Study author Hui Zheng said the findings suggested income inequality at any one point doesn't work instantaneously -- it begins to increase mortality rates five years later, and its influence peaks after seven years, before fading after 12 years.
Zheng used data from the U.S. National Health Interview Survey from 1986 to 2004 with mortality follow-up data from 1986 to 2006. His final sample involved more than 700,000 people age 30 and older.
The study measured income inequality using three different methods, including the most commonly used metric -- the Gini coefficient, calculated by the U.S. Census Bureau. All three methods resulted in similar findings, Zheng said.
The Gini coefficient has been steadily rising in the United States in recent decades.
The study, published in the journal Social Science and Medicine, found a 0.01 unit rise in the Gini coefficient increased the cumulative odds of death by 122 percent in the following 12 years.
"This finding is striking and it supports the argument that income inequality is a public health concern," Zheng said in a statement. "For the first time, we can clearly capture the long-term effect of income inequality on health."
Some hypothesize income inequality may lead to higher death rates because of cuts in social services and healthcare. Inequality reduces social cohesion and trust, which is important to individual health, and as the rich get richer many feel they have failed and depression increases, Zheng suggested.