Quantifying the contribution of social disconnection to the mortality gap associated with mental disorders: a decomposition analysis.

Individuals with mental disorders face a substantially higher risk of mortality and are more likely to be lonely, socially isolated, and with low social support compared to those without mental disorders. We aimed to quantify the extent to which the observed mortality gap associated with mental disorders could be explained by these social factors. This cohort study included 162,483 participants from the Danish National Health Survey in 2013 and 2017 who were followed for six years after survey participation. Survey data on social disconnection (loneliness, social isolation, social support in the form of perceived emotional support, and a composite measure) was linked with register data on hospital-diagnosed mental disorders and mortality. We applied G-computation-based causal decomposition to compare the sex-specific relative risk of mortality associated with mental disorders under a natural course to a counterfactual scenario in which all individuals had a distribution of social disconnection similar to individuals without mental disorders. We found that social disconnection and the distribution of loneliness, social isolation, and social support accounted for 10-34% of the mortality gap associated with mental disorders among men and 2-20% among women, assuming a causal effect of social disconnection on mortality. The largest contributions were found for social isolation and loneliness, whereas the smallest were found for social support. Our results highlight the possibility that different aspects of social disconnection, especially social isolation and loneliness, could explain part of the mortality gap associated with mental disorders, with larger contributions among men than women.