Urbanisation and psychosis

We agree that in strict terms the definition of incidence rates is in accordance with the definition described by for example Clayton & Hills (1993) and Last (1998).

However, we disagree when Pedersen & Agerbo state that we claim ‘this is the first admission ever of these people’. Nowhere in the article have we used the phrase ‘first admission ever’. In the Method we clearly described that the 4.4 million women and men were followed from 1 January 1997 until 31 December 1999 for first admission to hospital for treatment of psychosis or depression (i.e. during the study period). Individuals with previous hospital admissions for treatment of psychosis or depression from 1992 to 1996 were excluded (i.e. the ‘wash-out’ period was 5 years). We calculated the urbanisation measure for the year 1996 (i.e. the year when we defined our study population).

Pedersen & Agerbo also say that our article implies problems with causality. However, we did not claim that we have solved the causality problem. In addition, we discussed that we were unable to adjust our results for selective migration (i.e. migration from urban to rural areas). However, we did adjust our model for migration between neighbourhoods and the results remained almost unaltered.

We agree that urban-rural differences in admission rates could exist if, for example, the distribution of psychiatric beds differed between urban and rural areas. We checked this possible bias and found that the number of psychiatric beds per 1000 inhabitants differed very little across Sweden.

To sum up, our large-scale study sheds new light on the inconsistencies in previous research since we used the comprehensive Swedish registers and adjusted our results for several possible confounders. In addition, the urbanisation measure was calculated as actual numbers of inhabitants per area unit, which should reflect the level of urbanisation in the most appropriate way.