Social capital and cognitive decline in the aftermath of a natural disaster: a natural experiment from the 2011 Great East Japan Earthquake and Tsunami

Background No previous studies have examined that social capital mitigates impacts of natural disaster experiences on cognitive function. We aimed to examine prospectively whether social capital mitigates the adverse effects of natural disaster on cognitive decline. Methods In this natural experiment, the baseline for our study was established 7 months before the 2011 Great East Japan Earthquake and Tsunami in a survey of older community-dwelling adults who lived 80 km west of the epicentre (59·0% response rate). About 2 and a half years after the disaster, which occurred on March 11, 2011, the follow-up survey collected information about personal experiences of disaster as well as incidence of cognitive disability. Our primary outcome was cognitive disability (measured on an 8-level scale) assessed by in-home assessment. Findings We obtained 5058 respondents at the baseline survey (59·0% response rate) and re-contacted 3594 survivors in the follow-up survey (82·1% follow-up rate). The experience of housing damage was associated with risk of cognitive impairment (coefficient 0·05 [95% CI 0·03 to 0·07]). Factor analysis of our analytical sample (n=3566) established two subscales of social capital: a cognitive dimension (perceptions of community social cohesion) and a structural dimension (informal socialising and social participation). Fixed effects regression indicated that improved informal socialising and social participation mitigated the risk of cognitive decline due to housing damage (coefficient −0·10 [95% CI −0·14 to −0·05]) and deteriorating informal socialising and social participation aggravated the effect of housing damage on cognitive decline (coefficient 0·04 [0·01 to 0·07]). Interpretation Improved informal socialising and social participation reduces the risk of cognitive decline due to housing damage in the aftermath of natural disasters. Interventions to promote civic participation should be tried to promote cognitive resilience of older survivors. Funding National Institutes of Health, the Japan Society for the Promotion of Science, the Japanese Ministry of Health, Labour and Welfare, and the Japanese Ministry of Education, Culture, Sports, Science and Technology.