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Higher levels of income inequality in the United States actually lead to more deaths in the country over a period of years, a new study found.

According to the results of the study, income inequality at any one point doesn’t work instantaneously — it begins increasing mortality rates 5 years later, and its influence peaks after 7 years, before fading after 12 years.

“This finding is striking and it supports the argument that income inequality is a public health concern,” said Hui Zheng, author of the study and assistant professor of sociology at Ohio State University.

He used data from the U.S. National Health Interview Survey from 1986 to 2004 with mortality follow-up data from 1986-2006. His final sample included more than 700,000 people aged 30 and up.

It was found that income inequality has a substantial effect on mortality.   

In the study, the lead author was able to look at deaths in a particular year and control for income inequality for each of up to 21 years preceding the death. 

"For the first time, we can clearly capture the long-term effect of income inequality on health," Zheng said. "Previous studies are likely to miss the effect if the mortality follow-up period is too short for income inequality to exert its impact or too long for income inequality to maintain its influence." 

Other scholars have hypothesized why income inequality may lead to higher death rates. One reason may be that, in countries with rising income inequality, the interests of the wealthy tend to diverge from the rest of society. The wealthiest people may push government for more services for themselves, rather than invest in public goods like education or affordable medical services.

In addition, inequality reduces social cohesion and trust, which studies suggest is important to individual health.

Finally, inequality may develop a culture of upward comparison, where many people see the lifestyles of the rich and feel they can’t live up to expectations. That can lead to negative views of themselves, frustration and depression, which have been linked to sickness and mortality.

“None of these negative factors caused by income inequality will have an immediate effect on chronic illness and mortality,” Zheng said.

"But over time they take a toll on health, which can eventually lead to sickness and death. That's why this study found that it takes 5 years for the effects of income inequality to appear." 

The study appears online in the journal Social Science and Medicine and will be published in a future print edition.