Increased intraindividual variability is a marker of ADHD but also of dyslexia: a study on handwriting.

It has been suggested that intraindividual variability (IIV) in neuropsychological tasks may be a specific characteristic of Attention-Deficit Hyperactivity Disorder (ADHD), but previous research has not thoroughly examined whether IIV also concerns academic performance or other types of developmental disabilities. The present study investigates the role of IIV in 15 children with ADHD without reading difficulties, 15 children with dyslexia without associated symptoms of ADHD, and 15 typically developing children (TDC) in a simple response time (SRT) task and in a skill more directly related with school learning-handwriting. Results show that children with ADHD and those with dyslexia have a greater IIV than the TDC in both tasks. However, the pattern of the relationship between IIV in SRT and handwriting was different in children with ADHD and dyslexia: the IIV in the handwriting task was found to depend on IIV in the SRT task only in children with dyslexia. These findings support the crucial role of IIV not only in ADHD but also in other developmental disabilities, but suggest that in children with ADHD it may present specific aspects related with motor control.