Both disorders are worldwide, lifelong, and recurrent illnesses with periods of exacerbation and partial or full remissions. Lifetime prevalence of schizophrenia and bipolar disorder type I is around 1%. Women and men are affected at proximately equal rates and the typical age of onset is similar. Both disorders also share risk factors showing evidence for impaired prenatal development, such as birth seasonality, abnormal dermatoglyphs and higher incidence of perinatal complications. However, there is evidence for differences in prevalence in geographical isolates, in presence of minor physical abnormalities and possibly also in influence of psychosocial factors, urbanicity and use of cannabis. However, based only on similar epidemiological parameters, it is not possible to assume any degree of continuity between bipolar disorder and schizophrenia. The main evidence for partially shared pathophysiology is given by genetic studies.