Income differences in mortality: a register-based follow-up study of three million men and women.

OBJECTIVES: We analyse whether the relationship between net household income and mortality form a continuous linear gradient or is curvilinear, assess the attenuation of this association after adjusting for confounding and reverse causality, and assess the strength of the association by age and cause of death. DESIGN AND
SETTING: Prospective study of mortality in Finland among all men and women over 30 years old. Information on household income and sociodemographic factors was from the records of the Finnish tax authorities and the 1990 census. Income data were available for more than 95% of the cohort. Follow-up was by record linkage to death certificates in 1991-1996; altogether about 261 000 deaths.
RESULTS: The all-cause mortality ratio between the lowest and the highest household income decile is 2.37 (95% CI : 2.30-2.44) among men and 1.73 (95% CI : 1.67-1.80) among women. Adjusting for household structure, spouse's economic activity, social class, education and own economic activity attenuates the relationship by 61% among men and 52% among women. The association between income and mortality is mainly linear before and after adjusting for confounding, and the association is strong for all 5-year age groups below 60-64 years, after which it declines rapidly in strength.
CONCLUSIONS: The mainly linear nature of the relationship and the strong attenuation after adjustment for other socioeconomic factors and economic activity status, and the age pattern of the relationship indicate that a large part of the relationship is unlikely to be due to direct causal effects of poverty and material hardship. Rather, income seems to be related to accumulation of factors that increase mortality over the whole range of incomes.