Cognition has been widely studied in ADHD (for reviews, see Barkley
1997; Castellanos et al.
2006; Sergeant et al.
2003; Wilcutt et al.
2005). This research has shown, among others, problems with inhibitory control, working memory, vigilance, reward sensitivity (delay aversion), and time processing and currently focuses on different cognitive endophenotypes of ADHD (i.e. biologically based phenotypes that carry genetic loadings and index liability to disease; see Castellanos and Tannock
2002). Oppositional Defiant Disorder (ODD), Conduct Disorder (CD) and high levels of physical aggression have also been associated with cognitive difficulties such as language-based verbal skills and executive function (EF) (Henry and Moffitt
1997; Hill
2002; Séguin et al.
1999). Controversy still exists about whether verbal and, particularly, EF deficits are present in ODD/CD without comorbid ADHD (Pennington and Ozonoff
1996). Since Pennington and Ozonoff’s review on EF in different forms of developmental psychopathology, an increasing number of studies have taken into account the high comorbidity of ODD/CD and ADHD. This has resulted in reports of both qualitative and quantitative differences in EF-dysfunction between disorders. For example, Van Goozen and colleagues (
2004) reported the absence of problems with Working Memory (WM) and planning in a group of children with Oppositional Defiant Disorder (ODD) and a group with combined ODD/ADHD compared to healthy controls. Both are executive functions that have frequently been reported to be deficient in ADHD. Oosterlaan et al. (
2005) compared ADHD, ODD/CD, and ADHD+ODD/CD groups and found that WM and planning deficits were specific to ADHD (and explained by ADHD in the combined disorder). Distinguishing ADHD and ODD/CD is more difficult when it concerns inhibitory control (Oosterlaan et al.
1998). Inhibition has been defined and operationalized in a number of different ways, e.g. as stopping an ongoing response, interference control and inhibition of prepotent responses (Barkley
1997), and studies have generally not included tasks measuring such different forms of inhibition. Furthermore, inhibition has been listed as one of the executive functions (e.g. Pennington and Ozonoff
1996), but has also been described as an important cognitive ability underlying many different executive functions (Zelazo et al.
1997). Lack of inhibitory control has been proposed as the central deficit in both ADHD (Barkley
1997) and ODD/CD/physical aggression (e.g. Lau et al.
1995), but is not necessarily similarly dysfunctional in both types of disorders. Nigg (
2003) has suggested that ADHD is predominantly associated with dysfunctional executive inhibition, whereas conduct problems are predominantly associated with dysfunctional motivational inhibition. Nonetheless, a ‘motivational’ pathway (or endophenotype) to cognitive deficits (e.g. altered reward sensitivity) in ADHD has also been identified (Solanto et al.
2001; Sonuga-Barke
2002). ODD/CD and ADHD may be distinguished more easily on other aspects of emotional or social information processing. As a starting point it may therefore be good to use a theoretical framework that, in addition to motivation, accommodates more different aspects of emotional or social information processing. Such a model has been proposed by Zelazo and Müller (
2002).