Selection and mortality: a ten-year follow-up of income decile mortality in Norway

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Abstract
Background: There is universal agreement that higher mortality goes with lower income. Opinions differ on causality: the association may reflect the damaging effect of poverty on health and survival chances. Conversely, it may reflect selection/reverse causation: low income indicates health problems, and from health problems follow a higher risk of dying. Methods: We studied all deaths in Norway (111 504) during the 10-year period 1994–2003 among persons aged 25–66 years in 1993 (2 261 076). For each year, age-standardized mortality rates were calculated for each 1993 income decile for men and women separately. Income was calculated as family size-adjusted income after taxes but including cash welfare transfers. If the selection theory was correct, one would expect to see the excess mortality in the lower income fractiles decline as the bad risks, over-represented among the poorer, died away. Results: Large income decile variations in mortality remained at the end of the 10-year period: after 10 years, the age-standardized mortality rate for men and women was still much higher in the lower income deciles. Conclusion: As the excess mortality in the poorer income deciles was not much reduced during the 10-year period, excess mortality among persons in bad health in the lower income deciles does not explain the income inequality in mortality in our data set.

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