Background. Higher level of urbanicity of place of birth and of place of residence at the time of
illness onset has been shown to increase the risk for adult schizophrenia. However, because urban
birth and urban residence are strongly correlated, no conclusions can be drawn about the timing
of the risk-increasing effect. The current study discriminated between any effect of urbanization
before and around the time of illness onset.
Methods. All individuals born between 1972 and 1978 were followed up through the Dutch National
Psychiatric Case Register for first admission for schizophrenia until 1995 (maximum age 23 years).
Exposure status was defined by a combination of place of birth and place of residence at the time
of illness onset in the three most densely populated provinces of the Netherlands (the ‘Randstad’,
exposed) or in all other areas (the ‘non-Randstad’, non-exposed). The risk for schizophrenia was
examined in four different exposure groups: non-exposed born and non-exposed resident (NbNr,
reference category), non-exposed born and exposed resident (NbEr), exposed born and non-exposed resident (EbNr) and exposed born and exposed resident (EbEr).
Results. The greatest risk for schizophrenia was found in the EbNR group, without evidence for any
additive effect of urban residence (rate ratio (RR) for narrow schizophrenia in EbNr group, 2·05
(95% CI 1·18–3·57); in EbEr group, 1·96 (95% CI, 1·55–2·46)). Individuals who were not exposed
at birth, but became so later in life, were not at increased risk of developing schizophrenia (RR for
narrow schizophrenia in NbEr group, 0·79 (0·46–1·36)).
Conclusion. The results suggest that environmental factors associated with urbanization increase the
risk for schizophrenia before rather than around the time of illness onset.