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Data from the National Institute of Child Health and Human Development Study of Early Child Care were examined to test whether: attention deficit/hyperactivity disorder (ADHD) symptoms remain stable from 54 months through early elementary school; behavioral inhibition and attention deficits assessed at 54 months predict ADHD symptoms in elementary school, even after controlling for their temporal stability; and early behavioral inhibition and attention deficits moderate the longitudinal stability in ADHD symptoms. Data were examined using continuous and categorical measures of symptoms. Modest stability in ADHD symptoms from 54 months to third grade was found. Measures of inhibition and inattention predicted later teacher ratings uniquely, but no evidence was found for moderation. Measures of preschool behavioral inhibition also predicted “persistently at risk status” defined by elevated teacher ratings over time. Results are discussed in terms of executive and motivational facets of inhibition that may be related to early signs of ADHD.

Over the past two decades attention deficit/hyperactivity disorder (ADHD) has become one of the most commonly diagnosed and studied childhood disorders (see, for review, American Academy of Pediatrics, 2000 and Tannock, 1998). Major questions still remain, however, about its etiology and developmental course and about what early indicators may signal risk for the emergence of ADHD (Rowland, Lesesne, & Abramowitz, 2002). In the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) ( American Psychiatric Association, 1994), ADHD is defined by symptoms in two primary areas: hyperactivity–impulsivity and inattention. Childhood ADHD is associated with maladjustment in many domains of functioning over the course of development ( American Academy of Pediatrics, 2000), including academic achievement and social relationships. Overall, children with ADHD disproportionately use medical and mental health services compared with children without ADHD ( Rowland et al., 2002). The costly toll that ADHD takes on individual adjustment, family life, schools, and social services underscores the importance of understanding the developmental course of ADHD symptoms, with the ultimate goal of early identification and treatment. Identifying early markers of ADHD symptomatology and charting its developmental course involve three fundamental tasks. First, the longitudinal stability of ADHD symptoms must be established. Second, potential early markers of ADHD symptoms must be identified. This search should be guided by theoretical models of ADHD and by empirical data. Third, the predictive power of these hypothetical markers must be tested in a community sample across time to avoid the biases associated with clinic referral, especially in young children (e.g., Lahey et al., 2004). Examining the development of ADHD symptomatology in a community sample also necessitates investigating these questions at the symptom level rather than at the disorder level. Therefore, in the current study, we examine the longitudinal stability of ADHD symptoms from 54 months through first and third grades. Then, based on a review of the theoretical literature and empirical research, we explore whether behavioral inhibition and inattention in preschoolers predict ADHD symptoms as rated by teachers in first and third grades. In addition, we investigate whether poor behavioral inhibition or inattention in preschool-aged children exacerbates ADHD symptoms in early elementary school in children with higher levels of early symptoms. Finally, as we are ultimately motivated by our desire to understand attention deficit/hyperactivity disorder, we also examine these same questions as a function of elevated ADHD symptom levels across time. 1.1. Longitudinal stability of ADHD Although several studies have examined the longitudinal stability of early ADHD symptoms (Lahey et al., 2004, Lahey et al., 2005 and Pierce et al., 1999), they have not investigated potential mechanisms that may underlie this temporal stability. Lahey et al. (2004) examined the 3-year predictive validity of ADHD in children diagnosed between 4 and 6 years of age using DSM-IV criteria. They found that children who met full diagnostic criteria during their first assessment were likely to continue to meet diagnostic criteria for ADHD over the next 3 years (Lahey et al., 2004). Pierce et al. (1999) found that symptoms of ADHD identified in hard-to-manage preschool boys predicted continuing problems in middle childhood. The present study aims to examine both the stability in ADHD symptoms over time and also to advance current knowledge by examining the role played by potential early markers of ADHD symptoms: behavioral inhibition and inattention. 1.2. Theoretical models of ADHD: The role of inhibition and inattention Over the past 15 years, several theoretical models of ADHD have emerged (Sergeant, Geurts, Huijbregts, Scheres, & Oosterlaan, 2003). Despite different emphases, these models all posit deficient behavioral inhibition and attention as central features of ADHD, but they differ in the precise definitions and roles of inhibition and attention deficits in the emergence of ADHD. For instance, Barkley (1997) identifies behavioral inhibition as an executive function in the Behavioral Inhibition Model, whereas Sonuga-Barke, Houlberg, and Hall (1994) view poor behavioral inhibition as a symptom of the inability to wait in the Delay Aversion Model. The models differ not only in their definitions, but also in whether they ascribe a primary or secondary role to underlying inhibition and attention deficits (Nigg, 2001). For example, the Behavioral Inhibition Model asserts that ADHD is driven primarily by an inhibition deficit, which, in turn, is responsible for an attention deficit (Barkley, 1997), whereas inhibition and attention deficits are both relegated to secondary roles in the Cognitive-Energetic Model (Sergeant, Oosterlaan, & van der Meere, 1999). On the extreme end of this spectrum lies the Delay Aversion Model (Sonuga-Barke et al., 2002 and Sonuga-Barke et al., 1994), which ascribes peripheral roles to both behavioral inhibition and inattention; it suggests that a hypersensitivity to delay is responsible for ADHD symptoms and is partly manifest as impulsive and/or inattentive behavior. 1.3. Behavioral inhibition and inattention Despite the lack of agreement regarding the definitions of inhibition and inattention and the specific roles they play in the etiology of ADHD, as already noted, some form of inhibition and/or attention deficit is common to nearly all models of ADHD. In addition, the development of inhibition and sustained attention are important developmental tasks for preschool children who must learn to regulate behavior, control impulses, and attend in response to situational demands (Campbell, 2002). Thus, early indicators of poor behavioral inhibition and inattention are possible markers that can be studied in early childhood and may contribute to the development and maintenance of ADHD symptoms (Barkley, 1997). Based on the literature on ADHD, we use Barkley's (1997) definition of behavioral inhibition, that is, the ability to suppress a dominant response to an event. Suppression of the response may occur before the response is initiated or while the response is ongoing. Further, the failure to withhold dominant responses, despite either threat of punishment or loss of desirable rewards, is defined as a behavioral inhibition deficit. 1.4. Studies of inhibition and ADHD symptoms in children The emphasis on executive function deficits, such as deficient behavioral inhibition, in models of ADHD has progressed in tandem with empirical research exploring their interrelation (Barkley, 1997, Nigg, 2001 and Pennington and Ozonoff, 1996). To date, the overwhelming majority of studies assessing various aspects of inhibition have demonstrated a concurrent inhibition deficit in school-aged children exhibiting ADHD symptoms compared with children without signs of ADHD (see, for reviews, Barkley et al., 1992, Corkum and Siegel, 1993, Homack and Riccio, 2004, Losier et al., 1996, Oosterlaan et al., 1998, Pennington and Ozonoff, 1996 and Tannock, 1998). From these studies, we can conclude that behavioral inhibition deficits are related to ADHD symptoms. However, the majority of these studies have not examined the nature of this relationship over time or considered whether an early behavioral inhibition deficit, assessed at preschool age, may exacerbate the severity of ADHD symptoms in early elementary school. This study examines whether behavioral inhibition deficits moderate the longitudinal stability of ADHD symptoms. Behavioral inhibition has been operationalized using a variety of measures, including the Continuous Performance Task (CPT) (see, for reviews, Barkley et al., 1992, Corkum and Siegel, 1993 and Losier et al., 1996), delay of gratification tasks (DGTs) (Campbell et al., 1971, Pennington and Ozonoff, 1996 and Weyandt and Grant, 1994), and the Stroop Test (see for review, Homack & Riccio, 2004). Consistent with this literature, in the current study, behavioral inhibition is measured using CPT commission errors, DGT waiting time, and Stroop interference effects. An early marker of potential attention problems is also included, CPT omission errors, thought to index lapses in attention (Halperin, Sharma, Greenblatt, & Schwartz, 1991). These measures have been used extensively in research on school-aged children and they have also been used successfully with preschool-aged populations. This dual focus on deficits in both inhibition and attention is consistent with recent theoretical arguments suggesting multiple pathways to ADHD symptoms and implicating dysregulation in both inhibitory and attentional systems (Nigg, 2006). Overall, the findings from studies focusing on school-aged children buttress the theoretically posited relationship between inhibition deficits and ADHD (Barkley, 1997). However, the aim of this study is to explore whether early behavioral inhibition and/or inattention deficits predict later symptoms of ADHD, and also to examine the roles that behavioral inhibition and inattention play in the development and maintenance of ADHD symptoms in preschool-aged children. Therefore, we now discuss the literature examining behavioral inhibition, inattention, and ADHD symptoms in preschool-aged children. Initially stimulated by the work of Campbell and colleagues (Campbell et al., 1994 and Campbell et al., 1982), a growing body of studies have examined ADHD symptoms and behavioral inhibition in preschool children (Berlin et al., 2003 and Marakovitz and Campbell, 1998). The majority of these studies (9/11) have found a significant association between ADHD symptoms and deficient behavioral inhibition (Berlin and Bohlin, 2002, Berlin et al., 2003, Byrne et al., 1998, Campbell et al., 1982, Campbell et al., 1994, Hughes et al., 1998, Marakovitz and Campbell, 1998, Sonuga-Barke et al., 2002 and Sonuga-Barke et al., 2003), adding credence to the hypotheses that early behavioral inhibition predicts later ADHD symptoms and may influence the relationship between early and later ADHD symptoms. Only three of these studies, however, have specifically examined the predictive relationship between behavioral inhibition at preschool age and ADHD symptoms at school age (Berlin et al., 2003, Campbell et al., 1994 and Marakovitz and Campbell, 1998). Berlin and colleagues investigated the association between early inhibition deficits and later symptoms in a large community sample of boys and girls. Behavioral inhibition was measured at approximately 5 years of age and was operationalized using the go/no-go task. Teachers and parents rated ADHD symptoms when children were, on average, 8 years old. Preschool inhibition deficits predicted later ADHD symptoms both at school and home for boys and in the school context only for girls. Furthermore, Berlin et al. (2003) found that preschool inhibition and concurrent executive function measures contributed independently to the variance in ADHD symptoms in school for boys and the sample as a whole. Unlike the current study, Berlin and colleagues did not examine the predictive association between preschool inattention and ADHD symptoms. Campbell et al. (1994) examined the relationship between behavioral inhibition and ADHD symptoms in a sample of preschool boys identified by parents and/or teachers as “hard-to-manage.” These boys met approximate criteria for Attention Deficit Disorder with Hyperactivity according to DSM-III. A group of boys who did not meet these criteria and were matched with the hard-to-manage boys on classroom and age constituted the control group. An additional group of “problem boys” were referred to the study by parents complaining about their son's overactivity, inattention, and discipline problems. At preschool age, behavioral inhibition was assessed using a DGT and a resistance-to-temptation task. A Continuous Performance Task (CPT) was used to measure behavioral inhibition and inattention at the follow-up visit, when the boys were approximately 6 years old. A significant longitudinal relationship was found for the entire sample between preschool delay performance and later observations of behavior during structured tasks in the laboratory, including cooperation, restlessness, attentional focus, task involvement, out-of-seat behavior, and distraction. Inattention, measured by CPT omission errors, was examined concurrently at follow-up. No main effect was found; that is, CPT omission errors did not differ between the “hard-to-manage” and comparison boys. Marakovitz and Campbell (1998) followed these same children in elementary school and examined the relationship between measures of inhibition at ages 4 and 6 and a diagnosis of attention deficit disorder (ADD) at age 9. A significant association was found between latency to touch an appealing toy on the resistance-to-temptation task at ages 4 and age 9 ADD diagnostic status. Specifically, boys diagnosed with ADD at age 9 were less able to resist touching the forbidden toy at age 4 than were control boys, although performance on the DGT was unrelated to later ADD status. Impulsive errors on the CPT at age 6 were not found to differentiate between ADD, non-ADD, and control groups at age 9, although power to detect differences was low. Inattention, measured by CPT omission errors, was also examined at ages 6 and 9. The authors report a significant concurrent relationship between ADD status and inattention at age 9. However, age 6 inattention did not predict age 9 ADD status. Similar to the three studies discussed above, the current study also examines whether behavioral inhibition and/or inattention in preschool-aged children predict later ADHD symptoms. However, this study provides the unique opportunity to examine that relationship in more detail across three time points using multiple measures in a large community sample of girls and boys, thereby allowing for greater generalizability of the results. In addition, this study uses multiple measures of behavioral inhibition, which allows us to assess behavioral inhibition as an executive function (CPT commission errors) and as a motivational strategy (delay of gratification). Furthermore, by controlling early ADHD symptoms when examining the predictive relationship between preschool behavioral inhibition, inattention, and later ADHD symptoms, we will determine whether early behavioral inhibition and/or inattention predict later ADHD symptoms above and beyond the temporal stability in symptoms. Finally, these questions are also examined as a function of ADHD symptom level. In summary, this study aims to answer the following questions: 1. Are ADHD symptoms stable, despite different reporters, that is, teachers, from 54 months to third grade (age 9)? 2. Are behavioral inhibition and attention deficits associated both concurrently and longitudinally with ADHD symptom ratings? 3. Do behavioral inhibition deficits and inattention at 54 months predict ADHD symptoms in first and third grades, even after controlling for ADHD symptoms at 54 months? 4. Is the relationship between ADHD symptoms at preschool age and at first and third grades stronger among participants who exhibit deficits in behavioral inhibition or attention at preschool age? That is, does preschool behavioral inhibition and/or inattention moderate the association between symptoms of ADHD assessed at preschool age and later ADHD symptoms? 5. Are the relationships described in and still significant when examined as a function of ADHD symptom level, that is, within a sample of children exhibiting ADHD symptoms in the “at risk” range between 54 months and third grade? Overall, this study provides the rare opportunity to examine these questions with empirically validated measures and multiple reporters in a large, diverse community-based sample using a prospective longitudinal design.

We can conclude, based on our findings, that behavioral inhibition and inattention at 54 months (operationally defined by the DGT, CPT commission errors and CPT omission errors) predict ADHD symptoms at first and third grades. Furthermore, these findings suggest that behavioral inhibition and inattention at preschool can be used as independent markers of developing ADHD symptoms. These markers, if used in tandem with other early indicators, could be used to create a risk index to help identify children at risk for ADHD who could then be targeted for early intervention.