Change in income and change in self-rated health: Systematic review of studies using repeated measures to control for confounding bias

It is generally assumed that income is strongly and positively associated with health. However, much of the evidence supporting this assumption comes from cross-sectional data or analyses that have not fully accounted for biases from confounding and health selection (the reverse pathway from health to income). This paper reports results of a systematic review of panel and longitudinal studies investigating whether changes in income led to changes in self-rated health (SRH) in adults. A variety of electronic databases were searched, up until January 2010, and thirteen studies were included, using data from five different panel or longitudinal studies. The majority of studies found a small, positive and statistically significant association of income with SRH, which was much reduced after controlling for unmeasured confounders and/or health selection. Residual bias, particularly from measurement error, probably reduced this association to the null. Most studies investigated short-term associations between income and SRH or the effect of temporary (usually one year) income changes or shocks, so did not rule out possibly stronger associations between health and longer-term average income or income lagged over longer time periods. Nevertheless, the true causal short-term relationship between income and health, estimated by longitudinal studies of income change and SRH that control for confounding, may be much smaller than that suggested by previous, mostly cross-sectional, research.