Researchers compared 49 adults with persistent ADHD who had never taken stimulant medication (26 men, 23 women; average age 38.7) with 49 adults of similar age and gender who did not have ADHD (average age 38.1). Everyone completed a battery of tests. Some measured executive function, such as holding back an automatic response, ignoring distractions, and switching between tasks. Others measured non-executive skills.
Before any adjustments, the ADHD group did worse on several tests. The differences were large for holding back a response and for switching tasks, and medium for verbal working memory, which is holding and reworking spoken information in mind.
The researchers then adjusted for IQ and for the non-executive demands of each task. After that, the gaps in inhibition and task switching stayed strong (partial eta-squared .13 to .23), but the verbal working memory gap and one inhibition measure no longer held up. The authors suggest adult ADHD may be mainly a disorder of inhibition, while noting the task-switching gap also remained.
Some limits: the groups differed in IQ at the start (100.6 for the ADHD group, 107.71 for controls), no other clinical group was tested, so the inhibition finding may not be specific, and one inhibition task varied a lot.
Source: Boonstra A, Kooij J, Oosterlaan J, et al.. To act or not to act, that’s the problem: Primarily inhibition difficulties in adult ADHD.. Neuropsychology (2010). doi:10.1037/a0017670
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