Many disorders are the result of interaction between susceptibility genes and environmental influences. One example is cardiovascular disease; people with a family history of cardiovascular disease are more susceptible to heart problems, and environmental influences, such as diet, can increase this risk. Schizophrenia is also a complex disorder that can arise from both genetic and environmental influences, although heritability estimates are around 80%.

What is the evidence for genetic and non-genetic risk?

Moderate to high quality evidence found large increased risk of schizophrenia in people with one or two first-degree relatives with schizophrenia compared to people without a relative with schizophrenia. Moderate quality evidence from twin studies suggests genetic effects contributing to risk of schizophrenia are much higher than environmental effects, although both factors are involved. Genetic risk scores for schizophrenia are associated with increased symptoms of schizophrenia, particularly negative and disorganised symptoms.

Moderate to low quality evidence suggests a medium increase in prevalence rates of subclinical psychotic symptoms in people with a family history of mental illness. There is a large increased risk of schizophrenia in offspring of people with schizophrenia compared to offspring of people with no mental disorder. There is no increased risk of depression, anxiety, disruptive disorders, substance use disorder, or attention deficit hyperactivity disorder in offspring of people with schizophrenia. There is high familial coaggregation of schizophrenia and bipolar disorder, with first-degree relatives of people with schizophrenia or bipolar disorder showing an increased risk of developing either disorder.

April 2022

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