Reaction Time And ADHD
The signature of ADHD on RT data is variance, not speed.
TL;DR — People with ADHD post mean reaction times only slightly slower than controls (10–20 ms), but their standard deviation is 30–60% higher — they have more very-slow attempts.
If your average looks normal but every fifth attempt is 100 ms slower than the others, that pattern matches the published ADHD signature far more than it matches "slow reflexes".
What the meta-analyses show
Kofler et al. (2013) pooled 319 studies on RT in ADHD. Mean-RT effect size: d = 0.34 (small-to-moderate). Intra-individual variability (IIV) effect size: d = 0.76 (large). The variability finding has replicated in over 100 independent studies and is one of the most robust effects in clinical cognitive science.
Why variability matters more than mean
- Mean RT can be normalised by removing the slowest attempts — variability cannot.
- IIV correlates with real-world inattention better than any single mean number.
- Stimulant medication primarily reduces variability, not peak speed.
What this means for testing
A standard 5-attempt web test will frequently miss the ADHD signature because the slowest attempts get dropped. A 20–30 attempt test that reports SD alongside the mean is far more informative.
Important caveats
- No reaction-time test diagnoses ADHD. Variability is a marker, not a diagnostic.
- Sleep deprivation produces a similar IIV pattern in non-ADHD adults.
- Caffeine reduces IIV in both ADHD and non-ADHD populations.
Frequently Asked Questions
Can a fast reaction time rule out ADHD?
No. Mean speed can be normal in ADHD; the marker is consistency.
Why does Ritalin help reaction time?
It boosts dopaminergic tone in prefrontal circuits that govern attention regulation, narrowing the slow-tail attempts.
Is high variability always pathological?
No. Tired adults, caffeine-withdrawn adults and teens all show elevated IIV without any disorder.
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