How do socio-economic characteristics influence the effect of disability acquisition on mental health? An analysis of effect modification and mediation
AuthorAitken, Zoe Lisa
AffiliationMelbourne School of Population and Global Health
Document TypePhD thesis
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© 2019 Zoe Lisa Aitken
Background People with disabilities in Australia experience poorer mental health than people without disability. However, the mechanisms by which disability leads to poor mental health are inadequately understood. This PhD thesis aims to form a better understanding of how people’s socio-economic circumstances influence the effect of disability acquisition on mental health. Elucidating the causal mechanisms underpinning this relationship will inform the development of effective public health and social policies to improve the mental health of people with disabilities. Methods I used data from the Household, Income and Labour Dynamics in Australia Survey – a large nationally representative longitudinal dataset – to quantify the effect of disability on mental health and to examine the socio-economic mechanisms leading from disability acquisition to poor mental health. I identified adults who acquired a disability during their participation in the survey and used data both before and after they acquired the disability to estimate the causal effect of disability on mental health. Firstly, I conducted analyses of effect modification, examining a wide range of different socio-economic factors to determine whether they influenced the magnitude of the effect of disability acquisition on mental health, using inverse probability weighting and fixed effects models to better control for confounding. Secondly, I conducted causal mediation analyses to further examine the socio-economic mechanisms leading from disability to poor mental health, using sequential mediation analysis to examine a broad range of socio-economic characteristics and an interventional mediation approach to quantify indirect effects operating through two distinct socio-economic characteristics: employment and income. Finally, I examined the indirect effect mediated by employment in greater detail, further decomposing the natural indirect effect through employment to estimate the proportion attributable to interaction, mediation and their joint effects. Results There is a clinically significant and large effect of disability acquisition on mental health. The analyses of effect modification provided evidence that the magnitude of the effect differed according to people’s socio-economic characteristics, with greater effects observed for more disadvantaged groups. The mediation analyses provided additional evidence that socio-economic characteristics contribute to the effect of disability acquisition on mental health. A third of the effect was found to be mediated by material socio-economic factors such as employment, income, wealth, financial hardship and housing characteristics, and a further investigation of the indirect effect through employment and income highlighted employment (but not income) as an important contributing factor, explaining 11% of the effect alone. Finally, further decomposition of the indirect effect through employment suggested that the mediated effect was due to interaction between disability and employment, rather than pure mediation. Conclusion The findings of this thesis highlight the importance of the social determinants of health in generating mental health inequalities. Interventions should prioritise addressing the social determinants of health to improve the mental health of people with disabilities and reduce disability-related mental health inequalities. Furthermore, the evidence that employment is a key mediator of the effect of disability acquisition on mental health indicates that policy strategies are needed to target the causes of low employment rates for people with disabilities.
KeywordsMental health; Disability; Socio-economic characteristics; Epidemiology; Causal inference; Causal mediation analysis; Biostatistics
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