Associations between population density, neighbourhood deprivation and the risk of developing schizophrenia or depression can be attributed to familial selection factors rather than the local environment itself, research suggests.
These findings indicate that “[o]mission of adequate adjustments for familial confounding may have led previous authors to overemphasize the relative importance of the direct or moderated effects of these wider environmental risk factors”, say study authors Amir Sariaslan (Karolinska Institutet, Stockholm, Sweden) and colleagues.
The researchers looked at geographical data and medical records for more than 2.36 million people born in Sweden between 1967 and 1989, including approximately 1.72 million cousins within 559,270 extended families and approximately 1.67 million siblings within 719,666 nuclear families. The study participants lived within 7388 neighbourhoods, with 58% of cousins and 19% of siblings living in different neighbourhoods at age 15 years.
They found that 49% and 21% of the phenotypic variance in schizophrenia and depression, respectively, could be attributed to sibling similarities, compared with less than 1% and approximately 2%, respectively, that could be attributed to neighbourhood effects.
Further analysis showed that each percentage increase in population density score at age 15 years was associated with a significant 10% increase in the likelihood of later developing schizophrenia. This decreased to 6% when unobserved confounders shared by cousins were taken into account and was completely attenuated when familial risk factors shared by siblings were included. Similar results were observed for depression.
Neighbourhood deprivation at age 15 years was associated with a significant 43% and 13% increased likelihood of developing schizophrenia and depression, respectively. However, these associations were also reduced, to 19% and 9%, when risk factors from extended families were accounted for, and completely attenuated when sibling risk factors were included.
Writing in Schizophrenia Bulletin, Saraislan and co-authors conclude that their results “support a selection hypothesis; observed ecological risk increases in psychiatric morbidity in densely populated and socioeconomically deprived areas are primarily explained by unobserved familial risk factors.”
They say: “This implies that familial liabilities that explain the onset of schizophrenia and depression in individuals also explain their selection, indirectly via their parents, into densely populated or socioeconomically deprived areas.”
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