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.
Reference:
[2066]
(Submitted). Etiology and neuropsychology of comorbidity between RD and ADHD: The case for multiple-deficit models.
Cortex. 46(10), 1345 - 1361.
[2046]
(2010). Exploring How Symptoms of Attention-Deficit/Hyperactivity Disorder Are Related to Reading and Mathematics Performance.
Psychological Science. 21(11), 1708 - 1715.