Our meta-analysis shows that if a genuine association exists between the COMT gene and WCST performance, the likely effect size is a standardized mean difference of 0·10, accounting for less than 0·5% of the phenotypic variance. Therefore, given a Val allele frequency of 50%, a sample consisting of in excess of 1700 subjects would be required to detect this effect with 80% power at a relatively non-conservative alpha of 0·05.
