It is of substantial interest that the existence of one or more relatively discrete nosological entities with mixed mood–schizophrenia features is supported by latent class analyses (e.g. Kendler et al,17,18 McGrath et al,19 Sham et al20) and that genetic epidemiology supports a strong genetic component to schizoaffective illness (e.g. Andreasen et al,21 Bertelsen et al,22 Farmer et al,23 Gershon et al,24 Maier et al,25 Slater & Cowie,26 Cardno et al27) with similar heritabilities to those in schizophrenia and bipolar disorder. Our findings are consistent with molecular genetic evidence for the existence of relatively specific genetic susceptibility for a form of major psychiatric illness that has features of both bipolar disorder and prominent psychosis.28–30 This could be interpreted as specific support for a category of ‘schizoaffective’ illness or for the existence of a region of overlap of schizophrenia and bipolar disorder clinical spectra in which the genetic variants that influence susceptibility are easier to identify than are those that confer specific risk to bipolar disorder or schizophrenia alone. In either case, this clinical entity has genetic utility and merits explicit recognition.
