A twin study involving 457 pairs has found that ADHD on its own was associated with a reduced ability to inhibit responses to stimuli, while reading disabilities were associated independently with weaknesses on measures of phoneme awareness, verbal reasoning, and working memory. Both disorders were associated with a slow processing speed, and there was a significant genetic correlation between RD and ADHD.
However, just to remind us that genetics are rarely solely the answer, another twin study, involving 271 pairs of 10-year-old identical and fraternal twins, has found evidence that the associations between ADHD symptoms, reading outcomes and math outcomes are a product of both genetic and common environmental influences. The researchers speculate that such environmental influences may include aspects of the classroom and homework environment.
(Submitted). Etiology and neuropsychology of comorbidity between RD and ADHD: The case for multiple-deficit models.
Cortex. 46(10), 1345 - 1361.
(2010). Exploring How Symptoms of Attention-Deficit/Hyperactivity Disorder Are Related to Reading and Mathematics Performance.
Psychological Science. 21(11), 1708 - 1715.