Common genetic influences for ADHD and reading disability

A new twin-study has found that their co-occurrence may be linked to slow processing speed.

Milan, Italy, 8 December 2010 – Attention Deficit Hyperactivity Disorder (ADHD) and developmental reading disability (RD) are complex childhood disorders that frequently occur together; if a child is experiencing trouble with reading, symptoms of ADHD are often also present. However, the reason for this correlation remains unknown. A new study reported in the latest special issue of Cortex (http://www.sciencedirect.com/science/journal/00109452), dedicated to “Developmental Dyslexia and Dysgraphia”, has suggested that the disorders have common genetic influences, which may also lead to slow processing speed – the brain taking longer to make sense of the information it receives.

The researchers looked at 457 pairs of twins from the Colorado Learning Disabilities Research Center (CLDRC) twin study – an ongoing study of the causes of reading disabilities, ADHD, and related disorders. Dr Erik Willcutt and colleagues compared groups of participants with and without RD and ADHD, using a variety of tests to measure general cognitive ability, processing speed, reading and language skills, and then analysed results from pairs of twins within those groups to determine the genetic causes of any correlations. The use of identical twins, who share all their genes, and non-identical twins, who share only half their genes, allowed the researchers to distinguish between genetic and environmental influences on the participants’ cognitive abilities.

The findings showed that both RD and ADHD are complex disorders, influenced by many factors; ADHD on its own was associated with a reduced ability to inhibit responses to stimuli, while reading disabilities were associated with various weaknesses in language and memory. However, both disorders were associated with a slow processing speed and the twin-analyses further revealed a significant genetic correlation between RD and ADHD, i.e., a participant with one of the disorders was more likely to show symptoms of the other. The authors of the study suggest that processing efficiency may therefore be a useful marker to look for in future studies of the connection between the two disorders.

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Notes to Editors:
The article is “Etiology and neuropsychology of comorbidity between RD and ADHD: The case for multiple-deficit models” by Erik G. Willcutt, Rebecca S. Betjemann, Lauren M. McGrath, Nomita A. Chhabildas, Richard K. Olson, John C. DeFries and Bruce F. Pennington, and appears in Cortex, Volume 46, Issue 10 (November/December 2010), published by Elsevier in Italy. Full text of the article featured above is available to members of the media upon request. Please contact the Elsevier press office, newsroom@elsevier.com. To schedule an interview, contact Dr Erik G Willcutt, willcutt@colorado.edu.

About Cortex
Cortex is an international journal devoted to the study of cognition and of the relationship between the nervous system and mental processes, particularly as these are reflected in the behaviour of patients with acquired brain lesions, normal volunteers, children with typical and atypical development, and in the activation of brain regions and systems as recorded by functional neuroimaging techniques. It was founded in 1964 by Ennio De Renzi. The Editor in-chief of Cortex is Sergio Della Sala, Professor of Human Cognitive Neuroscience at the University of Edinburgh. Fax: 0131 6513230, e-mail: cortex@ed.ac.uk. Cortex is available online at http://www.sciencedirect.com/science/journal/00109452.

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