ساختار نهفته اختلال نقص توجه/بیش فعالی در یک نمونه بزرگسال

Attention deficit/hyperactivity disorder (ADHD) is one of the most prevalent psychiatric conditions, affecting roughly 8% of children and adolescents (CDC, 2005) and 4.4% of adults (Kessler et al., 2006) in the United States. Despite considerable advances in the diagnosis, treatment, and etiology of ADHD, a number of unresolved issues remain, including questions about (a) whether ADHD is the consequence of a single core deficit (e.g., Barkley, 1997) or whether multiple pathways can lead to ADHD (e.g., Sonuga-Barke, 2002), (b) how to understand the high comorbidity between ADHD and other externalizing disorders (e.g., Waschbusch, 2002), and (c) when symptoms of inattention and hyperactivity/impulsivity are sufficiently severe to warrant treatment and medication. A clear understanding of the latent structure of ADHD can help inform many of these unresolved issues. In other words, knowing whether ADHD is a categorically distinct condition or “taxon” that is either present or absent (like Type 1 diabetes) or whether it is dimensional and exists on a continuum (like Type 2 diabetes), can inform questions of assessment, etiology, diagnosis, and treatment. In recent years a growing research literature has examined the latent structure of ADHD using a variety of statistical methods, including latent class analysis and Meehl's taxometric procedures (Waller and Meehl, 1998). Most (but not all) of these studies have yielded dimensional results. The findings from both Hudziak et al.'s (1998) and Neuman et al.'s (1999) latent class analyses of parent reports of adolescent symptoms suggested that these ADHD symptoms were continuously distributed and not taxonic. Similarly, using factor mixture modeling (which combines latent class analysis with factor analysis) to analyze parent reports of child ADHD symptoms in a national registry sample of male twins, Lubke et al. (2009) concluded that ADHD symptoms are dimensional and that children with ADHD are at the far end of this continuum. In contrast, Todd et al. (2001) latent class analysis of data from adolescent female twins yielded findings that could be interpreted as supporting a taxonic latent structure. Three studies have used Meehl's taxometric procedures to examine the latent structure of ADHD. Unlike latent class analysis, Meehl's taxometric procedures were specifically designed to test whether a construct has a taxonic or dimensional latent structure. Haslam et al. (2006) applied taxometric procedures to parent report data from two epidemiological samples. One sample consisted of children between the ages of 6 and 12, and the other was an adolescent sample (13–17). Their analyses yielded dimensional findings in both samples. Using parent and teacher reports as well as measures of cognitive ability, Frazier et al. (2007) examined the latent structure of each set of symptoms associated with ADHD (e.g., academic underachievement, sustained attention), as well as a combination of ADHD symptoms across these domains. Like Haslam et al., Frazier et al. found consistent evidence that both ADHD and its components had a dimensional latent structure. Most recently, Marcus and Barry (2011) used taxometric procedures to examine data from the NICHD Study of Early Child Care and Youth Development that were collected when the children were in third grade. This data set included parent reports, standardized tests of achievement, and laboratory measures. These taxometric analyses yielded dimensional results for inattention, hyperactivity/impulsivity, and ADHD as a whole. All of these ADHD studies that have yielded dimensional findings have used child or adolescent samples and the primary indicators in most of these studies have been parent reports of child symptoms. Given that ADHD is classified as a disorder “usually first diagnosed in infancy, childhood, or adolescence” (DSM-IV-TR; American Psychological Association, 2000), the focus on child and adolescent samples is appropriate. However, considering that the prevalence rate for adult ADHD is roughly half that for children and adolescents, it is conceivable that child and adolescent samples include individuals who do not belong in a putative ADHD taxon, but who have transient symptoms of inattention or hyperactivity/impulsivity, which might create the illusion of continuity across the ADHD spectrum. Perhaps a taxometric study of ADHD using an adult sample might reveal an ADHD taxon that was obscured in these child and adolescent samples. On the other hand, a replication of these dimensional results in an adult sample would strengthen the conclusion that ADHD and its various components are dimensional across the lifespan. Furthermore, whereas these other studies relied primarily on parent reports and not self-reports, a taxometric analysis of self-reported adult ADHD symptoms would test whether these dimensional findings are robust across multiple assessment methods.