"......Hensler and colleagues from Florida State University conducted a study of 1024 first grade twin pairs from the Florida Twin Project on Reading. In this study, all subjects in the study completed the Stanford Achievement Test-Reading subtest (SAT-10) a validated measure of reading ability.
Analyses of reading performance were compared for overall reading ability and presence of reading disorder defined by performance at the 15%tile or lower on the SAT-10. These analyses found the following contributions:
For general reading ability, 53% of the performance was due to genetic factors, 25% was due to shared environmental factors while 21% was due to unshared environmental factors
The concordance rates for reading disorder in the monozygotic twins was .58 while for the dizygotic twins it was .32 indicating a significant genetic contribution to dyslexia
As noted in the post on the epidemiology of reading disorder, understanding the overlap between reading disorder and ADHD is important. A twin study from the Colorado Learning Disabilities Research Center is informative for understanding the overlap in these two developmental disorders.
Erik Willcutt along with colleagues from the University of Colorado, Regis University and Massachusetts General Hospital conducted a analysis of 457 twin pairs. They conducted a study starting from six cognitive domains:
phoneme awareness (ability to hear, identify and manipulate the smallest units of sound)
verbal reasoning
working memory
inhibitory control
processing speed
naming speed
They found in their analysis confirmation of previous findings that "measures of single-word reading, inattention, and hyperactivity-impulsivity are highly heritable."
Additionally, their twin analysis found that reading disorder was linked to independent deficits in phoneme awareness, verbal reasoning and working memory. ADHD was linked independently to deficits inhibitory control, while reading disorder and ADHD shared a common deficit in processing speed....."
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