Researchers at the Institute for Behavioral Genetics, University of Colorado/Boulder, and the Colorado Learning Disabilities Research Center recently reported on some findings from their studies with reading-disabled students. In a study of 494 twins with a reading disability and 373 twins without a reading disability, Erik G. Willcutt and Bruce F. Pennington, found that individuals with a reading disability were more likely than those without one to meet the criteria for Attention Deficit Hyperactivity Disorder (ADHD). When these researchers evaluated the students in this study, they found that the association between reading disability and ADHD was stronger for symptoms of inattention than for symptoms of hyperactivity or impulsivity.
This was true for both boys and girls. However, boys with reading disabilities exhibited a higher prevalence of both inattention and hyperactivity/impulsivity. In terms of referrals, boys typically outnumber girls three or four to one. These researchers speculated that the hyperactive and impulsive behaviors exhibited more commonly by boys with reading disabilities are more disruptive than the inattentive behaviors exhibited by girls and therefore precipitate more referrals. Teachers and parents rated students similarly, indicating that the ADHD symptoms were seen both in school and at home and, therefore, are not attributable solely to academic frustration. No significant age differences in the relation between reading disabilities and ADHD were found.
Connection between reading disabilities and ADHD
In conclusion, Willcutt and Pennington report that reading disabilities and ADHD occur together significantly more often than would be expected on the basis of chance, but they have not found the cause of this connection. They recommend that future studies of the overlap between reading disabilities and ADHD examine the influence of gender, type of ADHD, and intelligence. Including these variables should narrow the diversity of the study population, enabling researchers to gain a better understanding of the nature of the association between reading disability and ADHD.
In a second study, researchers Sally J. Wadsworth, Richard K. Olson, Bruce F. Pennington, and John C. DeFries looked more closely at the relationship between IQ and reading disability. Previous research suggests that genetic factors may be more important as a cause of reading disabilities among children with higher IQ, and that environmental factors such as lead exposure may be responsible for reading problems in students with lower IQ. The purpose of this study was to test more rigorously whether the cause of reading disabilities differs as a function of IQ. To do this, they used a larger sample of same-gender twin pairs than previous studies did.
They compared 223 pairs of identical twins and 169 pairs of same-gender fraternal twins in which at least one twin in each pair had been diagnosed with a reading disability. The twins were between eight and 20 years of age and were reared in primarily English-speaking, middle-class homes. Overall, more than half the reading deficit of both groups of twins was due to heredity. However, when the students were split into two groups based on IQ above 100 and IQ between 85-100, there was a significant difference between the groups. Heredity played a greater role in reading disabilities in children with higher IQs. Wadsworth et al. conclude that there is a difference in the amount of influence heredity exerts on students with reading disabilities and that this difference is associated with IQ. Genetic influences appear to be more important as a cause of reading disability among children with higher IQ scores, but this study did not determine if there are different genetic influences related to IQ level.
Use of IQ can help classify reading disability
These researchers report that research into the relationship between IQ and reading disability is important because of the use of IQ scores in the classification of learning disabilities. Students who do not meet the criteria for a significant discrepancy between ability and achievement are often denied special-education services. Results of this study suggest that knowing a child’s IQ may tell us something about the cause of the reading deficit and, in the future, might help focus intervention and remediation efforts. Although it has been suggested that individuals with higher IQ may respond better to remediation, some studies have found that children with below average achievement for their age rather than those who have large ability-achievement discrepancies actually show greater progress. The use of IQ scores in the definition and diagnosis of reading disabilities may be relevant for suggesting differing causes for learning problems, but Wadsworth et al. write that this should not preclude the availability of services to those whose deficit is less specific.
“Comorbidity of Reading Disability and Attention-Deficit/Hyperactivity Disorder: Differences by Gender and Subtype” “Differential Genetic Etiology of Reading Disability as a Function of IQ” Journal of Learning Disabilities Volume 33, number 2, March/April 2000
Published in ERN May/June 2000 Volume 13 Number 5