The idea that schizophrenia and bipolar disorder stand as two distinct entities of mental illness came from German psychiatrist Emil Kraepelin in the late 1800s. So began the separation of the diseases that still marks today's psychiatry. However, in practice, it's not always easy to distinguish the two disorders. Schizophrenia and bipolar disorder have a number of symptoms and epidemiological characteristics in common, and both respond to dopamine blockade. Family, twin and molecular genetic studies suggest that the reason for these similarities may be that the two conditions share certain susceptibility genes.

The authors present a revision of several articles that reveal new evidences supporting the unitary psychosis theory. This theory has its origins in 1830s with Guislain, Zeller and Griesinger and defends a continuum of mental illness from schizophrenia to bipolar disorder. More recently genetic studies show overlapping of some predisponent genes of both diseases and some imagiological findings are also present in both. The existence of a continuum of mental illness may be useful for new treatment strategies and prognosis.