Over the past few decades, epidemiological evidence has accrued to establish variance in psychosis risk across both geographical locations and demographic characteristics such as the excess risk in migrants and their descendants. Yet, the causes of this variation in rates between places and ethnic groups are still unclear, and I aimed to address this in this thesis.

I conducted a systematic review and meta-analyses to synthesise existing literature on psychosis incidence in the six countries included in the EUropean network of national schizophrenia networks studying Gene-Environment Interactions (EU-GEI) study: England, The Netherlands, Spain, France, Italy and Brazil. I subsequently analysed data from two parts of the EU-GEI study: a 17-centre service-based incidence study of psychosis, and a case-control arm utilising community volunteers. In the latter, I aimed to explain excess risk in ethnic and religious minorities using a theoretical sociocultural distance model I developed using literature from the social sciences. Here, I proposed that culturally distant minorities were particularly at risk of social exclusion, and this outsider experience led to increased psychosocial disempowerment (a lack of control over one’s life), which increased psychosis risk. I also explored if this model could explain any excess risk in those with increased genetic African ancestry in England.

Incidence varied substantially between the studies included in the systematic review, although methodological differences could not be excluded as an explanation. The EU-GEI incidence study confirmed substantial variation by place, and demonstrated a higher incidence in ethnic minorities and for young men, as well as in areas characterised by a low percentage of owner-occupied housing. The sociocultural distance model could explain most of the excess psychosis risk in ethnic minorities, although some excess risk remained, particularly in the Black ethnic group. Social and cultural distance appeared to be more important predictors than psychosocial disempowerment, suggesting that chronic social injustices rather than acute stress play an important role. This model did not explain excess risk in religious minorities: those following any religion retained an excess risk. It could explain the excess risk in those with increased genetic African ancestry, although this was a small, exploratory sample and this will need replicating in larger studies.

This thesis demonstrated, for the first time, that excess risk in ethnic minorities could be explained by the sociocultural distance model. Overall, the findings from this thesis confirm substantial variation in psychosis risk by person and place, and suggest that the social reality of the environment plays a crucial role in explaining this.