Psychometric IQ (WAIS-III), onset and peak latency of the lateralized readiness potential (LRP), decision time, and accuracy were assessed during an Eriksen Flanker task in a young (149 families) and in an older (122 families) cohort of twins and their siblings. Stimulus-response incongruency effects were found on all measures of processing speed and accuracy. The effects on the percentages of wrong button presses and too slow (>1,000 ms) responses were larger in the older than in the younger age cohort. Significant heritability was found for processing speed (33-48%), accuracy (41%), and stimulus-response incongruency effects (3-32%). Verbal and performance IQ correlated significantly with stimulus-response incongruency effects on accuracy (-0.22 to -0.39), and this correlation was completely mediated by an underlying set of common genes. It is concluded that measures of the ability to perform well under conditions of stimulus-response incongruency are viable endophenotypes of cognitive ability.