 1990), then studies that indiscriminately combine these samples are misguided at best, and the conclusions that are drawn from these studies may be seriously flawed. The answer to this question can also have important implications for our understanding of ADHD, especially in terms of what constitutes the core features of the disorder. Finally, if it is the case that the inattentive subtype is a separate disorder and not part of the ADHD family, then studies focusing on the similarities of and differences between the inattentive and combined subtypes are unnecessarily narrow in scope, and attention could better be directed toward identifying the nature of the inattentive disorder. To address the question of whether the combined and inattentive subtypes are distinct and unrelated disorders, we first review the history of ADHD, including the changes in theory and diagnostic criteria. Next, we review the literature examining the correspondence of the different DSM diagnostic categories to examine whether and how to collapse across studies using different diagnostic criteria. Third, we discuss several taxonomic perspectives for determining whether two disorders belong to common or distinct diagnostic categories and use these taxonomic perspectives as guidelines to review the available literature on ADHD/C and ADHD/I. Finally, we identify significant gaps in the literature that preclude making definitive conclusions concerning this issue, and we make recommendations for future research. T H E H I S T O RY O F A D H D Attention-deficit hyperactivity disorder has been reconceptualized several times since it first appeared as a diagnostic category in the DSM-II (American Psychiatric CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • Association [APA], 1968), at which time it was classified as hyperkinetic reaction of childhood. The term indicates that the disorder was primarily conceptualized as having a core dysfunction of excess motor activity. However, with the publication of DSM-III (APA, 1980) came a new diagnostic label, attention deficit disorder (ADD), and a new conceptualization of the disorder as one of a dysfunction in attentional processes. Of greater historical significance, for the first time the disorder was subdivided into subtypes to distinguish individuals with hyperactivity, labeled as having attention deficit disorder with hyperactivity, from those without hyperactivity, labeled as having attention deficit disorder without hyperactivity. The absence of hyperactivity in the latter category further demonstrated that the role of hyperactivity in ADD had shifted from being a core component to being a nonessential, concomitant symptom. It is interesting to note that little research existed at the time DSM-III was published to support the validity of the ADD/WO diagnostic category. In fact, one of the primary reasons this subtype was included in the DSM-III appeared to be to stimulate investigations to determine whether this subtype was a valid diagnostic disorder. The clinical literature had suggested that children with ADD/ WO were being seen in clinics, and the originators of this category hoped that formalizing the disorder in the DSM would encourage researchers to undertake the necessary validation studies. Such validity studies began appearing almost immediately thereafter (e.g., Maurer & Stewart, 1980), and an early review of this literature (Carlson, 1986) suggested that ADD/WO was a valid behavioral category. Thus, it was surprising when the DSM-III-R was released in 1987 with ADD once again reconceptualized as a unidimensional category (APA, 1987) and given the term attentiondeficit hyperactivity disorder (ADHD). DSM-III-R did not provide for the subdivision of ADHD into subtypes to distinguish between individuals with or without hyperactivity, and it did not even require that a significant number of attention symptoms be present. A diagnosis of ADHD was made if an individual met any 8 or more of the possible 14 symptoms of hyperactivity, impulsivity, and inattention. A separate category, undifferentiated attention-deficit disorder (UADD), was also included in the DSM-III-R. However, no explicit diagnostic criteria were given for it, and the category was listed in a separate section of the manual. The result was that the DSM was perceived to have abandoned the inattentive subtype. V8 N4, WINTER 2001 464 While the UADD category provided a way to diagnose individuals who displayed no hyperactivity or impulsivity, it was also possible to meet criteria for ADHD without endorsing any symptoms of hyperactivity. Thus, the unidimensional approach to ADHD resulted in a diagnosed population that was more heterogeneous than before (Lahey et al., 1997). After publication of the DSM-III-R, evidence supporting the validity of a multidimensional approach to ADHD began accumulating. Several factor analytic studies found that symptoms of ADHD could be grouped into two factors: inattention and hyperactivity-impulsivity (Lahey et al., 1997). As a result of these and other studies, ADHD began to be reconceptualized once again, this time as a disorder having two distinct areas of dysfunction. Thus, the criteria for ADHD presented in the more researchdriven DSM-IV (APA, 1994) reflect this multidimensional conceptualization of the disorder. ADHD is now subdivided into three categories, reflecting the possible combinations of the two dimensions: ADHD/predominantly hyperactive-impulsive type (ADHD/HI), which is marked by maladaptive symptoms of hyperactivityimpulsivity; ADHD/predominantly inattentive type (ADHD/I), which is marked by maladaptive symptoms of inattention; and ADHD/C, which is marked by maladaptive symptoms of hyperactivity, impulsivity, and inattention. DSM-IV presents ADHD in a manner similar to that of DSM-III, returning to a subtyping system of classification. Conceptually, ADHD/C corresponds well to the DSMIII diagnosis ADD/H, as both are distinguished by symptoms of inattention and hyperactivity/impulsivity. ADHD/I corresponds well to the DSM-III diagnosis of ADD/WO, as both are distinguished by symptoms of inattention and a lack of hyperactivity. ADHD/HI shows the least correspondence to a prior ADHD diagnostic category and has the least empirical support at this time (Lahey et al., 1997). In fact, similar to the motivation behind the original identification of the ADD/WO subtype, the identification of the ADHD/HI subtype seems partially driven by the desire to encourage research examining the validity of this subtype.1 C O R R E S P O N D E N C E A M O N G A D H D C AT E G O R I E S I N DSM-III, DSM-III-R, AND DSM-IV To properly review the literature on the ADHD subtypes, it is necessary to determine which studies should be included in the review. Some articles have examined DISTINCT ADHD DISORDERS • MILICH ET AL. ADHD as it is classified in the DSM-III, while others have examined the DSM-III-R or DSM-IV version of ADHD. Several studies have explored the correspondence among the different DSM versions of ADHD. These results have helped researchers to determine if findings from studies examining one particular DSM version of ADHD can be assumed to generalize to other DSM versions of ADHD. A study by Biederman et al. (1997) examined the correspondence between DSM-III-R and DSM-IV ADHD. Out of 405 consecutive referrals to a pediatric psychopharmacology program, 302 were given a DSM-III-R diagnosis and 302 were given a DSM-IV diagnosis. Ninety-three percent of children with a DSM-III-R ADHD diagnosis also received a DSM-IV ADHD diagnosis. Of these children, 64% received a DSM-IV diagnosis of ADHD/C, 25% received a DSM-IV diagnosis of ADHD/I, and 11% received a DSM-IV diagnosis of ADHD/HI. Of the 21 children who met criteria for only DSM-IV ADHD, 90% were the inattentive type, 5% were the hyperactive-impulsive type, and 5% were the combined type. A study by Morgan et al. (1996) examined 62 children who had a previous DSM-III ADD diagnosis or a DSMIII-R ADHD diagnosis. The children were given a retrospective DSM-IV ADHD diagnosis; 30 were diagnosed as predominantly inattentive type and 26 were diagnosed as combined type. It was found that children with a DSMIII ADD/WO diagnosis received the predominantly inattentive type diagnosis more often than they received the combined type diagnosis. Conversely, the children with a DSM-III ADD/H diagnosis received the combined type diagnosis more often than they received the predominantly inattentive type diagnosis. No differences in receiving DSM-IV diagnoses were found for children with a DSM-III-R ADHD diagnosis, although more did receive a combined type diagnosis than a predominantly inattentive type diagnosis. In a third study by Baumgaertel, Woraich, and Dietrich (1995), teachers rated 1077 children in a school sample on DSM-III, DSM-III-R, and DSM-IV criteria for ADHD. The prevalence rates for the various ADHD diagnoses were examined and the congruence among these disorders compared. They found that 95% of children meeting criteria for DSM-III and 95% meeting criteria for DSMIII-R also met criteria for a DSM-IV ADHD diagnosis. One-hundred percent of children with ADHD/C, 70% with ADHD/HI, and 30% with ADHD/I met criteria for DSM-III-R ADHD. Ninety percent of children with 465 ADHD/C also met criteria for DSM-III ADD/H diagnosis. Only 32% of children with ADHD/I and 30% of children with ADHD/HI met DSM-III ADD criteria. Finally, in a DSM-IV field trial study, structured interviews were given to multiple informants to assess the presence of DSM-III, DSM-III-R, and DSM-IV ADHD in 380 clinic-referred children (Lahey et al., 1994). Ninetyseven percent of children meeting DSM-III ADHD criteria also met DSM-IV criteria. Of children who met criteria for DSM-III ADD/H, 88% also met criteria for DSM-IV ADHD/C or ADHD/HI. Of children who met criteria for DSM-III ADD/WO, 91.7% met DSM-IV ADHD/I criteria. Ninety-seven percent of children who met criteria for DSM-III-R ADHD also met criteria for one of the DSM-IV ADHD subtypes. For children meeting criteria for a DSM-IV subtypes, 100% of the combined type, 74% of the predominantly inattentive type, and 61% of the predominantly hyperactive-impulsive type also met DSM-III-R ADHD criteria. The DSM-IV ADHD criteria resulted in a net 15% increase in number of ADHD cases over the DSM-III-R criteria. The new cases of DSM-IV ADHD were more than twice as likely to be female, with most of these cases being in the predominantly inattentive type. These four studies yield consistent findings regarding the correspondence of the DSM-III, DSM-III-R, and DSM-IV ADHD diagnoses. There is a high correspondence among them, with 93–97% of DSM-III-R cases meeting DSM-IV criteria, and 95–97% of DSM-III cases meeting DSM-IV criteria. The DSM-III subtypes ADD/ H and ADD/WO correspond well to the DSM-IV subtypes ADHD/C and ADHD/I, respectively. The DSMIII-R ADHD diagnosis yields the most heterogeneous diagnostic group. The DSM-IV ADHD/C subtype corresponds more closely to ADHD than do the ADHD/I and ADHD/HI subtypes. However, 30–74% of ADHD/I cases meet criteria for DSM-III-R ADHD as do 61–70% of ADHD/HI cases. From these findings it can be concluded that results from studies based on DSM-III ADHD subtypes can be generalized with a good degree of accuracy to ADHD/C and ADHD/I subtypes. Results from studies based on DSM-III-R ADHD should only be generalized to ADHD/C and ADHD/I subtypes with a great deal of caution. The majority of studies we reviewed examined DSM-III and DSM-IV subtypes. Before proceeding to these studies to assess the validity of ADHD/ C and ADHD/I as distinct disorders, we discuss the most CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • appropriate approach to use in establishing the validity of a mental disorder. T H E C L A S S I F I C AT I O N O F PAT H O L O G Y There is debate over the most appropriate approach to use in establishing the validity of a psychiatric disorder. The most dominant and long-standing approach to classification is the neo-Kraepelinian approach (Widiger & Clark, 2000). This approach is based on the classification system originated by Emil Kraepelin, who believed that mental disorders represented disease states that could be identified through the careful grouping of patients with disorders of similar course and appearance. While he believed in the organic etiology of most mental disorders, he asserted that careful observation of patient behaviors was initially the most fruitful step toward classification. During the past few decades, this approach has been supported and developed by those within the neo-Kraeplinian movement who believe in the need to classify disorders and develop explicit diagnostic criteria for them. They view psychiatry as a branch of medicine whose task is to investigate the causes, diagnoses, and treatment of discrete mental disorders (Blashfield, 1984). This movement became more influential with the publication of a journal article titled “Diagnostic Criteria for Use in Psychiatric Research” (Feighner et al., 1972). This article set forth explicit diagnostic criteria for 16 diagnostic categories and led to the formulation of explicit diagnostic criteria in the DSM-III (APA, 1980). The authors also described criteria for establishing the validity of a psychiatric illness that are divided into five phases. In line with its Kraepelinian origins, the first phase involves obtaining a clinical description of the disorder. This includes not only describing associated symptoms, but also the distribution across race, sex, age at onset, and precipitating factors, among others. The second phase includes conducting reliable laboratory studies to help develop a more precise clinical description. Laboratory studies include the use of certain psychological tests as well as chemical, physiological, radiological, and anatomical examinations. In the third phase, exclusionary criteria are developed so that boundaries can be drawn between the disorder under investigation and other disorders that bear a resemblance to it. The fourth phase is to carry out follow-up studies of patients. This step helps illuminate whether the patients being examined form a homogenous group that is experiencing the same illness. Differential V8 N4, WINTER 2001 466 outcomes for patients suggest that they may not all have the same disorder. Although a given disorder may not have a definitive outcome or prognosis, differential outcomes at least call into question the validity of the diagnosis under study. The final phase involves investigating the family history of patients with the given disorder. The validity of the diagnosis is strengthened if an increased prevalence of the disorder is found among close relatives of the patients (Feighner et al., 1972). These five phases were expanded by Cantwell and Baker (1988; see also Cantwell, 1995) into a six-stage model to specifically assess the validity of childhood psychiatric disorders. Their stages include (a) studies of essential and associated features of the disorder and the exclusionary criteria, (b) physical and neurological studies, (c) laboratory studies, (d) family psychopathology studies and family interaction studies, (e) follow-up studies, and (f) treatment studies. The traditional psychiatric approach as described above is, of course, not the only way to classify mental disorders. Quantitative approaches have become increasingly popular as more sophisticated statistical techniques have been developed and as advanced computer technology has made the routine use of these techniques feasible. This approach involves the analysis of large data sets with advanced statistical tools such as factor analysis and cluster analysis to develop psychiatric groupings and symptom clusters. The research assessing the validity of many childhood disorders, including ADHD subtypes, has been based on the traditional psychiatric model of classification. The majority of studies examining the ADHD subtypes have focused on clinical descriptions and differentiation from other disorders, laboratory findings, and family history. As a result, in this review we mainly focus on this body of literature and examine ADHD subtypes from the perspective of the traditional psychiatric approach. However, attention is also given to the small number of studies that have taken a quantitative perspective and employed factor analytic and cluster analytic techniques in an effort to identify and categorize the ADHD subtypes. The statistical and categorical approaches can be viewed as offering different and complementary perspectives on the question of whether ADHD/I and ADHD/C are subtypes of the same disorder. The statistical approach is appropriate for determining whether meaningful subtypes exist, but this approach does not offer specific guidelines for ascertaining DISTINCT ADHD DISORDERS • MILICH ET AL. whether the subtypes are distinct or represent a common underlying disorder. Thus, empirically derived factors or clusters do not necessarily answer the question of interest. The psychiatric approach, as noted earlier (see Cantwell & Baker, 1988; Feighner et al., 1972) has offered specific criteria for determining whether distinct disorders exist. Thus, first we review the few empirically derived classification studies to look for the existence of separate factors and clusters, and then we review the categorical studies in an attempt to determine the distinctiveness of the subtypes. FA C T O R A N A LY T I C S T U D I E S ADHD has been viewed at times as a multidimensional disorder (e.g., DSM-III, DSM-IV), and at other times as a unidimensional disorder (e.g., DSM-III-R). Several factor analytic studies of DSM-III, DSM-III-R, and DSM-IV ADHD criteria have been carried out to help address this issue. Lahey et al. (1997) reviewed the initial studies in this area (see also Lahey & Willcut, 2000), and subsequent studies (e.g., Baumgaertel et al., 1995; DuPaul et al., 1997) have come to similar conclusions. Because the findings are generally consistent across all of the published studies, we will not systematically summarize each study. Instead, we review the Lahey et al. (1988) study in detail as a representative example and then mention additional studies that may have obtained different findings. Lahey et al. (1988) examined two separate samples, the first of which was composed of 677 nonreferred, schoolaged children and the second of which was composed of 663 school-aged children who were referred to a clinic. For children in both samples, teachers completed a checklist composed of DSM-III symptoms for ADD. For the clinic-referred sample, clinicians rated children on an experimental scale consisting of ADD/H and DSM-III-R ADHD items as well as five items frequently associated with ADD/WO (e.g., sluggish, forgetful). The teacher ratings for both samples and the clinician ratings for the clinic sample were separately subjected to principal components analysis with varimax rotation. The analyses of teacher ratings for both samples yielded nearly identical two-factor solutions: one factor was composed of items assessing inattention-disorganization, whereas the other factor was composed of items assessing hyperactivity-impulsivity. The inattention-disorganization factor consisted of items relating to concentration problems, distractibility, difficulty finishing tasks, and organizational difficulties. Three 467 of the purported impulsivity items from DSM-III actually loaded on this factor. The hyperactivity-impulsivity factor consisted of the remaining three impulsivity items from DSM-III (e.g., acts before thinking), as well as items reflecting motor overactivity. In addition, a three-factor solution resulted from the clinician ratings: a hyperactivity-impulsivity factor, an inattention-disorganization factor, and a sluggish tempo factor. A review of the other studies that report factor analyses of DSM-III, DSM-III-R, and DSM-IV ADHD criteria consistently yield a two-factor solution that differentiates symptoms of attention from symptoms of hyperactivityimpulsivity (Lahey et al., 1997). The primary exceptions to this conclusion arise from studies that added other internalizing or externalizing disorder symptoms to the analyses (e.g., Pelham, Gnagy, Greenslade, & Milich, 1992; Wolraich, Hanah, Pinnock, Baumgaertel, & Brown, 1996). In such cases, additional factors (e.g., oppositional defiant disorder [ODD], anxiety) are usually produced that reflect these additional symptoms. When additional symptoms of inattention are added (e.g., sluggishness, forgetfulness) that supposedly reflect the problems of the inattentive subgroup, inconsistent findings are obtained. In Lahey et al.’s (1988) study, a separate sluggish tempo factor was extracted in addition to an inattentiondisorganization factor for the clinician ratings but not for the teacher ratings. However, when Bauermeister, Alegra, Bird, Rubio-Stipec, and Canino (1992) performed a similar procedure, they failed to obtain a separate sluggish tempo factor. Instead, the associated symptoms of ADHD/I loaded onto the inattention factor. Despite these occasional inconsistencies, the results are quite reliable in documenting that, when the DSM ADHD criteria are factor analyzed, two distinct factors emerge reflecting inattention problems and hyperactivity/ impulsivity. It was from data such as these that the DSMIV proposed a two-factor conceptualization of ADHD (Lahey et al., 1997). However, as noted earlier, such factor analytic evidence is necessary but not sufficient in determining whether two distinct syndromes of ADHD exist. That is, distinct factors are implied by the conception of these problems as subtypes of the same disorder (necessity), but such results are also consistent with the conception of these problems as distinct disorders. C L U S T E R A N A LY T I C S T U D I E S Some authors (e.g., Lahey et al., 1997) have argued that the question of whether ADHD is better conceptualized CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • as two distinct syndromes is more adequately addressed through the use of cluster analytic techniques rather than through factor analysis. The former approaches group individuals based on their similar symptom picture, whereas the latter approach groups symptoms according to their coherence. Unfortunately, there are only two published studies that have used cluster analysis to better understand the nature of ADHD. In the study by Lahey et al. (1988) described above, three factors derived from clinician’s ratings (inattention-disorganization, hyperactivity-impulsivity, and sluggish tempo) were subjected to cluster analysis. Three clusters emerged: the first cluster was low on all three factors, the second cluster was high on inattention-disorganization and sluggish tempo and low on hyperactivity-impulsivity, and the third cluster was high on inattention-disorganization and hyperactivityimpulsivity and low on sluggish tempo. Seventy-five percent of children diagnosed as ADD/H fell into the third cluster, and 95% of children diagnosed as ADD/WO fell in the second cluster. Of the remaining ADD/H children, 20% fell in the second cluster and 5% fell in the first cluster. Of the remaining 5% of ADD/WO children, all fell in the third cluster. These results indicate that children with ADD/H are best characterized by inattentiondisorganization and hyperactivity-impulsivity, whereas the children with ADD/WO are best characterized by inattention-disorganization and sluggish tempo. Bauermeister et al. (1992) subjected two dimensions of ADHD (inattention and hyperactivity-impulsivity) derived from factor analysis to cluster analysis. A five-cluster solution yielded three ADHD profiles and two profiles reflecting a lack of clinical symptoms. The first cluster was characterized by high hyperactivity-impulsivity and moderately high inattention (H), the second cluster was characterized by very high inattention and very low hyperactivity-impulsivity (I), and the third cluster was characterized by both high inattention and high hyperactivity-impulsivity (HI). Seventy-three percent of children in the H cluster and 60% of children in the IH cluster received an ADD/H diagnosis. Children in the I cluster were equally likely to receive ADD/H, ADD/ WO, or no diagnosis. Children in these three clusters were compared on several other variables. Children in the IH cluster were found to be more active and impulsive, have a higher rate of mother-reported pregnancy complications, and were rated by teachers as more unpopular, anxious, aggressive, and self-destructive than children in the I cluster. The I cluster children were rated by teachers V8 N4, WINTER 2001 468 as more socially withdrawn than the other children and as having more reading difficulties than IH children. The types of inattentive problems appeared to differ for I and IH children. I cluster children were rated higher on moving slowly, blank staring, and concentration problems but lower on confusion, getting tired too much, and lack of persistence. H cluster children were more likely to be male and from lower socioeconomic status than IH children. They were rated by teachers as less socially withdrawn than I and IH children. The results of these studies provide support for a distinction between children exhibiting problems with inattention and hyperactivity-impulsivity (ADHD/C) and those exhibiting predominantly problems with inattention (ADHD/I). Although it is not possible to draw definitive conclusions based on cluster-analytic work, the results do provide support for the distinctiveness of the syndromes of ADHD/C and ADHD/I. Again, however, these results can be interpreted as supporting the conceptualization of these two disorders being distinct and unrelated or supporting the belief that they are subtypes of the same disorders. To further explore this issue, it is necessary to examine the evidence from a categorical perspective, in which guidelines for identifying the distinctiveness of disorders have been offered. R E V I E W O F S T U D I E S C O M PA R I N G A D H D / C A N D ADHD/I SUBTYPES As noted earlier, there are no universally agreed-upon criteria for determining whether two disorders are distinct and unrelated or instead represent subtypes of a common disorder. However, Feighner et al. (1972) and Cantwell (1995; Cantwell & Baker, 1988) offered guidelines for determining the validity of a disorder, and these criteria have been used by framers of the DSM and others to identify distinct disorders. Extrapolating from these guidelines, the more of these criteria that differentiate the two disorders, the more likely they are to represent distinct, unrelated disorders. To facilitate understanding of this issue, we review the available studies comparing ADHD/C and ADHD/I samples according to criteria similar to those proposed by Cantwell and Baker (1988). Thus, in the review that follows, we have sections devoted to essential features; associated features; demographics and epidemiology; laboratory studies; family history and family psychopathology studies; treatment response; and prognosis and follow-up studies. In selecting studies appropriate for this review, care was DISTINCT ADHD DISORDERS • MILICH ET AL. taken to include only articles that identified ADHD groups using DSM criteria. Although several studies used strict DSM criteria, others included in this review used experimental criteria that were based on the DSM. Additionally, some studies using explicit DSM criteria adhered to only criteria for symptoms, ignoring criteria for age of onset and impairment in multiple domains. Studies of all of the above types were included in the review. In addition, some of the studies were carried out using a clinical population, while the remainder were carried out using a community or school population. Studies examining both types of samples are important for gaining a better understanding of ADHD. Specifically, studying community samples may provide more accurate estimates of demographic characteristics, such as prevalence, age of onset, and comorbidity, among others. Alternatively, clinic samples may provide more accurate information about expected treatment outcomes. Thus, although both types of samples are profitable to study, their findings may be discrepant with each another. This factor should be considered when evaluating their results. Essential Features There are three essential features in the diagnosis of ADHD: level of impairment on the inattention symptoms, level of impairment on the hyperactive/impulsive symptoms, and age of onset. We reviewed the evidence concerning subtype differences for these three essential features. It seems reasonable to expect that if ADHD/C and ADHD/I are simply subtypes of the same general disorder, we would find both groups to exhibit the same patterns of inattention problems, but the ADHD/C group would show symptoms of hyperactivity and impulsivity as well. A review of the literature, however, suggests that the reality is not so straightforward. Attention Problems. Although there are few studies specifically examining the nature of the attention problems for the two ADHD subtypes, the available studies suggest that the two subtypes may experience dramatically different types of attention problems. This possibility was first raised in studies examining the DSM-III subtypes, in which the performance of the ADD/H group was characterized by distractibility and impulsivity, whereas the behavior of the ADD/WO group was characterized by what has come to be called a “sluggish cognitive tempo” (SCT), consisting of behaviors such as drowsiness, leth- 469 argy, and hypoactivity (Carlson & Mann, in press). Although these latter items sound as though they have to do with activity level, they tend to load on the inattention factor in factor analytic studies (see Lahey et al., 1997). Achenbach has reported two similar inattention dimensions in his factor analyses of both his rating scales (Achenbach, 1996) and his Direct Observation Form (Achenbach, 1991; McConaughy, Achenbach, & Gent, 1988). Items relating to this sluggish cognitive tempo were included in the DSM-IV field trials for ADHD (Frick et al., 1994), but they were not included in the final symptom lists for ADHD, primarily because the DSM-IV work group wanted to keep the inattention symptoms identical for all of the subtypes (Lahey et al., 1994). This decision by the work group may have artificially attenuated differences between the combined and inattentive subtypes because it eliminated those attention symptoms that may have shown the clearest differentiation between the two groups. Following up on work begun by McBurnett and Pfiffner (1999), Carlson and Mann (in press) tested this possibility by subdividing a sample of diagnosed ADHD/I children into two groups depending on whether they showed elevated scores on the two SCT items of “daydreams or gets lost in his/her thoughts” and “underactive, slow moving, or lacks energy.” This dichotomization resulted in 28% of the inattentive group falling in the high SCT group and 72% in the low group. Comparing the two groups on teacher ratings revealed that the high SCT group was rated higher on internalizing problems and lower on externalizing problems than the low SCT group. On measures relating to internalizing problems, the ratings of the low SCT group were more similar to those of the ADHD/C group than they were to the high SCT group. Similar results have been reported previously in three studies comparing the inattentive and combined subtypes on teacher ratings of inattentive symptoms. Lahey, Schaughency, Frame, and Strauss (1985) examined a school sample of ADD/H, ADD/WO, and normal control children using teacher ratings on the Revised Behavior Problem Checklist (RBPC). Groups were compared on teacher responses to items on the Attention ProblemImmaturity (API) factor of the RBPC. Both ADD groups were rated as exhibiting greater attention problems than controls on 12 of the 16 API items. However, the two ADD groups differed from each other on seven of the items. ADD/H children were rated higher on the irre- CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • sponsibility, distractibility, impulsivity, sloppiness, and answering without thinking items. The ADD/WO children were rated higher on the sluggishness and slowness items. Lahey et al. (1985) suggested that the patterns of behavior for the two groups may differ so markedly that ADD/WO could be characterized as an internalizing disorder. In another study examining differences in cognitive tempo among ADHD groups, Lahey, Schaughency, Hynd, Carlson, and Niever (1987) compared ADD/H and ADD/WO children using teacher ratings. On the Conners Behavior Rating Scale, teachers rated ADD/ WO children as having a more sluggish tempo than ADD/H children. Finally, Barkley et al. (1990) also used teacher rating scales to examine differences in patterns of attention among ADD/H, ADD/WO, learning disabled, and normal control groups. On the Teacher Rating Form, ADD/WO children were rated higher than ADD/H children on items “lost in a fog,” “daydreaming or getting lost in thought,” and “apathetic or unmotivated.” Thus, consistent with the results of Carlson and Mann (in press), the results of these three studies all suggest that items dealing with a sluggish cognitive tempo may well be the most sensitive items for differentiating the attention problems of the combined and inattentive groups. The results of these studies raise some interesting possibilities concerning the inattentive subtype. First, as Carlson and Mann (in press) note, this subtype as identified by the DSM-IV criteria appears to be a somewhat heterogeneous group. Thus, within this subtype there may be at least two different manifestations of the problem; a group high on items relating to the sluggish cognitive tempo and high on internalizing problems, and a group that may be more similar to the combined type but having fewer hyperactive/impulsive symptoms, resulting in these children incorrectly receiving an inattentive subtype diagnosis. Although drawing definitive conclusions from these few studies is premature, if our speculation is correct, there are several important implications for our understanding of the ADHD subtypes. First, it appears that the DSM-IV work group may have erred by excluding items that tap into the sluggish cognitive tempo. These items may be necessary to identify a pure inattentive group. Second, the diagnostic system needs to put a limit on how many hyperactive/impulsive symptoms a child can demonstrate and still receive an ADHD/I diagnosis. For V8 N4, WINTER 2001 470 example, a child who meets criteria for the inattention symptoms but evinces three to five H/I symptoms might be considered subthreshold for the combined type rather than receiving an ADHD/I diagnosis. As it stands now, a child can have five H/I symptoms and still receive an ADHD/I diagnosis. This obviously produces a heterogeneous group of children, some of whom may be considered purely inattentive and some of whom probably could be classified as subclinical combined type (see Carlson & Mann, in press). Thus, the studies examining the inattentive subtype are probably examining a heterogeneous group of children, depending on how carefully the investigators have tried to obtain pure inattentives and exclude subclinical combined children. This heterogeneity may explain why there is considerable inconsistency in this literature, as will become apparent later on in our review. A surprising omission in the literature examining the validity of the inattentive subtype is the lack of studies using laboratory or observational procedures to identify the nature of the attention problems for the different subtypes. In fact, we found only a few studies addressing this issue. Paternite, Loney, and Roberts (1996) failed to find differences between ADHD/C and ADHD/I groups on omission errors from a continuous performance test, a vigilance task that purportedly measures both sustained attention (omission errors) and impulsivity (commission errors). Similarly, Carlson, Lahey, and Neeper (1986) compared a sample of ADD/H, ADD/WO, and normal control children obtained from a school population. These groups were compared on a variety of cognitive measures designed to assess aspects of attention problems, including the Detroit Visual Attention Span for Objects and the Stroop Color Distraction test. The results revealed no differences between the two ADD groups on these measures, although both groups were slower than controls on time to complete the Stroop task. Finally, Barkley et al. (1990) compared groups of ADD/H, ADD/WO, learning disabled, and normal control children using the Continuous Performance Task. No significant differences were found between the ADD groups, although the combined group did exhibit more ADHD-consistent behaviors (e.g., off-task, fidgeting, out-of-seat) during the task. In addition, the ADD/H group made more omission and commission errors than learning disabled and control groups, while the ADD/WO group also made more commission errors than the learning disabled group. The dearth of laboratory or observational studies DISTINCT ADHD DISORDERS • MILICH ET AL. examining possible attention differences between the two subtypes is especially surprising given the many laboratory studies devoted to identifying the nature of the attention problems among ADHD children in general. Barkley (1997) and others have speculated that the combined and inattentive subtypes may exhibit different types of attention problems, with the former exhibiting more distractibility and the latter more passive-inattentive behavior. Although this is a plausible and even intriguing hypothesis, to date the only data supporting such a differentiation come from ratings made by parents and teachers. Such information does not offer the specificity necessary to directly test this hypothesis. Hyperactivity/Impulsivity. Similar to the area of attention problems, there is a long history of using laboratory and observational measures to examine the overactivity and impulsivity problems associated with ADHD (see Tannock, 1998). Given this history, it is therefore surprising to note how few studies have specifically examined these symptom constructs as they relate to the combined and inattentive subtypes of ADHD. Paternite et al. (1996) failed to find group differences on commission errors on the Continuous Performance Task, a purported measure of impulsivity. Paternite et al. (1996) also compared the validity of DSM-IV subtypes on the Kagan Matching Familiar Figure Test (MFFT). Although few group differences were found, the ADHD/C group displayed significantly shorter response latencies than the clinical control group. Conte, Kinsbourne, Swanson, Zirk, and Samuels (1986) compared performances of ADD/H, ADD/WO, and control groups on the MFFT, a purported measure of impulse control. Somewhat surprisingly, it was found that the ADD/WO group had shorter latencies than the ADD/H and control groups. Additionally, the ADD/WO group made more errors than the control group, suggesting that the ADD/WO group was the most impulsive of the groups examined. Carlson et al. (1986) found their ADD/WO group to be less accurate than controls on a visual matching task analogous to the MFFT. Barkley et al. (1990) also examined differences among ADD/H, ADD/WO, LD, and normal control groups using the Kagan MFFT. No differences were found among groups on this measure. Finally, Trommer, Hoeppner, Lorber, and Armstrong (1988) administered the go/no-go paradigm to ADD/H, ADD/WO, and normal control children. In this paradigm, children were 471 required to raise and lower their index finger in response to hearing a single tap (go signal) and refrain from responding to a double tap (no-go signal). All children demonstrated the ability to respond properly to both signals. ADD/WO children made more commission errors (which is suggestive of impulsivity) as well as more overall errors on the first trial than did ADD/H children and normal control children. However, ADD/WO children improved more on the second trial than did ADD/H children, who committed an equal number of commission errors on both trials. Taken together, these studies offer few reliable differences between the combined and inattentive subtypes on laboratory measures of impulsivity, with results from the MFFT being contradictory. However, the construct validity of the MFFT has been questioned, so it is not clear exactly what this test is measuring (Milich & Kramer, 1984). Perhaps the most carefully done and compelling study examining this issue is a recent investigation by Nigg, Blaskey, Huang, and Rappley (2000). The authors compared combined and inattentive subtypes along with a normal control group on a stop-signal task, a wellvalidated measure of behavioral inhibition (Oosterlaan, Logan, & Sergeant, 1998). Nigg et al. found a deficit in behavioral inhibition for the combined group, even after controlling for comorbid ODD/CD symptoms. No such deficit was found for the inattentive group. The Nigg et al. (2000) study appears to be the only investigation specifically documenting that the combined group may have problems in disinhibition that are not present in the inattentive group, despite the fact that such problems are the major defining feature in differentiating the two groups. Clearly, a great deal more research using well-validated laboratory measures is needed to document that the two groups do differ in this defining feature, as well as to elucidate the underlying factors that differentiate these two subtypes. As noted earlier, such specificity is not possible when group differences are limited to behavioral ratings made by parents and teachers. Age of Onset. Identifying the specific age of onset of ADHD can be difficult for several reasons (Campbell, 1990). First, very young children are often not placed in situations that require sustained attention, so that it is almost impossible to identify attention problems among preschoolers. Second, parental ratings of ADHD among CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • young children tend to be quite high, with up to half of the parents of preschoolers identifying their children as having problems in the areas of overactivity, distractibility, and impulsivity (Campbell, 1990). Nevertheless, it is widely agreed upon that the problems with ADHD should be evident once the child is placed in a structured school setting. For this reason, the DSM requires an onset of the disorder by age seven, or approximately by the time the child has finished first grade. Although only a few studies have examined this issue, evidence is beginning to appear that suggests the combined subtype has an earlier onset and age of referral than the inattentive subtype. Thus, the DSM-IV field trials for ADHD found that only 57% of youths meeting symptom criteria for ADHD/I also met the criterion for age of onset before seven years, whereas 82% of youths meeting symptom criteria for ADHD/C met the age of onset criterion (Applegate et al., 1997). Similarly, the field trials found the ADHD/C group on average to be more than one year younger than the inattentive type at age of referral (Lahey et al., 1994). Similarly, Paternite et al. (1996) reported age of onset for ADHD/C children as 3.22 and for ADHD/I children as 3.61; age at referral was reported as 8.25 for ADHD/C children and 9.04 for ADHD/I children. Finally, Faraone, Biederman, Weber, and Russell (1998) found that the combined group had significantly earlier age of onset (2.9 vs. 4.0 years) and referral age (6.4 vs. 9.2) than did the inattentive group. In contrast to these significant differences, Barkley et al. (1990) failed to find a difference in age at time of study between their ADD/H group (8.3 years) and their ADD/WO group (9.0 years). Further, one community-sample study found no differences in age of onset (Gaub & Carlson, 1997). Thus, although only a few studies have examined subtype differences in age of onset, the evidence suggests that the combined type is identified and referred at an earlier age. It is possible that the earlier onset ages for the combined type represent a referral bias, in that the problems of the combined type may be more evident or at least more disturbing at an earlier age (Gaub & Carlson, 1997). For this and related reasons, Barkley and Biederman (1997) have critiqued the DSM-IV criterion of requiring an age of onset before age 7. However, the field trial data make it clear that this criterion works fine for the combined type and hyperactive/impulsive types; it is only the inattentive subtype that fails to meet this criterion at a significantly high rate. V8 N4, WINTER 2001 472 Associated Features In addition to the core or essential features identified with ADHD, this disorder is noted to co-occur with a number of associated problems. In fact, a number of these associated features (e.g., conduct disorders, learning disabilities, social relations problems) show such strong relations with ADHD that authors have speculated whether these associated features should be considered part of the disorder itself (see Greene et al., 1996; Jensen, Martin, & Cantwell, 1997; Lynam, 1996). In the following section of the review we examine associated features of ADHD in three broad categories: comorbid disorders, social problems, and academic difficulties. Comorbidity. Different patterns of comorbidity among ADHD/C and ADHD/I subtypes have been indicated in the literature. In terms of externalizing problems, the literature is quite consistent in documenting that the ADHD/C group is associated with higher rates of conduct disorder and oppositional defiant disorder compared with the ADHD/I group (see Lahey & Willcutt, 2000). Several studies have found that children with ADHD/C are generally rated higher than children with ADHD/I by parents and teachers on scales measuring aggression, delinquent behavior, and conduct problems (Barkley et al., 1990; Eiraldi et al., 1997; Gaub & Carlson, 1997; King & Young, 1982; Lahey et al., 1987; Maedgen & Carlson, 2000; Morgan et al., 1996; Nigg et al., 2000; Paternite et al., 1996). Similarly, many studies have found higher rates of CD and ODD among ADD/H children compared with ADD/WO children (Barkley et al., 1990, Eiraldi, Power, & Nezu, 1997; Faraone et al., 1998; Hynd et al., 1991; Morgan et al., 1996; Wolraich et al., 1996). Thus, for example, Faraone et al. found that the combined group showed significantly higher rates of comorbid conduct disorder (24.3%) and oppositional defiant disorder (68.1%) than did the inattentive group (8.7% and 44.0%, respectively). Although the evidence concerning group differences in comorbid externalizing disorders is quite consistent, no clear distinction in patterns of comorbidity of internalizing problems between groups has emerged from the literature. In fact, elevated rates of internalizing disorders often have been found in both ADHD/I and ADHD/C groups (Eiraldi et al., 1997). Although Eiraldi et al. (1997) found that parent and teacher ratings of internalizing problems were higher for ADHD/C children than for DISTINCT ADHD DISORDERS • MILICH ET AL. ADHD/I children, no difference between groups was found in rates of co-diagnoses of mood and anxiety disorders. Faraone et al. (1998), Morgan et al. (1996), and Wolraich et al. (1996) also found that children with ADHD/ C and ADHD/I did not differ in rates of anxiety and depression disorders. In contrast, there have been at least three studies that have found differences between ADHD/C and ADHD/I in rates of comorbid internalizing disorders. Lahey et al. (1987) found that children with ADD/WO (43%) were more likely than children with ADD/H (10%) to have a comorbid internalizing disorder. Similarly, Barkley et al. (1990) found that the ADD/WO group was more likely to receive a diagnosis of major depressive disorder than the ADD/H group, whereas the latter had a higher rate of separation anxiety disorder than the former. However, it should be noted that in all cases the occurrence of these comorbid disorders was less than 2% for each group. In this study, the two groups did not differ on any parent or teacher ratings of internalizing problems. Finally, Faraone et al. (1998) report a somewhat controversial finding (see Stein, Roizen, & Leventhal, 1999): the combined group had a higher rate of comorbid bipolar disorder (26.5%) than the inattentive group (8.7%). In summary, the literature regarding the presence of comorbid disorders in the combined and inattentive groups is clear in documenting important differences in terms of the externalizing problems of conduct disorder and oppositional defiant disorder. The results concerning internalizing problems are less clear cut, with some suggestion that these problems may be more evident in the inattentive group, although the majority of studies fail to find such differences. In the next section we compare the groups in terms of academic difficulties, including comorbid learning disabilities, to further document important group differences in the rates of associated features. Academic Achievement. Academic underachievement is a problem historically related to ADHD (Barkley, 1998). On tests of academic achievement, both ADHD/C and ADHD/I groups have been found to perform more poorly than normal control groups (Barkley et al., 1990; Faraone et al., 1998; Carlson et al., 1986). Studies examining differences between ADHD groups offer mixed results. Several studies have found no differences between groups on tests of academic achievement (Barkley et al., 1990; Faraone et al., 1998; Frick et al., 1991; Morgan et 473 al., 1996; Paternite et al., 1996). Barkley et al. found that equal percentages of ADD/H (31.7%) and ADD/WO (31.9%) children were held back at least once in school, but significantly more of the ADD/WO children had a school placement of learning disabled. In contrast, 15% of the ADD/H and none of the ADD/WO were suspended from school at least once, and the former (12%) were significantly more likely than the latter (0%) to receive a behavior disorders school placement. One study using teacher ratings to assess academic performance obtained a similar lack of differences (Baumgaertel et al., 1995). However, Lahey, Schaughency, Frame, and Strauss (1984) used teacher ratings to assess academic achievement and found that normal control children were rated higher in academic achievement than ADD/H children, who in turn were rated higher than ADD/WO children. Carlson et al. (1986) found that although both ADD/WO and ADD/H groups performed worse than controls on measures of spelling and reading achievement, only the ADD/ WO group performed worse than controls on a measure of math achievement. Similarly, Hynd et al. (1991) and Marshall, Hynd, Handwerk, and Hall (1997) found that children with ADD/H scored higher than children with ADD/WO on a test of mathematics achievement. Additionally, several studies have addressed the issue of associated academic problems by comparing the combined and inattentive groups in terms of their comorbid diagnoses for specific learning disabilities. Both ADHD/ C and ADHD/I subtypes have been associated with elevated rates of learning disorders. Faraone et al. (1998) found that ADHD/C children were more likely than ADHD/I children to have a co-diagnosis of language/ stuttering disorders, but no differences in the rates of identified reading or math disabilities. Nigg et al. (2000) also found the combined group to have a higher rate of comorbid learning disorders (17%) than the inattentive (7%) and control (0%) groups, which did not differ significantly. Morgan et al. (1996) found that ADHD/I children were more likely than ADHD/C children to have a math learning disability. However, no differences between groups emerged when both reading and math learning disabilities were analyzed together. In contrast, studies by Hynd et al. (1991) and Marshall et al. (1997) found that ADD/WO children were more likely than ADD/H children to have some type of developmental learning disorder. In summary, both children with ADHD/I and CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • ADHD/C have been found to perform more poorly on measures of academic achievement than children without ADHD, and both groups are also consistently more likely to receive some form of a learning disabilities diagnosis. Furthermore, when subtype differences are found, children with ADHD/I have been found to perform more poorly on achievement measures than children with ADHD/C, particularly on measures of math achievement (Carlson et al., 1986; Hynd et al., 1991; Marshall et al., 1997). This latter finding suggests that the processing deficits in the combined and inattentive groups may be qualitatively different, with the inattentive group perhaps showing more deficits in right hemispheric functioning. Social Functioning. Poor peer relationships have long been identified among children with ADHD (Milich & Landau, 1982). King and Young (1982) examined classroom peer perceptions of ADD/H, ADD/WO, and normal control children. Both ADD groups received fewer “most liked” peer nominations and more “least liked” peer nominations than the control group. No differences were found between the ADD groups. Lahey et al. (1994) found additional evidence indicating that ADHD groups exhibit social impairment. In a study comparing DSM-IV ADHD subtype groups and a clinic control group, ADHD/C and ADHD/I groups were rated more as “liked least” and less as “liked most” compared with clinic controls. No differences were found between the ADHD groups. Lahey et al. (1984) found similar results when examining a school sample of ADD/H, ADD/WO, and normal control children. However, it was also found that ADD/H children received even more “liked least” nominations than ADD/WO children. Additionally, ADD/H and ADD/WO children were more frequently nominated than control children for “frequently fight,” “poor leaders,” and “not cute,” although ADD/H children were even more likely than ADD/WO children to be nominated for “frequently fight.” Carlson, Lahey, Frame, Walker, and Hynd (1987) examined the sociometric status of ADD/H, ADD/WO, and normal control children using peer nominations. When children without co-diagnoses were compared, ADD/H children received more “liked least” nominations than control children and ADD/WO children received fewer “liked most” nominations than control children. When children with co-diagnoses were included (primarily conduct disorder), both ADD groups V8 N4, WINTER 2001 474 received more “liked least” nominations and fewer “liked most” nominations compared with the control group. Additionally, ADD/H children were more likely to be nominated as “fights most.” Gaub and Carlson (1997) used teacher ratings to assess the social functioning of ADHD/I, ADHD/HI, ADHD/ C, and normal control children. The control group was rated as more liked, less disliked, and less regarded neutrally than all ADHD groups. The ADHD/C group was rated as less liked and more disliked than the ADHD/I group. Additionally, the children with ADHD/C were rated higher on a social problems scale than the children with ADHD/I, who in turn were rated higher than the control group. Edelbrock et al. (1984) compared the social functioning of ADD/H, ADD/WO, and clinic control boys using teacher ratings on the Child Behavior Checklist-Teacher Report Form. ADD/H boys were rated as more unpopular than ADD/WO boys. However, ADD/WO boys were rated as more socially withdrawn. Hynd et al. (1991) used parent ratings on the Personality Inventory for Children to assess ADD groups’ social functioning. It was found that ADD/H children were rated as having more inappropriate social skills than ADD/WO children. Maedgen and Carlson (2000) compared combined and inattentive groups on measures of social knowledge and performance, as well as on their behavior following a disappointment. Parents rated the combined children as more disliked and less liked than the inattentive group, whereas teachers rated the combined group as less liked. On a measure of social knowledge, both groups showed deficits relative to controls, but the combined group showed more aggressive solutions to social problems, whereas the inattentive group showed more passive behavior. Finally, there was a tendency for the combined group to display more negative behavior following disappointment than children in the inattentive group. Distinct behavioral differences between the inattentive and combined subtypes were reported in a peer nomination and observational study by Hodgens, Cole, and Boldizar (2000). Although both subtypes of boys with ADHD received lower social preference scores (“liked most” nominations minus “liked least” nominations) than controls, the two ADHD subtypes were observed to engage in different patterns of behavior in playgroups. The inattentive boys were observed to engage in high levels of solitary, on-looking behavior and low levels of sus- DISTINCT ADHD DISORDERS • MILICH ET AL. tained interactions compared to the control boys and boys with the combined subtype. In terms of peer nominations, the inattentive group received significantly more “shy” nominations than the combined group, whereas the latter received more “starts fights” nominations than did the other two groups. These findings document that even though both the inattentive and combined subtypes have significant peer problems, these difficulties appear to reflect different behavioral etiologies. Together, the findings from studies assessing ADHD children’s social functioning indicate that, although both ADHD/I and ADHD/C groups exhibit social impairment, ADHD/C children appear to display more social problems, such as fighting (Hodgens et al., 2000), and are more likely to be actively rejected by their peers than ADHD/I children (Wheeler & Carlson, 1994). In contrast, there is some evidence to suggest that the inattentive group is more likely to exhibit passive, withdrawn behavior (Hodgens et al., 2000; Maedgen & Carlson, 2000). This pattern of group differences is consistent with findings from the peer relations literature (Rubin, Bukowski, & Parker, 1998), which suggests that disruptive, aggressive behavior is associated with both low popularity and active rejection, whereas inattentive, passive behavior is associated only with low popularity. Demographics and Epidemiology An important indicator of diagnostic distinctiveness can be found in the epidemiology of ADHD, especially differential gender ratios and differences in population prevalence. A number of studies examine such differences for the combined and inattentive groups. Prevalence. In comparing the reported prevalences of the combined and inattentive groups, an interesting differential pattern emerges depending on whether population or clinic samples are examined. For the community samples, ADHD/I is the more prevalent subtype (Carlson & Mann, 2000), with the ADHD/I subtype approximately twice as common as the ADHD/C subtype (Baumgaertel et al., 1995; Gaub & Carlson, 1997; Wolraich et al., 1996). In contrast, in clinical samples, ADHD/C is approximately 1.5 times more prevalent than ADHD/I (Eiraldi et al., 1997; Faraone et al., 1998; Lahey et al., 1994; Morgan et al., 1996). Thus, although ADHD/I appears to be more prevalent in the population, ADHD/C children are 475 much more likely to be referred to clinics for treatment. This most likely reflects the greater amount of disruptive behavior found in ADHD/C children. Gender, Race, and Socioeconomic Status. Males are more prevalent in all ADHD subtypes throughout both clinical and community samples (Carlson & Mann, 2000; Lahey et al., 1997). However, there is an even greater proportion of males in the ADHD/C subtype than in the inattentive subtype in both clinical and community samples. In community studies, the male-to-female ratio for the ADHD/ C subtype ranges from 2.8:1 (Gaub & Carlson, 1997) to 3.5:1 (Wolraich et al., 1996), with an average of 3.2:1. In clinical studies, the ratio for the ADHD/C subtype ranges from 1.2:1 (Faraone et al., 1998) to 8.4:1 (Lahey et al., 1994), with an average of 4.1:1. For the ADHD/I subtype, ratios in the community studies range from 2:1 (Baumgaertel et al., 1995) to 2.3:1 (Gaub & Carlson, 1997), with an average of 2.1:1. Ratios in the clinical studies range from 1.5:1 (Faraone et al., 1998) to 3.7:1 (Lahey et al., 1994), with an average of 2.5:1. Race within ADHD subtypes was examined by only one community-based study (Gaub & Carlson, 1997) and by two clinic-based studies (Eiraldi et al., 1997; Lahey et al., 1994). In all cases, no difference between subtype groups was found. Socioeconomic status was only reported in three clinic-based studies. Faraone et al. (1998) found that ADHD/I children were of higher socioeconomic status than other ADHD subtypes. Paternite et al. (1996) found a similar trend among ADHD groups, although the results did not reach significance. In contrast, Barkley, Grodzinsky, and DuPaul (1992) found no differences in socioeconomic status between the combined and inattentive groups. Overall, ADHD/C and ADHD/I groups can be distinguished on several important demographic variables, especially gender and prevalence. Alternatively, they appear to be indistinguishable on the demographic variables of race and socioeconomic status. Children with ADHD/C are more prevalent in clinical samples, while children with ADHD/I are more prevalent in community samples. This most likely reflects the greater disruptiveness and aversiveness of the behavior of the combined group, leading to a greater likelihood of being referred and is consistent with the earlier age of onset for this group noted earlier. Children with ADHD/C are also more likely to be male, which raises interesting questions CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • about the possible underlying mechanisms (e.g., genetic, biological, referral biases) that may differentiate the combined and inattentive groups. Laboratory Studies There are a relatively large number of studies that have examined differences between the combined and inattentive groups in terms of laboratory measures. Most of these studies have examined various forms of cognitive functioning or performance on neuropsychological measures, although a few have looked at more direct measures of brain functioning. Despite the relatively large number of studies, this is a difficult area to review for several reasons. First, it is difficult to identify reliable findings because there is often little consistency across studies in the measures used. Thus, there are numerous cases in which a specific measure was used in only a single study. Second, many of the studies appear to take an atheoretical approach to differentiating the two groups, so that it is difficult to know which findings may reflect chance differences. In a related vein, several of the studies examine results from batteries of measures, with no attempt to correct for the large number of analyses, further calling into question the reliability of any significant findings that may emerge. Third, many of these studies suffer from small sample sizes, so that it is possible that the frequent failure to obtain significant group differences may merely reflect insufficient statistical power. Finally, these studies frequently do not control for differences in IQ. Thus, it is not always clear whether the results reflect group differences in the specific construct being measured or in overall intellectual functioning. Bearing in mind these cautionary concerns, we attempt to review this disparate literature in a somewhat parsimonious fashion. Therefore, we will merely mention the numerous studies and measures for which group differences were not found, recognizing that low power may well be compromising many of these studies. We summarize the measures for which significant group differences were obtained, with a special focus on measures for which reliable differences across studies are available. A number of studies have assessed the possibility of different levels of intelligence between ADHD/C and ADHD/I groups. The consistent finding from the studies is that there is no difference in intelligence between these V8 N4, WINTER 2001 476 subtypes (Barkley et al., 1990, 1992; Faraone et al., 1998; Frick et al., 1991; Holcomb, Ackerman, & Dykman, 1985; Hynd et al., 1991; King & Young, 1982; Morgan et al., 1996; Nigg et al., 2000; Paternite et al., 1996; Schaughency, Lahey, Hynd, Stone, & Piacentini, 1989). Two studies found significant group differences in IQ, but the results are not consistent across these studies. Carlson et al. (1986) found that ADD/WO and control groups obtained higher IQ scores than the ADD/H group. Additionally, they found that the ADD/WO group obtained a higher verbal IQ score than the ADD/H group. Although Marshall et al. (1997) found no difference in full-scale IQ or verbal IQ among DSM-III-R ADHD and DSM-III ADD/WO groups, they did find that the ADD/H group obtained higher performance IQ scores than the ADD/ WO group. Among studies comparing ADHD/C and ADHD/I groups with control groups, most have found that both of the ADHD groups score lower on IQ measures than do controls (Barkley et al., 1990; Faraone et al., 1998; Holcomb et al., 1985; King & Young, 1982; Schaughency et al., 1989; Paternite et al., 1996). In addition to measures of intelligence, a few studies have examined group differences on extensive batteries of neuropsychological measures. Schaughency et al. (1989) administered the Luria-Nebraska Neuropsychological Battery-Children’s Revision (LNNB-CR) to groups of ADD/H, ADD/WO, and clinic control children. When full-scale intelligence was included as a covariate, no significant differences emerged among the three groups on the LNNB-CR battery scales. Similarly, Lorys, Hynd, and Lahey (1990) failed to find ADD/H and ADD/WO differences on several neurocognitive measures. The groups were administered selected variables from the LNNB-CR and reaction-time and speeded classification tasks. Analysis of covariance using age, full-scale IQ, and number of conduct disorder symptoms as covariates revealed virtually no differences among groups on the neurocognitive measures. In contrast to these null findings, Frank and Ben-Nun (1988) studied differences between boys with ADD/H and boys with ADD/WO using a neuropsychological battery. Children with ADD/H were more likely to have a history of perinatal or neonatal abnormality. Furthermore, they were more likely to have abnormal findings on neurological examinations of motor skills. While both ADD/ H and ADD/WO groups performed below normal on almost every cognitive psychometric test administered, DISTINCT ADHD DISORDERS • MILICH ET AL. the ADD/H group performed below the ADD/WO group on test of visual perception, visual sequential memory, and writing performance. For a number of years now, ADHD has been theorized to be associated with deficits in frontal lobe functioning (Barkley, Grodzinsky, & DuPaul, 1992). Several recent studies have examined whether these deficits also extend to the inattentive group. Barkley et al. (1992) compared 12 boys with ADD/H and 12 boys with ADD/WO on a variety of neuropsychological measures of frontal lobe functioning, including pegboard performance, trailmaking test, Porteus Mazes, the Stroop, and the Wisconsin Card Sorting Test. There were no group differences on any of these measures. Similarly, Nigg et al. (2000) did not find a deficit for the ADHD group on the Tower of London, a measure of executive functioning. Nigg et al. found a deficit for the ADHD/I group on this measure, but this was accounted for by IQ differences. In contrast to the null results for measures of executive functioning, Klorman et al. (1999) compared 102 children with ADHD/I and 207 children with ADHD/C on two measures of executive functioning: the Tower of Hanoi and the Wisconsin Card Sorting Test. The ADHD/C group exhibited deficits in executive functioning on both tasks compared to the ADHD/I group. The former group made more nonperseverative errors on the Wisconsin Card Sorting Test and solved fewer puzzles and broke more rules on the Tower of Hanoi. Further, these differences were not accounted for by comorbid ODD or reading disorder. Finally, Nigg et al. (2000) found the ADHD/ I group to exhibit a deficit in set shifting on the trailmaking test. Several studies have attempted to examine group differences on measures of automatic and effortful processing. Ackerman, Anhalt, Dykman, and Holcomb (1986) compared ADD/WO, ADD/H, reading disorder, and normal control groups on tasks assumed to measure effortful processing. These tasks included semantic and acoustic encoding, list learning of high- and low-imagery words, memory of two printed words, rapid addition and subtractions, and paper-and-pencil arithmetic tests. No differences were found between ADD/WO and ADD/H groups. The control group generally performed better on the tasks than all three clinical groups. Using the same sample of children, Ackerman, Anhalt, Holcomb, and Dykman (1985) studied differences involving innate and acquired automatic processing. The sample 477 of children completed tasks involving frequency of occurrence sensitivity, naming and temporal sensitivity, speed of writing O’s and one’s name, rapid addition and subtraction, and paper-and-pencil arithmetic test. Once again, no differences in processing were found between ADD/ WO and ADD/H groups. However, on a task of arithmetic computation, the two groups showed delayed automatization compared with controls. In contrast, group differences were found by Hynd et al. (1991), who assessed 10 ADD/WO and 10 ADD/H children using measures of rapid alternating naming (RAN) and rapid alternating stimulus (RAS) naming. These are generally assumed to assess automatized processes underlying confrontational naming. Results indicate than ADD/WO children were slower than ADD/H children on these tasks, suggesting a deficit in automatized processing. Other studies have examined other aspects of cognitive functioning. Conte et al. (1986) compared performances of ADD/H, ADD/WO, and control groups on paired associate tasks. The tasks were presented under both fast and slow presentation rates and under both mixed and fixed rates. Although the control group made more correct matches during paired associate learning than the two ADD groups, these latter two groups did not differ from one another. Similarly, Carlson et al. (1986) did not find differences between ADD/H and ADD/WO groups on a variety of cognitive measures, including a rapid naming task, a visual match-to-sample task, Detroit Visual Attention Span for Objects, and Stroop Color Distraction test. Hynd et al. (1989) compared differences in reaction time and cognitive processing speed among ADD/H, ADD/ WO, and clinical control groups. The groups did not differ on simple reaction time, mean physical match classification time, and mean name-match classification time. There were also no differences among groups on error rates for these tasks. Finally, one study examined group differences on direct measures of brain functioning. Holcomb et al. (1985) examined the differences in event-related brain potentials among ADD/H, ADD/WO, reading disabled, and normal control males. No differences were found between ADD groups. Overall, the studies of neurocognitive differences between ADHD/C and ADHD/I do not allow definitive conclusions to be drawn. Although the studies consistently support the conclusion that neurocognitive deficits CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • exist within both ADHD groups, few reliable differences were found between groups. A few studies indicated possible differences between groups that need to be further explored. Specifically, Klorman et al. (1999), using an appropriately powerful design, did find executive functioning deficits to be specific to the combined group. This is consistent with previous theorizing (see Barkley, 1997) as well as research on behavioral inhibition (Nigg et al., 2000), but no other study found such a differentiation. A few studies pointed to processing deficits in the inattentive group, including set shifting on the trail-making test (Nigg et al., 2000) and automatized processing on the RAN/RAS tasks (Hynd et al., 1991). Finally, Frank and Ben-Nun (1988) found that ADD/H children had a greater history of perinatal or neonatal abnormality as well as higher amounts of abnormal motor skills than ADD/ WO children. Despite these few positive findings, it is disappointing that there is not one reliable group difference in all of the studies examining cognitive and neuropsychological differences between the inattentive and combined groups. Family History Although researchers have explored the family psychiatric history of children with ADHD fairly extensively (see, e.g., Faraone, Biederman, Chen, Milberger, Warburton, & Tsuang, 1995), only a few studies have explored differences between ADHD subtypes on this variable. In 1990, Barkley et al. compared ADD/H, ADD/WO, learning disabled, and normal control groups in terms of parental reports of psychopathology among maternal and paternal relatives. It was found that ADD/H children were more likely than all other groups to have paternal relatives with attention deficits and hyperactivity as well as maternal relatives with substance-abuse problems. ADD/ WO children were more likely than all other groups to have maternal relatives with anxiety disorders. Additionally, both ADD/WO and learning disabled children were more likely than the other two groups to have siblings with a learning disorder. The mothers of the four groups did not differ, in self-reported depression, although the mothers in all three clinical groups reported greater psychological distress than did the mothers in the control group. The results of the Barkley et al. (1990) study offer intriguing group differences in terms of family history of psychopathology, differences consistent with earlier reported V8 N4, WINTER 2001 478 evidence that the combined group is more associated with externalizing problems, whereas the inattentive group is more associated with internalizing problems. However, this differential family history has not been replicated in two other studies that reported such data (Faraone, Biederman, & Friedman, 2000; Frank & Ben-Nun, 1988). Specifically, Faraone et al. failed to find that the subtypes “bred true”—in other words, that there were significant associations between the child’s disorder and the type of ADHD present among first-degree relatives. The relatives of the two subtypes both showed higher rates of ADHD/ C than did the control relatives, but the three groups did not differ in terms of rates of ADHD/I among the firstdegree relatives. Thus, it is impossible to draw any firm conclusions about differences in psychopathology among relatives of probands of the inattentive and combined groups. Future studies in this area are highly desirable because the results of these investigations could be especially informative concerning possible genetic differences among the subtypes, as well as implications for understanding the differential mechanisms that may underlie these disorders. Treatment Response Only one study has compared of the response ADD groups to medication. No studies have compared the response of ADD groups to other forms of treatment, such as behavior management or psychotherapy. Barkley, DuPaul, and McMurray (1991) compared the responses of ADD/H and ADD/WO groups to three doses of methylphenidate (5-, 10-, or 15-mg doses). Using a tripleblind, placebo-controlled crossover design, medication effects were assessed using parent and teacher ratings, laboratory tests, and direct observations during academic tasks. An examination of all of the dependent variables revealed that both the combined and inattentive groups showed positive medication effects, with no variable showing a significant group-by-dose interaction. This would suggest that the groups are equally sensitive to the medication across the range of doses. However, a somewhat different conclusion was drawn when medication recommendations were made. Based on all of the drugresponse data available, a psychologist and a pediatrician determined whether each child should be maintained on medication at the end of the study and, if so, at what dosage. More ADD/WO children (24%) were judged nonresponsive to methylphenidate compared with ADD/H DISTINCT ADHD DISORDERS • MILICH ET AL. children (5%). Additionally, more ADD/WO children responded best to the lowest dose (35%), while more ADD/H children responded best to the moderate (52%) or highest (19%) dose of methylphenidate. Barkley et al. (1991) suggest that the differential response of the two groups to the medication are most likely due to the greater severity of symptoms exhibited by the combined group, allowing more room for the higher doses of medication to have an effect. Given that this is the only study examining medication responsivity for the two groups, it is impossible to know whether symptom severity accounts for this difference or whether this may reflect some underlying biological difference. Clearly this is an area where greater research is necessary. Prognosis and Follow-up Studies Numerous long-term follow-up studies of children with ADHD have been reported in the literature. However, we could find no study that has followed-up a sample of children with the inattentive subtype for an appreciative length of time. This no doubt is due to some degree to the relatively new status of this disorder. However, it probably also reflects the fact that most studies in this area have been concerned with establishing the validity of this subtype, so that follow-up studies might appear premature. A few studies do suggest that the hyperactive/ impulsive dimension is a greater predictor of negative outcomes than is the inattention dimension (see Lynam, 1996), suggesting that the combined group would be at greater risk. Such a conclusion would also be consistent with the greater association of the combined group with aggression and conduct disorders. However, without definitive follow-up studies it is premature to draw any firm conclusions about the possible differential outcomes of the combined and inattentive groups. C O N C L U S I O N S A N D I M P L I C AT I O N S A number of review papers have appeared of late summarizing the evidence concerning the validity of the inattentive group (Carlson & Mann, 2000; Carlson et al., 1999; Lahey et al., 1997). However, these reviews generally have started with the assumption that the inattentive subtype belongs in the ADHD category, and the authors have concluded that this disorder is a valid subtype of ADHD. In contrast, this review started with a different basic assumption: the possibility that the inattentive group is a valid disorder but perhaps is not a member of the ADHD fam- 479 ily. When viewed from this perspective, the evidence seems quite consistent: ADHD/C and ADHD/I are distinct and unrelated disorders. According to the traditional psychiatric approach, the validity of a psychiatric disorder is substantiated by demonstrating that it can be distinguished from other disorders by a clear clinical description of essential and associated features, neurological and physical studies, laboratory studies, family psychopathology studies, follow-up studies, and treatment studies. Although there is no definitive way to determine when a distinct disorder has been found, the validity of a disorder increases as the body of evidence grows in support of it. The review of literature comparing ADHD/C and ADHD/I groups reveals that the preponderance of evidence supports the conclusion that they are best conceptualized as distinct disorders rather than as subtypes of the same disorder. ADHD/C and ADHD/I can be distinguished from each other by the essential features used to differentiate the two disorders (i.e., hyperactive/impulsive symptoms), and even in terms of the features they supposedly share (i.e., inattention symptoms). From this perspective, the two disorders have no defining features in common, which challenges the basic assumption that the disorders share some common core deficit. This conclusion is enhanced when one examines the symptoms used to describe the disorders. For the inattentive type, “sluggish,” “hypoactive,” and “daydreaming, lost in space” are the symptoms associated with this disorder (Carlson & Mann, in press), whereas for the combined group the relevant symptoms are “disinhibited,” “hyperactive,” and “distractible.” The distinction is enhanced when the evidence regarding associated features and demographics is examined. There is reliable evidence that the combined group, compared with the inattentive group, is much more likely to be male, to have an earlier age of onset or referral, to be actively rejected by their peers, and to have comorbid externalizing disorders. This last finding is also supported by the limited family history data available. In contrast, the inattentive group is reliably more likely to have a math disability, suggesting that there is a difference in the core processing deficits in the two groups. In addition, there is suggestive, albeit inconsistent, evidence that the inattentive group is more likely to have internalizing problems and to have relatives with internalizing problems. Similarly, the inattentive group is more likely to be shy and CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • withdrawn in their peer relations rather than outright rejected. This last finding is also consistent with the increased prevalence of internalizing problems in the inattentive group. Finally, there is suggestive evidence that the inattentive group may be less responsive to stimulant medication and exhibit an effective response at a lower dose than the combined group. Looking at these findings in their entirety, without being wed to the belief that the combined and inattentive groups are subtypes of the same disorder, it is difficult to imagine how one could conceive of them as variants of the same disorders. Not only do they have virtually nothing in common, but in many ways they appear to fall at opposite ends of a continuum-disinhibited versus inhibited, overactive versus hypoactive, externalizing versus internalizing, energetic versus sluggish. In fact, there are several other disorders (e.g., CD, ODD) that would appear to have much more in common with ADHD/C than does ADHD/I. Implications Removing the inattentive group from the ADHD family of disorders clearly would cause some initial problems and confusion. A new name for the disorder would be necessary, preferably one that did not suggest any association with the ADHD disorders. Thus, attention deficit disorder (ADD) has been used occasionally to refer to the inattentive group, but this may be so similar, both historically and currently, to the name for the combined group that it would still carry connotations of the two disorders being somehow related. Thus, a completely distinct name would be desirable. Deleting the inattentive group from the ADHD diagnosis would also cause some confusion and problems for the general public. For example, according to the regulations of the Department of Education, ADD is referred to as qualifying for special services, and this term is used for both the inattentive and combined groups. Obviously, the Department of Education can ignore any changes made in the DSM, just as they have ignored the switch to the ADHD terminology. Nevertheless, problems may arise if educators and clinicians are using completely different terminology to describe the inattentive group. Parents may also experience some confusion, believing their child has an attention deficit disorder only to discover that their child has a disorder with a different name that has no relation to ADHD. V8 N4, WINTER 2001 480 Despite these relatively minor confusions and inconveniences, redefining the inattentive group as a separate disorder can only have long-term benefits in terms of adding clarity to the field. This is true for practical, theoretical, and methodological reasons. Practically, redefinition will help eliminate some of the unnecessary debate about whether ADHD constitutes a disruptive disorder, as claimed by the DSM. Parent support groups (e.g., CHADD) raised concerns about considering ADHD as a disruptive disorder, primarily because many of the parents in this organization had children with the inattentive type, and thus they objected to characterizations of their children as having a disruptive disorder. Children meeting criteria for the combined group clearly engage in disruptive behavior, so such a characterization is not a problem. Theoretically, this change in the diagnostic system should bring greater clarity to our understanding of the underlying mechanisms involved in both disorders. Right now it is difficult, if not impossible, to come up with a unifying theory that can account for the problems experienced by both types of children. For example, Barkley’s (1997) model for explaining ADHD in terms of problems in inhibition does a nice job of accounting for many of the problems experienced by children with ADHD/C. However, Barkley (1997) explicitly states that his behavioral inhibition model of ADHD refers only to the ADHD/C and ADHD/HI subtypes. This is because he presumes their primary impairment to be response inhibition, while he presumes the primary impairment of ADHD/I to be inattention. Thus, his theory is not appropriate in attempting to account for the problems of children with ADHD/I. Alternatively, there may be models or theories that would better account for the problems associated with the inattentive group. Viewing ADHD/I as a distinct disorder will encourage the growth of theoretical literature regarding the development and dysfunction of children with ADHD/I. No strong theoretical model exists that provides a framework for understanding this disorder. Not only would such a theoretical model inform our conceptualization of the subtype, but it would inspire research to evaluate its validity. It probably is impossible to develop a unifying theory to account for both disorders. Recognizing the distinct nature of these two disorders makes this a moot point. Methodologically, removing the inattentive group from the ADHD category should bring greater clarity to the research being undertaken on both disorders. First, it DISTINCT ADHD DISORDERS • MILICH ET AL. will encourage differentiating ADHD/C and ADHD/I in research studies. Currently, the subtypes of ADHD are often regarded as part of one homogeneous group and not differentiated within studies; but they are, in fact, heterogeneous. This may be especially problematic for research on the molecular genetics of ADHD and neuroimaging studies, which rely on relatively small samples; both are predicated on the notion that the phenotype or group under study is relatively pure. Removing ADHD/I from the ADHD umbrella will refine the phenotype and presumably help move this important research forward. Of greater significance, perhaps, this change should bring about a more focused research agenda concerning the inattentive group. To date, all validity studies concerning this group have involved, as a comparison group, ADHD/C. Further, almost all of the relevant research questions addressed have arisen from what is known about ADHD/C and how the two groups may differ along these dimensions. For example, studies have been undertaken to see if the two groups may differ in terms of measures of behavioral disinhibition. However, under the current reconceptualization of the inattentive group, there is no reason to assume that behavioral disinhibition is a relevant construct to study for this group. Similarly, when treatment studies for the inattentive group are planned, we need no longer be locked into the mind set of identifying potential treatments that have worked for the combined group. Instead, viewing ADHD/I as a distinct disorder should positively impact the quality of treatment provided to children with ADHD/I. Specifically, it will encourage the development of treatments, such as psychotherapy and pharmacotherapy, that are specifically designed for the unique needs of ADHD/I children. It will also encourage physicians, clinicians, and educators to provide more tailored interventions for children with ADHD/I. The problems these children experience may begin to be more specifically addressed than they are currently, leading to greater improvements for these children, both behaviorally and academically. Finally, perhaps the best example of the benefits associated with this diagnostic reconceptualization is related to the attention symptoms used in the DSM-IV criteria. As noted earlier, the work group was so intent on using the same criteria for attention problems for all of the subtypes that they eliminated potentially some of the most relevant attention problems for the inattentive group. However, if the inattentive group is thought of as a separate disorder, 481 then investigators can focus on which specific attention problems are most relevant for this group, regardless of their relevance for the combined group. Reconceptualizing the inattentive group as a separate disorder does raise one classification problem: Where in the nomological net does this disorder fall? Unfortunately, researchers have not addressed this issue. It is possible that the inattentive group may be better conceptualized as a form of a learning disability. It is even possible, based on the comorbidity and family history findings, that the inattentive group may have more in common with internalizing disorders. It may even be that it will need to find its own niche. What does appear to be true from this review is that it does not belong in the ADHD family. L I M I TAT I O N S A N D F U T U R E D I R E C T I O N S In reviewing the literature on the inattentive group, one can identify a number of problems that make it difficult to draw firm conclusions about the inattentive disorder and its relation to ADHD. In this section we review the many difficulties in the hope that this will help guide and improve future studies in this area. Diagnostic Confusion. Although the inattentive disorder is only 20 years old, the diagnostic criteria have already undergone two major changes. The disorder disappeared with DSM-III-R and reappeared with DSM-IV. Although the evidence reviewed suggests that the correspondence between DSM-III and DSM-IV versions of the inattentive group is acceptable, problems exist in trying to derive inattentive groups from samples obtained under the DSM-III-R criteria. Thus, it is difficult to know how well the samples identified in earlier studies correspond to those samples being identified under the current diagnostic criteria. Of greater significance is the confusion concerning the appropriate criteria for determining attention problems. The DSM-IV work group used identical attentional symptom criteria for the combined and inattentive groups, despite suggestive evidence that the nature of the attention problems exhibited by these two groups may differ. The result is that the inattentive group as currently defined is heterogeneous and actually reflects at least two different manifestations of the disorder; a sluggish inattentive group and a subthreshold combined group (Carlson & Mann, in press). Much of the inconsistency in the literature may well reflect the different distributions of these groups in the inattentive samples used. CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • Sampling Issues. In addition to diagnostic problems, we also found that the results can be affected by sampling procedures, particularly whether clinical or community samples are used (Carlson & Mann, 2000). The effect of these sampling differences was most evident in the prevalence rates reported, in which the combined type is more prevalent among clinical samples but the inattentive group appears to be more prevalent in community samples. Although perhaps not as dramatic, sampling also appears to affect the findings concerning gender ratios and the prevalence of comorbid disorders. Dramatic or not, sampling differences among studies add noise to the research literature. Comorbidity. Another methodological factor to consider in assessing the results of ADHD subtype studies is the presence of comorbid psychiatric diagnoses among children with ADHD. Both internalizing and externalizing disorders have been found to co-occur with ADHD, and there is fairly consistent evidence that the inattentive and combined groups may differ in the degree to which they exhibit comorbid externalizing, internalizing, and learning disability disorders. On the one hand, this can be seen as a valuable lead in identifying underlying differences between the groups. On the other hand, comorbid conditions can be viewed as unfortunate confounds that make interpretation of group differences difficult if not impossible. Most articles in this review did not differentiate between ADHD children who did and did not have a psychiatric codiagnosis. However, a few of the studies examined ADHD groups with and without comorbid psychiatric disorders. Carlson et al. (1987) found that when children with codiagnoses (primarily conduct disorder) were removed from analyses of peer sociometric nominations, children with ADD/H were no longer nominated more than children with ADD/WO for “fights most.” When Lahey et al. (1987) removed children with a codiagnosis of CD from analyses of teacher ratings, ADD/H children were no longer found to have more conduct problems or to be more impulsive than ADD/ WO children. Similarly, Nigg et al. (2000) noted that several of their results and conclusions about group differences changed when comorbid conditions were statistically controlled. These results suggest that some of the impairment associated with particular ADHD subtypes may be more related to co-occuring diagnoses than to ADHD. Although such results are open to many interpretations (Angold, Costello, & Erkanli, 1999), at the very V8 N4, WINTER 2001 482 least they underscore the importance of taking comorbidity into account in all studies. Angold et al. (1999) present an excellent discussion of both the problems inherent in disentangling effects associated with comorbid conditions, as well as the ways in which analyses of comorbidity patterns can inform us about both the potential etiology of disorders and the validity of subtypes of these disorders (see also Caron & Rutter, 1991). Paucity of Studies. Although a considerable number of studies have appeared over the past 20 years examining the validity of the inattentive diagnosis, the studies are not evenly distributed across the classification criteria used in the present review. Thus, for example, although there are a number of studies examining issues relating to comorbidity or cognitive functioning, there are few if any studies in the areas of treatment response, family history, genetics, and long-term outcome. Why these areas have been ignored is unclear. Certainly long-term follow-up studies are more difficult, especially given the relative recency of the disorder, but their value is extremely high. Similarly, family history and genetic studies are expensive and labor intensive, but they probably bring us closer to understanding etiology than any other type of study. However, treatment-response studies are not nearly so difficult, and hundreds of such studies exist concerning ADHD/C. Whatever the explanation for the dearth of studies in these areas, such studies will yield important information regarding the distinctiveness of the inattentive group. Type 1 and Type 2 Errors. One of the more disappointing problems in the literature is the consistently small sample sizes. It was not unusual to find studies samples with 10 or 12 children per group. When other factors adding to group heterogeneity are included (e.g., comorbid status, gender, age, diagnostic heterogeneity), it is clear that many of the studies reviewed were grossly underpowered. Few studies ever corrected for the large number of comparisons undertaken. This was especially a problem in the studies using laboratory measures, in which entire batteries were administered and examined with no correction for inflated alpha levels. Thus, most of the studies reviewed suffered in some fashion from high Type 1 and/ or Type 2 error probabilities, making it impossible to know if unreplicated findings reflect chance findings or weak statistical power in subsequent studies. These methodological problems were compounded by the fact that most of the studies reviewed were not DISTINCT ADHD DISORDERS • MILICH ET AL. engaged in theory-driven hypothesis testing (see Nigg et al., 2000, for an exception) in examining group differences. Instead, a variety of purportedly relevant measures were collected to see which might differentiate the two groups. One can have more confidence in the results when they are consistent with well-thought-out theoretical hypotheses than when they reflect a small number of significant effects resulting from the analysis of a battery of measures. Confirmatory Bias. In addition to the atheoretical nature of much of the available literature, the research also reflects a confirmatory bias. Specifically, the overwhelming majority of studies reviewed accept the views offered by DSM-IV that the inattentive group is a subtype of ADHD and that the proposed diagnostic criteria (especially, the inattention symptoms) are appropriate for both groups. Different lines of investigation would have been generated if, for example, the inattentive subtype were assumed to reflect a developmental disorder rather than a disruptive disorder. However, because assumptions such as these were never considered, these lines of investigations were not pursued. Further, studies in which differences between ADHD/C and ADHD/I are found are interpreted only from the DSM subtype perspective, despite the fact that the results are consistent with the distinct disorders position advanced here. Future Directions. Although there are compelling reasons to view ADHD/I and ADHD/C as distinct disorders, additional research is needed to continue to build support for the validity of these groups as distinct and unrelated disorders. As noted earlier, there are many gaps in the literature concerning the inattentive group. One area to be addressed is the formulation of a theoretical model that addresses the development and dysfunction of ADHD/I. While theoretical models for ADHD exist, the ADHD/I subtype is not generally included. A theoretical model of ADHD/I based on its deficit in cognitive control has yet to be well developed. Several lines of research could help in developing such a theoretical model. Specifically, behavioral genetic and family history studies can be especially useful in identifying underlying, etiological mechanisms that may account for the problems experienced by the inattentive group. In addition, studies using advanced technology, such as neuroimaging methods, to examine differences in brain metabolic processes or brain morphology can be very valuable in this regard. Further, family, 483 genetics, and epidemiological studies of comorbidity (see Angold et al., 1999) will help determine where in the diagnostic nomenclature the inattentive group really belongs. As noted earlier, much more information is needed on the specific attentional problems associated with the inattentive group. Most of what we know about this comes from studies examining responses on rating scales (see Carlson & Mann, in press), rather than from systematic laboratory investigations. Although the rating scale studies offer some promising leads in describing the types of attention problems these children experience (e.g., the sluggish tempo described earlier), these instruments are too global and too broad to offer specificity concerning the actual patterns of attention exhibited by these children. Systematic investigations using well-validated laboratory and cognitive processing measures are needed to precisely identify the specific deficits experienced by these children. In a similar vein, such studies may offer valuable insights to the diagnostic criteria employed for the inattentive and combined groups. Expanding and/or altering the diagnostic criteria to identify attention problems for the inattentive group may serve to increase the accuracy of identifying true cases of this disorder. Effective treatments and long-term outcomes are two other areas where research is badly needed concerning the inattentive group. To date, only one treatment study involving ADHD/I children has appeared (Barkley et al., 1991), and this involved medication only. It is interesting to note that the large-scale, multisite treatment study for ADHD (Arnold et al., 1997) actually excluded the inattentive group from the study, so we cannot assume that the results reported would generalize to this group. Further, as noted above, it is not clear that the treatments identified to be effective for the combined group would automatically be the treatments of choice for the inattentive group. For example, the fact that this group reliably exhibits greater math deficits suggests that more focused academic interventions may be necessary for effective intervention. By identifying the inattentive group as a distinct disorder, this will enable researchers and clinicians to approach the problem of effective interventions from a fresh perspective and not be limited by what is known about the combined group. Longitudinal studies assessing long-term outcomes for the inattentive group also need to be conducted. These investigations would help us better understand the developmental trajectories of these children. Gaining knowl- CLINICAL PSYCHOLOGY: SCIENCE AND PRACTICE • edge in this area is necessary for developing appropriate prevention plans and targeting children who are at risk for particular problems, such as academic difficulties, underemployment, and interpersonal difficulties. In sum, we believe that the field would benefit from a shift in perspective. ADHD/C and ADHD/I are not variants of a single disorder, but should be considered distinct disorders. Extant evidence supports this claim, and we are confident that future evidence will as well, if we only begin to look for it. NOTE 1. Given the paucity of research on the predominantly hyperactive-impulsive subtype, issues relating to this subtype will not be discussed further in this review. 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