Community-level social capital and cognitive decline after a natural disaster: A natural experiment from the 2011 Great East Japan Earthquake and Tsunami.

We examined prospectively whether community-level social capital can mitigate the adverse effects of natural disaster on cognitive decline in the aftermath of the 2011 Great East Japan Earthquake and Tsunami. The baseline for our natural experimental study was established seven months before the disaster in a survey of older community-dwelling adults who lived in Iwanuma City, Japan, located 80 km west of the epicenter. Two and a half years after the disaster, we conducted a follow-up survey of survivors to gather information about their personal experiences during the disaster (n = 3560; 82.1% follow-up rate). Our primary outcome was the level of cognitive disability (measured on an 8-level scale) assessed within people's homes. Factor analysis established two subscales of community social capital: a cognitive dimension (perceptions of community social cohesion) and a structural dimension (informal socializing and social participation). The prevalence of cognitive decline at follow-up (11.5%) was three times higher than at baseline (4.2%). Our multiple membership multilevel model indicated that pre-versus post-disaster increases in community-level informal socializing and social participation were associated with lower risk of cognitive decline (coefficient = -0.12, 95% confidence interval: -0.20 to -0.04). In addition, social capital mitigated the risk of cognitive decline due to housing damage (interaction effect coefficient = -0.07, 95% confidence interval: -0.14 to -0.01). Community-level informal socializing and social participation buffers the impact of housing damage on cognitive decline in the aftermath of natural disaster. Relocating residents together with other community members may help to preserve community social capital and improve the cognitive resilience of older survivors.