Quasi-experimental evidence on the causal effects of physical health on mental health

Methods The study design employs exogenous injuries in bus accidents together with a matching procedure to simulate a random exposure to physical health shock. The study was conducted among travellers on state-owned buses in Karnataka, India. Exposure occurred between July and December 2005. Outcomes were assessed from a household survey conducted in November-December 2006. Eighty-four injured passengers identified from bus accident compensation records were interviewed along with 336 unexposed individuals enrolled from passengers on the same accident bus routes, matched on age group, gender and village/neighbourhood of residence. The main outcome of Psychological Distress was measured using the Kessler-10 scale. Results Exposure to the health shock increases psychological distress by 1.5 standard deviations (SD) 1 year later (P < 0.01). Physical disability is a key mediating mechanism, accounting for 65% of the observed effect. After controlling for disability, odds of having distress levels commensurate with moderate/severe mental illness was 3.01 [95% confidence interval (CI) 1.26-7.19]. Indebtedness resulting from the health shock did not mediate the association between shock and distress. Conclusion Evidence from this quasi-experimental study supports the hypothesis that acute physical health shocks can cause long-term mental health problems.