We have so far considered examples where we consider there to be sufficient genetic data available to arrive at an estimate of the effect size. Our conclusions, that the effect sizes found in genetic studies of endophenotypes are not considerably greater than those found in psychiatric diseases, may be biased because we have been limited to studying the COMT locus, or because so far no large effects have been found. The first issue is difficult to tackle. Significant associations reported for other genetic loci are based on either a single, or a very small number, of studies, so that we do not yet know how robust these findings are.
