Many risk factors, mostly of small effect, have been identified in schizophrenia epidemiology. But some – such as winter- or urban-birth - may have a relatively high population attributable fraction. Eg if a risk factor was weak (say a relative risk of 1.5-2.0) but prevalent (say 40-70%), then this could account for up to 20% to 40% of all cases. This means that if the underlying prevalent risk factor was removed then this would prevent a substantial number of cases of schizophrenia from occurring.
Risk factors may be modified by time place and person – such as season- or place of birth or the individual’s sex; and there may be a relationship between risk-factors.