A study compared 49 adults who had ADHD and had never taken stimulant medication with 49 adults of the same age and gender who did not have ADHD (mean age about 38). Both groups took a set of thinking tests. Some tested executive function, the mental control needed to hold back a response or switch between tasks. Others tested simpler skills.
At first the ADHD group did worse on several tasks. The biggest gaps were in holding back an automatic response, in ignoring distracting information, and in switching between rules, each a large effect (partial eta-squared .18, .17 and .16). There were smaller gaps in verbal working memory.
The researchers then adjusted for IQ and for how each group did on the simpler tasks. After this, the gaps in holding back a response stayed (partial eta-squared up to .23), and the task-switching gap also stayed (partial eta-squared .13). The verbal working memory gap and one inhibition measure no longer held up. The authors suggest adult ADHD may be mainly a problem with inhibition.
The two groups differed in IQ (100.6 versus 107.71), which the authors flag as a limit that needs testing in IQ-matched groups. The study also could not show the inhibition problem is unique to ADHD, because no other clinical group was included.
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