The shape of the relationship between income and self-assessed health: an international study.

BACKGROUND: The relationship between income and health is usually thought to be curvilinear, but previous studies have yielded inconsistent results. We therefore examined the shape of the relationship between household equivalent income and self-assessed health in seven European countries.
METHODS: Data were obtained from nationally representative health, level of living, or similar surveys in Belgium, Denmark, England, Finland, France, The Netherlands, and Norway and applied to men and women aged 25 years and older in the 1990s. Smooth nonparametric curves were fitted to the data, as well as a spline regression function with three linear pieces connected by two knots.
RESULTS: A higher household equivalent income is associated with better self-assessed health among men and women in all countries, particularly in the middle-income range. In the higher income ranges, the relationship is generally curvilinear and characterized by less improvement in self-assessed health per unit of rising income. In the lowest income ranges, the relationship is found to be curvilinear in four countries (Belgium, Finland, The Netherlands, and Norway), where the usual deterioration of health associated with lower incomes levels off or even reverses into an improvement.
CONCLUSIONS: Further research is necessary to investigate the background of differences between countries in the shape of the relationship between income and self-assessed health, and should focus on both methodological and substantive explanations. Assuming causality, the results of our study lend some support to the notion of decreasing marginal health returns of a unit increase in income at the higher income ranges.