Commentary: Income and health: why are curves so appealing?

Abstract

In this issue of the International Journal of Epidemiology Martikainen et al.1 report a linear relationship between household income and subsequent mortality. The linear trend accounts for ‘96% of the variation between income deciles’. Departures from linearity at the top and bottom of the income range are modest and variable—the most consistent being a flattening of the relationship at very high incomes. These results contrast markedly with the most directly comparable study of Backlund et al.2 which found pronounced curvilinearity, flattening at much lower income levels. The first point to stress about this difference is that it is very unlikely to be due to statistical artefacts or methodological differences between the studies. Both studies have huge sample sizes with very high response rates. Whilst income data are frequently problematic, particularly at the top and bottom of the range, both results are consistent with other findings from their respective countries. In short, we can be confident that they reflect a real difference and one which may be informative about the underlying causal processes at work.

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