five disorders whose diagnostic criteria are fairly distinct. Third, the five disorders we examined were limited to those for which large-scale GWAS datasets have been assembled by the PGC and processed through a uniform quality-control process. As further datasets become available, more comprehensive analyses of cross-disorder genetic effects on psychiatric illness should be pursued. Fourth, we restricted our analyses to individuals of European ancestry. Whether our findings apply to other populations is unknown. Finally, GWAS designs are suited to identify common variant aspects of genetic architecture; further studies (including analyses of copy-number variants and rare mutations) will be needed to account more completely for shared genetic contributions across disorders. As in almost all GWAS of complex disorders reported so far, the effect sizes of genome-wide significant loci are individually quite small and the variance they account for is insufficient for predictive or diagnostic usefulness by themselves. However, our study is the first large-scale effort to characterise allelic effects across five psychiatric disorders, incorporating single locus, multilocus, and pathway analyses.
