Common causes of schizophrenia and bipolar disorder

Schizophrenia and bipolar disorder have the same genetic causes, according to a study from Karolinska Institutet published today in the highly respected journal The Lancet. The results throw the current separate classification of the diseases into question.

Schizophrenia and bipolar disorder (also known as manic-depressive illness) are the two most common psychotic disorders. 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 scepticism 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 analysed 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.


Paul Lichtenstein, Benjamin H Yip, Camilla Björk, Yudi Pawitan, Tyrone D Cannon, Patrick F Sullivan, Christina M Hultman

Common genetic determinants of schizophrenia and bipolar disorder in Swedish families: a population-based study

The Lancet, 16 January 2009