Despite the low explained variance, the results of this study indicate some interesting issues regarding the etiology of MDD and especially BD. Although studies investigating the phenotypic association between personality and MDD, as well as BD suggest that both disorders are related to high N and that, in addition, BD is related to high O, the genetic association shows a different picture of shared genetic risk factors for N and MDD on the one hand and for E and BD on the other. As previous studies have already pointed to differences in personality profiles between depression-prone and manic-prone BD patients,6, 55 these results imply that BD is a heterogeneous disorder with different expressions related to different, genetically influenced, personality profiles. This view is supported by the finding that an association with polymorphisms in the GABA receptor β1 subunit gene is most significant in cases fulfilling the Research Diagnostic Criteria for schizo-affective disorder. Cases fulfilling the Research Diagnostic Criteria for Bipolar Disorder type II showed a similar allele frequency as did controls.61 Future studies in larger samples, for example, the Psychiatric
