A new study has pointed out that Attention Deficit Hyperactivity Disorder (ADHD) and developmental reading disability (RD) mostly occur together in children.

If a child is experiencing trouble with reading, symptoms of ADHD are often also present. However, the reason for this correlation remains unknown.

The new study has suggested that the childhood disorders have common genetic influences, which may also lead to slow processing speed - the brain taking longer to make sense of the information it receives.

The researchers looked at 457 pairs of twins from the Colorado Learning Disabilities Research Center (CLDRC) twin study - an ongoing study of the causes of reading disabilities, ADHD, and related disorders.

Erik Willcutt and colleagues compared groups of participants with and without RD and ADHD, using a variety of tests to measure general cognitive ability, processing speed, reading and language skills, and then analysed results from pairs of twins within those groups to determine the genetic causes of any correlations.

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 inhibit responses to stimuli, while reading disabilities were associated with various weaknesses in language and memory.