We searched for relevant publications in PubMed, PsycInfo and Medline, up to 31 May 2006, using the search terms ‘Wisconsin’, ‘card sorting’, ‘WCST’, ‘COMT’, ‘catechol O-methyl transferase’. Bibliographies of all articles obtained in this way were hand-searched for additional references. Data extraction and analysis procedures were similar to those employed in our previous meta-analyses (Munafo et al. 2005). Briefly, a fixed-effects framework, using inverse variance methods, was employed initially, and the assumption that the effect of allele frequency is constant across samples checked using a χ2 test of goodness of fit for homogeneity. When there was evidence of significant association in the presence of significant between-sample heterogeneity, the analysis was re-run using a random-effects framework, using Der Simonian and Laird methods.
