ADHD and Reading Difficulty: Nature of Their Relationship

Introduction

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 or factors is likely responsible.

Greven and colleagues (2012) agree that the evidence is indeed strong and that these traits therefore have a genetic origin, but they disagree with the theory that a single stable genetic factor could be responsible. To support their argument, they point to evidence suggesting these traits are unstable over time. Although the authors discussed a recent twin study that revealed these traits to be stable between the ages of 6 and 9, they argue that this is too short a time span — and possibly too small 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 the two are expressed in the same children so frequently.

Study Background and Hypotheses

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 paper critiques their research methods, findings, and conclusions in order to determine whether they found sufficient evidence to support their hypotheses. If these traits are unstable and ADHD is a better predictor of reading difficulty than reading difficulty is of ADHD, then this may have a significant impact on how reading problems are addressed in these children in the future.

The hypotheses being tested are described in a clear manner. First, the directionality of the traits was tested to see whether one trait occurs significantly more often than the other. If evidence of directionality were found, 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 they would find evidence for a partial genetic association. The third hypothesis tested was whether reading difficulty had a stronger association with attention-deficit traits or with hyperactivity traits, with the authors predicting a stronger association between inattention and reading difficulty.

Research Methods and Design

The use of a twin cohort as an experimental model is a common and widely accepted approach for genetic analysis, as Greven et al. (2012) discussed in their article. This can also be informally supported by a keyword search on PubMed using "twin study," which retrieves thousands of citations.

Approximately 7,000 children between the ages of 7 and 8 were enrolled in the study and followed for four years. Both monozygotic and dizygotic twins with all gender pairings were enrolled. 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. It also controls for pre-existing associations between traits, as well as shared and non-shared environmental conditions.