Each person has their own unique style of handwriting, whether it is everyday handwriting or their personal signature. Even identical twins who share appearance and genetics do not have the same handwriting. The place where one grows up and the first language one learns meld together with the different distribution of force and ways of shaping words to create a unique style of handwriting for each person.

Children with ADHD have been found to be more likely to have less legible handwriting, make more spelling errors, more insertions and/or deletions of letters and more corrections. In children with these difficulties, the letters tend to be larger with wide variability of letters, letter spacing, word spacing, and the alignment of letters on the baseline. Variability of handwriting increases with longer texts. 

Fluency of the movement is normal but children with ADHD were more likely to make slower movements during the handwriting task and hold the pen longer in the air between movements, especially when they had to write complex letters, implying that planning the movement may take longer.

Children who have ADHD were more likely to have difficulty parameterising movements in a consistent way. This has been explained with motor skill impairment either due to lack of attention or lack of inhibition. To anticipate a change of direction between strokes constant visual attention is essential. With inattention, changes will occur too late, resulting in higher letters and poor alignment of letters on the baseline. The influence of medication on the quality of handwriting is not clear.

On a purely mechanical level, the symptomatic mechanisms of ADHD are generally understood. ADHD is generally the result of neurological dysfunction, more specifically, processes related to the production and use of dopamine in the brain. With that said, most ADHD cases are of unknown causes.

Graphology is the pseudoscientific study and analysis of handwriting in relation to human psychology. Graphology is primarily used as a recruiting tool in the applicant screening process for predicting personality traits and job performance, despite research showing consistently negative results for these uses.