For over a century, the two diseases have been treated as distinct by clinical practitioners and researchers as regards definitions and risk factors.
However, such strict classification has met increasing skepticism over the years, partly owing to the results of modern genetic science, which has shown that certain genes seem to affect both disorders.
To study whether schizophrenia and bipolar disorder have the same genetic causes, Swedish scientists analyzed the records of two million families, including 35,985 patients with schizophrenia, 40,487 patients with bipolar disorder, and the blood relatives of both.
Their results show that members of families in which someone has either schizophrenia or bipolar disorder run an increased risk of developing the same condition.
The results also show that this is chiefly the result of genetic factors, and only slightly due to shared environmental factors.
The scientists also found that patients with schizophrenia are also more prone to bipolar disorder, and that relatives of patients with one of the diseases are more likely to have relatives with the other.
According to the researchers, the results, taken as a whole, provide convincing proof that schizophrenia and bipolar disorder are very much hereditary diseases, and that they share, in part, a common genetic cause.
They also argue that it is important for clinicians and researchers to take this common genetic background into account when studying and treating schizophrenia and bipolar disorder.
The study was funded by the Swedish Council for Working Life and Social Research and the Swedish Research Council.
Findings are published in the journal The Lancet.
Source: Karolinska Institutet