Children who are having difficulties reading and focusing in school are commonly diagnosed with both Attention-Deficit/Hyperactivity Disorder (ADHD) and a Reading Disorder (RD). Both disorders share many common traits and appear very similar when children are assessed. The reason for this correlation has remained unknown. However, a new study in the latest issue of Cortex, dedicated to "Developmental Dyslexia and Dysgraphia," has suggested that the disorders have common genetic influences. In addition, both disorders often lead to slow processing speed (i.e., the inability to complete tasks quickly).
The researchers in the study looked at 457 pairs of twins from the Colorado Learning Disabilities Research Center (CLDRC) twin study. The CLDRC twin study is an ongoing study of the causes of reading disabilities, ADHD, and other related disorders. Dr. Erik Willcutt and his colleagues compared groups of participants with and without RD and ADHD, using a variety of tests, including tests assessing cognitive ability, processing speed, reading, and language skills. The researchers then analyzed results from pairs of twins within those groups to determine the genetic causes, if any, of the correlations found. The use of identical twins, who share all of their genes, and non-identical twins, who share only half of their genes, allowed the researchers to distinguish between genetic and environmental influences on the participants' various abilities assessed.
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 exhibit self-control. RDs were associated with various weaknesses in language and memory. however, both disorders were associated with a slow processing speed. the twin analyses further revealed a significant genetic correlation between RD and ADHD. The researchers suggest that processing speed my therefore be a useful marker to look for in future studies of the connection between the two disorder.
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