OBJECTIVE: It has been suggested that variation in catechol O-methyltransferase (COMT) activity associated with variation in COMT Val158Met genotypes may result in enhanced or reduced cognitive performance, depending on whether the phenotype requires cognitive stability or cognitive flexibility. The authors’ goal was to determine whether, in confirmation of this prediction, performance on a measure of cognitive stability would be associated with Met loading. METHOD: COMT genotyping was investigated in relation to a measure of reaction time variability on the Continuous Performance Test, Identical Pairs Version, in a large and representative sample of 527 young men (mean age=21 years). RESULTS: Met loading was associated with reduced reaction time variability. CONCLUSIONS: Met genotype loading may confer enhanced "tuning" or greater stability in performance, possibly by stabilizing active neural representations in the prefrontal cortex during tasks involving working memory.