Bipolar disorder shares genetic components with other psychiatric disorders. To help understand the etiology of this disorder, Song and colleagues studied the familial aggregation of bipolar disorder and the co-aggregation with schizophrenia, depression, anxiety disorders, attention-deficit hyperactivity disorder, drug abuse, personality disorders, and autism spectrum disorders.
Familial risks were found to be 5.8-7.9 in first-degree relatives, and decreased with genetic distance. Co-occurrence risks for other psychiatric disorders were 9.7-22.9 in individuals with bipolar disorder and 1.7-2.8 in full siblings of bipolar probands. Heritability was estimated at 58%. The correlations between bipolar disorder and other psychiatric illnesses were considerable (0.37-0.62) and primarily due to genetic effects. Correlation with depression was the highest at 0.62; correlation with schizophrenia was 0.44.
The high familial risks are consistent with genetic factors playing an important role in the etiology of bipolar disorder; the shared genetic determinants suggest pleiotropic effects across different psychiatric disorders. The study concluded that bipolar disorder is in both the mood and psychotic spectra, but possibly more closely related to mood disorders.

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Song J, Bergen SE, Kuja-Halkola R, Larsson H, Landén M, Lichtenstein P: Bipolar disorder and its relation to major psychiatric disorders: a family-based study in the Swedish population. Bipolar Disorder [Epub ahead of print, August 13, 2014; doi: 10.1111/bdi.12242].
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http://www.ncbi.nlm.nih.gov/pubmed/25118125