Urban birth and residence as risk factors for psychoses: an analysis of 1880 data.

It is known that social drift to cities increases the urban prevalence for severe mental illnesses. Recent studies in England and Sweden have reported that being born in, or raised in, an urban area is also a risk factor for later developing schizophrenia. The present study utilized 1880 census data, the most complete enumeration of severely mentally ill individuals ever done in the United States, to examine the association of urban residence and severe mental illnesses. Individuals identified as having one of seven forms of 'insanity' (n = 91959) were allocated to their county of origin (n = 2661) in the census. Rates of 'insane' persons per 1000 population were calculated for each county. The counties were then divided by degree of urbanization based on the largest cities and the percentage of population living in towns of 4000 or more. The point prevalence of 'insanity' in the United States as a whole in 1880 was 1.8 per 1000. There was a significant association between 'insanity' by county and degree of urbanization, with odds ratios of 1.66 for urban, 1.46 for semi-urban, and 1.44 for semi-rural, and 1.37 for rural, using completely rural counties as a baseline. Most completely rural counties with high rates included mining camps, lumbering camps, or fishing villages, and not farming areas. In addition to urban drift, urban birth and residence appear to be risk factors for developing severe mental illnesses. Psychological and biological explanations have both been proposed. However, recent studies reporting winter birth and urban birth or residence as synergistic risk factors favor the latter.