Psychosis is characterized by distortions in thinking (e.g. fixed, false beliefs), in perception (e.g. hearing voices or less commonly seeing things that are not there), emotions, language, sense of self and behaviour. Although it used to be thought that schizophrenia was a discrete entity, much recent evidence has shown that this is not so. Schizophrenia does not have clear boundaries; rather, it merges into schizoaffective disorder and bipolar disorder on the one hand and into schizotypal and paranoid personality on the other. It is best considered as the severe form of psychosis. The different psychotic disorders share some of the same risk factors and are sometimes associated with cognitive impairments, co-existing mental health conditions, substance misuse and physical health problems; the latter often develop over the course of the illness.

In this chapter, we review genetic and then environmental risk factors for psychosis. Much knowledge has accumulated regarding both in the last two decades. We now know that the aetiology of psychosis is multifactorial. Genetic and environmental factors occasionally act alone but usually in combination as well as operate at a number of levels and over time to influence an individual’s likelihood of developing psychotic symptoms.