Although it is known that people living in densely populated areas are at higher risk for psychiatric illnesses, the reason for this remains unknown.
Swedish researchers have attempted to figure out what might be the reasons behind this association using health records from more than 2.4 million people, including separate datasets for siblings and cousins.
Researchers at the Karolinska Institute in Stockholm, led by Amir Sariaslan, used Swedish registry data to look at all children born between 1967 and 1989. Association between population density and “neighborhood deprivation,” a term used to descirbe poorer neighborhoods, and risk for a schizophrenia diagnosis was investigated.
Familial pedigree structures were generated using the Multi-Generation Registry and identified study participants with schizophrenia and depression using the National Patient Registry. Fixed-effects logistic regression models were used to study within-family estimates. Results were published in Schizophrenia Bulletin.
Population density, measured as (population size/km2), at age 15 predicted subsequent schizophrenia in the population (OR = 1.10; 95% CI: 1.09; 1.11). Unobserved familial risk factors shared by cousins within extended families lowered the association (1.06; 1.03; 1.10), and the link disappeared entirely within nuclear families (1.02; 0.97; 1.08). Similar results were found for neighborhood deprivation as predictor and for depression as outcome.
"Excess risks of psychiatric morbidity, particularly schizophrenia, in densely populated and socioeconomically deprived Swedish neighborhoods appear, therefore, to result primarily from unobserved familial selection factors,” the researchers concluded. “Previous studies may have overemphasized the etiological importance of these environmental factors.”
People living in densely populated and socially disorganized areas have higher rates of psychiatric morbidity, but the potential causal status of such factors is uncertain.
The nature of the associations between population density and neighborhood deprivation and individual risk for a schizophrenia diagnosis was investigated while adjusting for unobserved familial risk factors (through cousin and sibling comparisons) and then compared with similar associations for depression.
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