urbanicity

The risk of psychotic disorder in the population exposed to urbanicity alone was 1.59% (31/1946), the risk in those exposed to family history, broadly defined, alone was 3.01% ...

Thus, between 60% and 70% of the individuals exposed to both urbanicity and family history had developed psychotic disorder because of the synergistic action of the two proxy causes.

To assess this hypothesis, the urbanicity of a boy's place of residence was run against enlistment.