Deeper and Wider

Income and Mortality in the USA Over Three Decades

Jennifer B Dowd; Jeremy Albright; Trivellore E Raghunathan; Robert F Schoeni; Felicia LeClere; George A Kaplan

Disclosures

Int J Epidemiol. 2011;40(1):183-188. 

In This Article

Results

Table 1 shows descriptive statistics for our sample.

Median income for those 35–64 years of age increased by 22.6% in constant year 2000 dollars across the three samples; in 1970 sample it was $38 680, rising to $43 840 in 1980 and $47 430 in 1990. Each sample is roughly 56% male with an average age of 48 years. Figure 1 illustrates the income distribution for the three samples in constant dollars. While there has been some movement of the entire income distribution to the right over the three periods, there has also been a movement to greater density in the right tail of the distribution, reflecting an increase in income inequality over this time period.[15]

Figure 1.

Income distribution across decades, year 2000 dollars

Results from the spline models are shown in Table 2 and illustrated in Figure 2.

Figure 2.

Graphs of income HRs (in reference to median income)

The knot for the 1970s was estimated to fall at $14 180, or the ninth income percentile for the 1970 sample. The hazard ratio (HR) for income below the knot in 1970 indicates that a $10 000-increase in income was associated with a 50.4% lower risk of death over the 10-year follow-up. Above the ninth income percentile, the association of increased income was much reduced, corresponding to a 21.5% reduced risk of mortality for each additional $10 000 of income. In 1980, the knot location was estimated to fall higher in the income distribution, at $22 360 (20th percentile). Below this threshold in 1980, an additional $10 000 of income was associated with roughly the same mortality reduction as the pre-knot estimate for 1970, 54.6%, but importantly, this steeper association applied to a greater percentage of the population. For income above the knot threshold, the income–mortality association was much weaker, corresponding to a 9.7% reduction in mortality risk for each additional $10 000 of income. The 1990 estimate found the location of the knot moving out even further to $33 080 (32nd percentile). Below this threshold, each additional $10 000 of income was still associated with a 54.7% reduction in mortality risk as in previous decades, with an even flatter relationship above the threshold—a 5.3% reduction in risk for each additional thousand dollars of income. Thus, over these three decades, the increased risk of death associated with having less income applied to an increasing proportion of the US population.

The estimated patterns by decade are further illustrated in Figure 2, which shows the predicted HRs during all three decades, with median income at the beginning of each decade as the reference. With each decade, the portion of the income distribution subject to a strong association between income and mortality gets larger, whereas the relationship between income and mortality for those beyond the estimated threshold gets weaker over time. The relative risk for the poorest individuals also increased across decades, such that those in the bottom 5% of the income distribution in 1970 had roughly 2.6 times the risk of mortality compared with individuals at median income, whereas in 1980 had 3.5 times the risk, and in 1990 this risk had increased to >7.6-fold.

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