Psychoses

Abstract Psychoses are serious psychiatric disorders characterised by the inability to differentiate reality from unreality. Major symptoms are disorganised thoughts, delusions, hallucinations and persistent profound sadness or euphoric mood without apparent reason. They are split into a group of known organic and a group of unknown causes. The latter are termed major psychoses and affect approximately 2% of the human population. The subdivision of major psychoses into schizophrenia and bipolar disorder is a matter of debate. In search of the aetiology of these disorders, different models have been applied, mainly medical, Darwinian, Mendelian, psychogenic and genomic paradigms. These approaches, their results and hypotheses of schizophrenia are summarised in this article. More details can be found in other articles in this encyclopedia, references and in a list for further reading. Key Concepts Psychoses are caused by known or postulated brain disorders. The classification of psychoses is based on aetiology and course. Schizophrenia and bipolar disorder I (manic‐depressive illness) are two major psychoses of postulated but unknown organic origin. Bipolar II is not a psychosis per se but assumed to be a mild form of bipolar I and has been found to overlap in part with borderline personality disorders. Major psychoses are polygenic disorders with heritability estimates of approximately 80%. Genome research has identified several genes associated with schizophrenia or bipolar disorder which are, in part, identical. Brain imaging revealed partly overlapping volume reductions in schizophrenia as well as in bipolar disorder. Genes associated with schizophrenia or bipolar disorder play a role in neurodevelopment, synaptic and vascular regulation. Dopamine hyperactivity, neurodevelopment, inflammation, neurodegeneration, mild ischaemia and synaptic dysregulation have been implicated in the aetiology of major psychoses.