Because general cognitive ability (g) is among the most heritable behavioural traits, it is a reasonable target for a search for quantitative trait loci (QTLs). We used a selected-extremes design to test candidate genes for allelic association with g. Polymorphisms in four genes in the dopamine system (DRD2, DRD3, DRD4, DAT1) were genotyped for 51 high g children with IQ scores > 130 and for 51 average g control children. No significant allelic or genotypic differences were found between the high g and average g groups for these markers of the dopamine system, even though the selected-extremes design provides power to detect QTL associations that involve a relative risk of about 1.5.