Does Urban Living Cause Mental Illness?

Bret S. Stetka, MD; Amir Sariaslan

Disclosures

August 27, 2014

Genes vs Environment

Medscape: What did you find?

Mr. Sariaslan: We found that individuals who had been exposed to urban and socioeconomically deprived neighborhoods experienced elevated risks of developing both schizophrenia and depression. These risks were, however, substantially attenuated within extended families when we compared differentially exposed cousins, and it disappeared within nuclear families when we compared differentially exposed siblings.

Our results therefore suggest that it is not the adverse neighborhood conditions that cause the morbidity. Instead, it seems as if there are familial selection effects that draw high-risk individuals into densely populated/socioeconomically deprived neighborhoods. In other words, the same factors that explain residence in such neighborhoods also explain the increased risks for psychiatric morbidity.

Medscape: What do you presume are some of those factors?

Mr. Sariaslan: What our study highlights in the importance of accurately adjusting for familial influences in epidemiologic studies of psychiatric disorders. Our research design takes the approach of accounting for an aggregate of all familial influences shared between relatives. This implies, unfortunately, that we cannot identify specific genes or environmental risk factors that explain the nature of the confounding.

However, the literature has identified several interesting putative risk factors for schizophrenia (eg, childhood trauma, infections, and illicit drug use) that have not been thoroughly tested using this family-based approach.

Medscape: How influential do you believe genetic influences are in schizophrenia risk?

Mr. Sariaslan: In this specific study, we were not able to distinguish between genetic and environmental confounders. However, 2 factors suggest that genes may be more influential than previously expected:

  1. Within extended families, we observed a large attenuation of associations between cousins, who only share 12.5% of their genes.

  2. Etiologic studies on using have suggested a very small influence of shared environment.

We are currently working on projects that focus on elucidating the nature of these mechanisms.

Medscape: How important do you think gene/environment interactions are in development of schizophrenia? Do you believe that some people probably have a genetic predisposition that is brought out by certain environments (eg, an urban setting)?

Mr. Sariaslan: Although we cannot entirely exclude the possibility of potential gene/environment interactions, our results suggest that such influences are minimal at best. Four important points to note are:

  1. None of the gene/environment studies have taken familial confounding into consideration -- at least not in the urbanicity/schizophrenia literature.

  2. We found no causal main effects of either population density or neighborhood deprivation on schizophrenia or depression.

  3. In terms of schizophrenia, unrelated individuals who lived in the same neighborhoods did not share any characteristics that increased their risks of developing schizophrenia. It seems therefore unlikely that moderated effects would be that influential.

  4. I think that it is important to distinguish between 2 processes here: social causation and social drift. In the former case, social environments cause the individual to develop the disorder (via stress mechanisms). In the latter case, the individual drifts downward in the social hierarchy owing to their disorder (reverse causation). We have a manuscript under review that focuses on social drift in patients with schizophrenia.

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