Pre-disaster income inequality and post-disaster mental health: A natural experiment from the 2011 Great East Japan Earthquake and Tsunami.

Background: We sought to test the hypothesis that communities with a high degree of income inequality are ill-equipped to deal with crises. Specifically, we tested whether pre-disaster higher income inequality increases the risk of worse mental health conditions in the aftermath of disaster.

Methods: We examined the association between pre-disaster community income inequality and post-disaster mental health outcomes in a prospective cohort study of older adults (≥65 years) who survived the 2011 Great East Japan Earthquake. The baseline survey of participants (completed in 2010) predated the disaster by seven months. Survivors completed three waves of post-disaster surveys in 2013, 2016, and 2019. To assess the level of income inequality, the Gini index was calculated using individual income data in 2010 aggregated to 98 communities (range 0.05-0.50: median = 0.32) and categorized into tertiles (low, middle, high). Depressive symptoms and post-traumatic stress symptoms (PTSS) were assessed in the three post-disaster surveys using validated instruments. Multilevel analysis was conducted adjusting for pre-disaster characteristics.

Results: Higher levels of community income inequality were associated with higher PTSS scores across time (2013: β = 0.14, 95%CI = -0.01, 0.29; 2016: β = 0.16, 95%CI = 0.03, 0.29; 2019: β = 0.12, 95%CI = -0.01, 0.24). The association between income inequality and mental health was partly attenuated by adding the change in social capital to the model (2013: β = 0.14, 95%CI = -0.01, 0.29; 2016: β = 0.15, 95%CI = 0.03, 0.28; 2019: β = 0.11, 95%CI = -0.01, 0.22).

Conclusion: Our findings suggest that pre-disaster income inequality may explain PTSS symptoms in the aftermath of a disaster, and the relation may be mediated by disaster-related change. Further study to reveal its mechanism is needed for the policy recommendation.