Van Os (Reference Van Os2004) persuasively argues for a greater recognition of the urban environment as a justifiable and empirically sound aetiological factor in psychotic illness. The unanswered question, however, remains about the mechanism through which this environment increases the risk for psychosis. It seems necessary to suggest that perhaps psychiatric illness cannot be assessed under the generally accepted cause and effect rubric that defines other medical illnesses. This is mostly because there are no definitive or specific markers that can define the presence of the illness and, although genetic factors are associated with the risk for developing psychosis, the expression of illness is clearly an interaction with environmental factors (Reference Tsuang, Stone and FaraoneTsuang et al, 2001).

Van Os notes that the medium of risk exposure is likely to be widespread and cumulative over the course of development. This further suggests that the presentation of psychosis represents a culmination of an ongoing interaction between an individual and his/her environment. This remains the only reasonable explanation for the variation in incidence rates, particularly those reported for migrant populations in Britain and Europe (Reference Hutchinson and HaasenHutchinson & Haasen, 2004). Interactions between perceptions of self, cognitive processes and the features of a modern urban environment underlie social development. The relative weighting of vulnerability and resilience factors is a function of this interaction and must in turn be affected by wider social issues such as racism, socioeconomic opportunity and perceived social isolation. There is also the generational transfer of unfulfilled expectations and distrust of institutional structures. The problems in mental health for migrants in Britain are mirrored in the education and criminal justice systems (Reference Modood, Berthoud and LakeyModood et al, 1997). This suggests a developmental trajectory that is affected by social and generational realities and at the same time increases the risk of presentation with psychotic symptoms.

This would mean that the risk exposure for psychosis lies not specifically in the urban environment but in the way this environment generates and/or facilitates a life course that ultimately disadvantages those whose vulnerability is not compensated for by the support of their social environment. This is also influenced by the individual's perception of the negative experiences of the ethnic and socio-cultural groups with which they identify in both the narrow family and community sense as well as the wider national and international sense.

There might therefore be a need to reconstruct the neurodevelopmental model which has led to a preoccupation with the biology of psychosis to include a social developmental model that can demonstrate how the neurobiological endpoint of psychosis can have both biological and social origins.