This study tested adults with persistent ADHD who had never taken stimulant medication. There were 49 of them, 26 men and 23 women. They were compared with 49 adults without ADHD, matched for age and gender. Both groups took a set of executive-function tests and some simpler tests.
At first, the ADHD group did worse in several areas. The largest gaps were in holding back an automatic response, in ignoring distracting information and in switching between tasks. Smaller gaps showed up in verbal working memory.
The ADHD group had a lower average IQ. So the researchers ran stricter checks that took IQ and the simpler task demands into account. One response-control task and the distraction task still showed clear gaps. A second response-control task and both working memory tasks no longer passed the study's strict cutoff. The task-switching gap held, though it fell just short of large. Even so, the authors' overall summary says only the inhibition effects proved robust. They suggest adult ADHD may be mainly a disorder of inhibition.
Tests of planning, fluency, spatial working memory, card sorting and slow circle drawing showed no group differences.
The authors note limits. The IQ gap means the results need repeating in IQ-matched groups. No other clinical groups were included, so it is unclear whether the inhibition problem is specific to ADHD. One task varied widely in performance, which raises doubts about its reliability.
The numbers Stats and what the terms mean
The statistics behind the plain-language claims above.
- Boonstra, 2010: the cutoff a result had to pass to count, p <.004 with an effect size greater than .06.
- Boonstra, 2010: holding back an automatic response (Continuous Performance Test), before controls, F(1, 96) =21.69, p =.000, partial eta-squared =.18.
- Boonstra, 2010: the same task after controlling for IQ, F(1, 95) =18.41, .000/.16; after controlling for simpler task demands, F(1, 95) =28.41, .000/.23.
- Boonstra, 2010: ignoring distracting information (Stroop Color-Word Test), before controls, F(1, 96) =20.17, .000/.17; after controlling for IQ, F(1, 95) =16.56, .000/.15.
- Boonstra, 2010: switching between tasks (Change Response of the Change Task), before controls, F(1, 96) =18.45, .000/.16; after controlling for IQ, F(1, 95) =14.75, .000/.13; after controlling for simpler task demands, F(1, 95) =14.26, .000/.13.
- Boonstra, 2010: the second response-control task (Change Task Stop Signal Reaction Time), before controls, F(1, 96) =10.56, .002/.10; after controlling for IQ, F(1, 95) =7.20, .009/.07.
- Boonstra, 2010: verbal working memory (Digit Span-Backwards), before controls, F(1, 96) =10.06, .002/.10; after controlling for IQ, .012/.06; after controlling for simpler task demands, .185/.02.
- Boonstra, 2010: verbal working memory (Letter and Number Sequencing), before controls, F(1, 96) =9.76, .002/.09; after controlling for IQ, .015/.06; after controlling for simpler task demands, .195/.02.
- Boonstra, 2010: distraction task after controlling for simple word-naming speed, F(1, 95) =17.48, p =.000, partial eta-squared =.16. What the terms mean:
- F A test score comparing the two groups; a bigger number means a clearer difference between them.
- p How likely a difference this size would be if it were only down to chance; smaller means less likely to be chance.
- partial eta-squared The share of the variation in scores that is linked to being in the ADHD group or not; larger means a bigger difference. Pairs written as ".000/.16" give the p value first and the partial eta-squared second.
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