A fixed-effects model indicated evidence of association between COMT genotype and WCST performance (k=24, Z=1·97, p=0·048), with increased perseverative errors in the Val/Met+Val/Val group compared to the Met/Met group (d=0·10). There was no evidence of between-sample heterogeneity [χ2(23)=30·06, p=0·148]. These data are presented graphically in Fig. 1.
