According to new study findings published in the JAMA Intern Medicine, multivariable analyses that include the risk factors suggest that the variations in life expectancy are largely explained by behavioural and metabolic risk factors, with socioeconomic and race/ethnicity factors no longer being statistically significant.

Human health has improved dramatically over the last couple centuries. Yet, disparities remain, and recent work suggests that these disparities are widening for life expectancy. These disparities are owing to a combination of socioeconomic and race/ethnicity factors, behavioural and metabolic risk factors, and health care factors.

Indeed, in bivariate analysis, income is the strongest predictor of life expectancy. Given these somewhat incongruous results, the researchers wanted to compare the rise of income inequality, as well as racial inequality, over the same time frame as the rise in life expectancy inequality.

Available US Census data was used to chart income inequality by county over time, using the ratio of the 99th to the 1stpercentile level similar to the method used by Dwyer-Lindgren. Similar charting was performed for racial inequality over time by analyzing the ratio of counties with the fewest percentage of minorities to counties with the highest percentage of minorities.

Inequality in median household income between the top and bottom percentiles appeared to vary randomly over time. The ratio of the 99th percentile to the 1st percentile ranged from 4.12 at its highest in 1989 to 3.64 in 2010 at its lowest. Racial inequality between counties rose over time. The ratio of the counties with the fewest percentage of minorities and counties with the highest percentage of minorities went from 4.38 in 1990 at its lowest to 5.48 in 2010 at its highest.

The analysis showed that income inequality did not rise over time but rather varied over the years, suggesting that the rise in life expectancy inequality indeed is not only related to the rise in income inequality. However, racial inequality has been rising over time—in fact, at a rate very similar to that of life expectancy inequality. This is a visible demonstration that it is not simply behavioural and metabolic risk factors that play a role in longevity.

There are likely multiple potential routes (including socioeconomic) to more equitable health outcomes, and many of these factors are integrally connected to each other. Furthermore, income equality shows only a slice of the disparities in economic status; the gap in assets and wealth—in particular between white individuals compared with black and Hispanic individuals. Combining this with the increasing residential segregation occurring based on wealth shows how difficult it is to unchain all these variables from each other.

The various factors associated with life expectancy inequality, then, are not necessarily confounders, but perhaps rather mediators between class and health. Likely, a multifaceted approach is needed in reducing inequalities in longevity and beyond, the researchers concluded.