Comorbid behaviour and learning difficulties and their influence on educational performance

Despite the consistency of the association between externalising behaviour and educational performance, the nature of the association has been more difficult to determine. This is partly to do with the interrelationships between hyperactivity and LD, as well as between hyperactivity and CD. A number of epidemiological studies have reported that these relationships are unlikely to occur purely by chance (eg Rutter et al., 1970; McGee & Share, 1988). A study by Rapport, Scanlan & Denney (1999) reported that between 9-48% of children with ADHD have co-occurring learning problems, although this relationship is strongly determined by the criteria used to define both conditions. A similar relationship between CD and LD is estimated to be between 11-61% (Rutter & Yule, 1970), and children with SRR are five times more likely to exhibit antisocial behaviours than children in the general population. As such, it would be difficult to consider the relationship between

externalising behaviours and educational performance without considering the influence that comorbid LD might have on the outcomes.

Studies investigating the associations between behaviour and LD have used cross-sectional, follow-up or longitudinal methodologies. Cross-sectional methodologies frequently compare a group with both disorders against groups with one or other disorder, and most of these tend to focus on ADHD/hyperactivity and RD which are measured against cognitive deficits and/or neuropsychological impairment (Chadwick et al., 1999). Some studies have used this methodology to differentiate between different types of externalising behaviour (eg McGee et al., 1984) or between attention deficit problems either with or without hyperactivity (eg Barkley et al., 1990a).

For instance, using a cross-sectional methodology McGee et al (1984) compared four different groups of 7 year old boys from the Dunedin New Zealand birth cohort study, using threshold scores on the Rutter questionnaires, which were completed by parents and teachers. They found that boys who were hyperactive only or aggressive-hyperactive performed worse on cognitive tasks than those who were aggressive only or had no problems. They also found that both hyperactive groups showed evidence of early cognitive impairment, which was not found in the aggressive only group. Problem behaviours were still apparent in a follow-up study two years later, although only boys in the two hyperactive groups performed significantly worse on spelling and reading tasks, and those in the mixed group were significantly likely to show specific reading retardation. From this McGee et al (1984) concluded that the hyperactive and aggressive behaviours were distinguishable from one another according to behaviour and impaired educational performance. Further support comes from their later review of the literature, from which they concluded that the interrelationship between ADD (with and without hyperactivity) and LD (comprising spelling, reading, mathematics and general literacy problems) was sufficient to suggest that the two conditions are interdependent (McGee & Share, 1988). Moreover, the authors also suggested that the two

disorders were not likely to be causally related, and any association was likely to be a consequence of some other common factor.

Other studies have found that hyperactivity and LD result in similar outcomes, and it has been suggested that the two are so highly correlated that this may interfere with functioning in a similar way. A study by Barkley et al (1990a) reported that those with disorder (ADD or LD) differed significantly from controls on academic attainment measures of reading, writing and mathematics tests. However, there were no significant differences in the performance of groups of children with either ADD (with or without hyperactivity) or LD, lending support to an overall association between behaviour and LD in contributing to impaired educational performance. Support for this conclusion comes from a study by Rowe & Rowe (1992), who also found that the negative effects of ADD and LD could be moderated by the use of home reading and improved attitudes to reading, a finding which has important implications for treatment of both conditions.

A number of studies have attempted to evaluate the underlying mechanisms associated with both disorders, to ascertain whether or not there are differences in the way in which they impact on educational performance. The most commonly reported outcome suggests that whilst mixed disorder groups (typically comprising hyperactivity and RD) perform worse on outcome measures related to educational performance, the pattern of deficits are most alike the RD alone group (eg McGee, Williams, Moffitt & Anderson, 1989; Shaywitz, Fletcher, Holahan et al., 1995). The extent to which these mixed disorder groups are similar to pure ADHD groups is less clear. A study by August & Garfinkel (1990) attempted to look at this relationship in a sample of boys (aged 7-17) referred for ADHD, of whom 39% had a co-occurring specific reading disability. They found that the comorbid group (ADHD + RD) and pure ADHD group performed worse on a number of tasks, such as word recognition and sequential processing tasks (using words and letters), than the control group. As these cognitive skill deficits are generally characteristic of RD, they concluded that a RD group

would also have performed similarly to the comorbid group. On executive type cognitive tests, they identified an ordering of responses, with the comorbid group performing worst, followed by the pure ADHD group and with the normal controls performing best. The authors concluded that the two types of cognitive deficit in the comorbid group reflected different levels of processing theories, whereby the RD group showed deficits in basic skills, whereas the ADHD group had difficulties in skill acquisition.

An alternative approach to identifying the relationship between behaviour and RD was used by Willcutt & Pennington (2000). Their study used concordant and discordant twin pairs for RD, whereby the twin with no RD formed part of the familial control group. Measures of psychopathology included the Child Behaviour Check List (CBCL, Achenbach, 1991), and age- appropriate standard IQ tests across 8-18 year olds in community and clinic referred samples. They found statistically significant relationships between RD and all measures of psychopathology, although this relationship was particularly marked for ADHD and more commonly reported in males in both clinic and nonclinic groups. Similarly to Frick et al (1991), they concluded that the relationship between RD and other externalising behaviours was mediated by their comorbidity with ADHD and other familial factors, such as low SES. As internalising behaviours were only related to RD, the authors suggested that these may be a consequence of the negative experience of poor school performance in children with RD, causing them to become depressed which was similarly expressed by Rutter et al (1970).

Despite the associations with RD, comparatively few studies have investigated the outcome between behaviour and mathematics ability. The findings have been variable, and contribute to some of the confusion surrounding what underlying causal mechanisms result in poor educational performance. A study by Morgan et al (1996) using DSM-IV criteria for ADHD subtypes found clinically referred boys with ADHD-I had more problems with mathematics tasks when compared to those with ADHD-CT. Unfortunately no comparisons were made with boys in the ADHD-HI group.

as this group was too small to be statistically comparable, which somewhat weakens their findings of differences amongst subtypes and relationships to different underlying cognitions, and makes comparisons with other studies problematic. As such, a later cross-sectional study by Marshall et al (1997) showed that poor mathematics performance in children aged 6-12 referred to neuropsychiatrie clinics for behaviour and LD, differed according to DSM-III ADD subtype (either with or without hyperactivity). Specifically, they found that those with ADD (no hyperactivity) performed worse on the mathematics tests than those with hyperactivity. This finding is in conflict with the majority of studies, which tend to report that children with hyperactivity are more impaired. The authors suggest that as this study measured mathematical ability, which is a specific cognitive skill, it might be that this is not influenced by the relationship between hyperactivity and learning problems. However Marshall et al (1997) also suggest that as there are few studies examining the relationship between educational performance and ADD (no hyperactivity), the lack of supporting evidence may reflect the comparatively lower levels of diagnosis of this subtype, rather than assuming that the relationship does not exist.

Clearly there are a number of studies suggesting that the association between behaviour and educational performance is strongly influenced by the overlap between hyperactivity and LD (particularly in relation to RD), as alluded to in some of the studies mentioned previously. This relationship is also recognised in the diagnostic criteria for ADHD, although it is not essential for diagnosis. The extent to which this relationship exists is heavily dependent on the methods used to identify LD, which tend to vary between studies making direct comparisons unreliable as criteria vary considerably between studies. This is well illustrated in a study by Semrud-Clikeman et al (1992) who showed the importance of how outcomes vary with differently defined LD criteria. They compared three groups of children (< 12 years), being those referred for ADHD (according to DSM-III criteria) or academic problems, and normal controls using three different methods of ascertaining LD. They found that when LD was loosely defined (“WRAT-R reading and

arithmetic > or equal to 10 standard score points below full scale IQ”), all three groups showed significantly high levels of RD and MD, although those with disorder performed marginally worse than those without overall. The two other methods of measuring LD were better at distinguishing between those with and without disorder, although only one method found those with disorder performed worse on measures of RD and MD (“WRAT-R reading and arithmetic of > or equal to 20 standard score points below full scale 10”).

Overall, cross-sectional studies have predominantly found that mixed disorder groups (ADHD and LD) perform worse overall on the various measures of educational performance. There are also some findings indicating that not all the relationships between poor educational performance and behaviour are accounted for by its comorbidity with LD. There is also considerable support for the interdependence of these two disorders, even though they appear to be highly correlated.

2.5.5 Comorbidity of externaiising behaviours and different