However, more recent studies have suggested that the genetic relationship between BP and SCZ might be greater than previously realized. Researchers have found increased risks of affective disorder in the families of schizophrenia patients (8) as well as the reverse (9). The largest of these studies, including data on more than 75,000 affected Swedish families, found that the risk of SCZ was substantially increased in the relatives of BP and vice-versa (10). Because of the availability of information on twin and half-sibling relationships and adopted-away relatives the authors were able to show a substantial genetic correlation between SCZ and BP. However, this study assigned diagnoses on the basis of chart diagnoses, which may be less reliable than direct interviews using standardized instruments. The only twin studies that have examined the genetic correlation between SCZ and BP diagnosed using direct patient interviews have been conducted in the Maudsley twin series (11). However, these studies used non-hierarchical diagnoses, which confound manic syndromes in the course of SCZ with BPD, which might have different genetic influences.
