This is a review that pulled together neurocognitive and brain imaging studies of how people with ADHD handle timing. It looked at children, teenagers and adults, so it is not one single group of people. ADHD is described as affecting about 5% of the population.
The authors group timing into three areas: motor timing, judging how long something lasts, and thinking ahead in time. They report that people with ADHD tend to do worse in all three. The most consistent problems were in keeping time with a beat, telling how long a sound or event lasted, reproducing a length of time, and choosing a smaller reward now over a larger one later.
The evidence is counted rather than pooled. Of 14 motor-timing experiments listed in a table, all but one found a real difference between the ADHD and comparison groups. Twelve studies of judging duration also found group differences.
The authors say these timing problems seem separate from, but linked to, skills like working memory, attention and self-control. Brain imaging is reported to show weaker activity in frontal, striatal, cerebellar and parietal networks tied to timing. The authors report that the stimulant methylphenidate eased most of the timing problems. The findings are not fully consistent, and one beat-keeping study found no group difference.
Source: Noreika V, Falter C, Rubia K. Timing deficits in attention-deficit/hyperactivity disorder (ADHD): Evidence from neurocognitive and neuroimaging studies. Neuropsychologia (2013). doi:10.1016/j.neuropsychologia.2012.09.036