¶ … ADHD and Reading Difficulty Learning Disabilities

The Nature of the Relationship between ADHD and Reading Difficulty

The Nature of the Relationship between ADHD and Reading Difficulty

There is substantial evidence showing a strong association between reading difficulty and attention problems in children and adolescents (reviewed by Greven, Rijsdijk, Asherson, and Plomin, 2012). This learning-related 'comorbidity' is believed to be primarily genetic in nature and several studies have presented evidence consistent with this theory. Since the association is so strong, some scientists have argued that the same genetic factor(s) is likely responsible.

Greven and colleagues (2012) agree that the evidence is indeed strong and therefore have a genetic origin, but not necessarily with the theory that a single stable genetic factor could be responsible. To support their argument, they point to evidence that suggests these traits are unstable over time. Although the authors discussed a recent twin study, which revealed these traits to be stable between the ages of 6 and 9, they argue that this is too short of a time span, and possibly too small of a sample, to determine whether the expression of these learning traits can be modified by genetic or environmental factors over time. In addition, they are concerned that ADHD could be playing a stronger role in the emergence of comorbidity, compared to reading difficulty, which may explain why they are expressed in the same children so frequently.

Summary

To further investigate these possibilities, Greven and colleagues (2012) analyzed a larger cohort of twins across a longer period of time to better understand how stable these traits are in relation to each other. This report will critique their research methods, findings, and conclusions in order to determine whether they have found sufficient evidence to support their hypotheses. If these traits are unstable and ADHD is a better predictor of reading difficulty than reading difficulty, then this may have a significant impact on how reading problems are addressed in these children in the future.

Critique

The use of a twin...

This claim can also be informally supported by a keyword search on Pubmed using 'twin study', which retrieved 7,434 citations.
The hypotheses being tested are also described in a clear manner. First, the directionality of the traits will be tested to see if one trait occurs significantly more often than the other. If there is evidence of directionality, then a reasonable conclusion would be that only one trait has a genetic origin. The authors predicted bidirectionality. The second hypothesis tested was how strong the genetic association is between ADHD and reading difficulty in the twin cohort. The authors predicted that they would find evidence for a partial genetic association. The last hypothesis tested was whether reading difficulty had a stronger association with attention-deficit or hyperactivity traits. The authors predicted a stronger association between inattention and reading difficulty.

Approximately 7,000 children between the ages of 7 and 8 were enrolled into the study and followed for 4 years. Both monozygotic and dizygotic twins with all gender pairings were enrolled in the study. Twin relationships for this cohort had already been confirmed by genotyping in a previous study. Parents rated their children in terms of ADHD symptoms and teachers rated reading difficulty. One of the models used to analyze the raw data was the 'cross-lagged twin model.' This model is useful because it is quantitative and can determine relative trait stability across time. This model also controls for preexisting associations between these traits, as well as shared and non-shared environmental conditions.

The results revealed that traits individually were stable across 4 years of development, with ADHD having a correlation of 0.67 and reading 0.48. The between trait correlations were 0.23 in middle childhood and 0.25 in early adolescence (see Table 1) and therefore stable across time. The 95% confidence intervals for these between trait correlations all overlapped, which means there was no significant difference.

Using the cross-legged model,…