Saturday, February 06, 2016

Twin Studies of Reading Disorder (Dyslexia)

Bill Yates
Brain Posts
Posted 16th October 2012

In a previous post, I reviewed recent research into the prevalence of reading disorder, commonly called dyslexia in children and adolescents.  The estimated prevalence for dyslexia in girls is estimated at about 7%, while in boys it is estimated as high as 14%.  Research confirms high rates of dyslexia in both boys and girls with a diagnosis of ADHD.

Twin studies can provide valuable insight into the contribution of genetic and environmental differences in a variety of developmental disorders.  The role of genetic influences can be determined by analyzing concordance rates between monozygotic twin pairs compared to dyzygotic twin pairs.

Support for a genetic contribution is found when the rates of concordance for a disorder is higher in monozygotic twins than in dizygotic twins.

There are several excellent twin cohorts that have been studied for reading ability and reading disorders.  I will summarize some of the recent research on this topic from two recently published free access manuscripts.

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.

The note in the discussion, that this finding might inform study design using molecular genetic approaches to reading disorder and ADHD.  Molecular genetic analyses might benefit from the collection of measures of cognitive function.   Specifically, processing speed measures may aid identification of genes that increase risk of both reading disorder and ADHD.

In the next post, I will review some of the brain imaging findings linked to reading disorder to better understand the anatomical and brain functional correlates of this developmental disorder.

Photo of the Florida scrub jay at Oscar Scherer State Park from the author's files.

Hensler BS, Schatschneider C, Taylor J, & Wagner RK (2010). Behavioral genetic approach to the study of dyslexia. Journal of developmental and behavioral pediatrics : JDBP, 31 (7), 525-32 PMID: 20814252.

Willcutt EG, Betjemann RS, McGrath LM, Chhabildas NA, Olson RK, DeFries JC, & Pennington BF (2010). Etiology and neuropsychology of comorbidity between RD and ADHD: the case for multiple-deficit models. Cortex; a journal devoted to the study of the nervous system and behavior, 46 (10), 1345-61 PMID: 20828676.

See the original article:

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