Inequality is at an all-time high in America. Since the 2008 crash, recent IRS figures show, the wealth of the top 1 percent grew 31 percent while the rest of American incomes grew by less than 1 percent. But although it might appear that income disparities affect only the poor and have primarily an economic impact, dozens of studies now link extreme inequality with poor health and shorter lives, across the entire socioeconomic spectrum.

Overall, the United States has among the largest social and economic inequalities of any rich country. Japan and the Scandinavian countries have the smallest. The more equal countries also have the longest life expectancies — and the richest American men only have the life expectancy of an average Japanese man, which is 4.5 years longer than the U.S. average, according to Sir Michael Marmot, a leading researcher on inequality and professor of epidemiology at University College London. He notes that residents of affluent suburban Maryland live, on average, 17 years longer than people in inner city Washington, D.C.

Marmot’s own research focuses on the UK, where a national healthcare system provides all socioeconomic classes with quality care. He has compared low- and high- ranking British civil servants over the course of their lives on a variety of health measures, ranging from cancer to obesity to alcohol addiction. For virtually all conditions except breast and prostate cancer (it is not clear why these are exceptions), Marmot found that those at the bottom are at dramatically greater risk, with overall mortality up to three times higher, depending on the specific condition. Increased levels of unhealthy behavior among the less-affluent — like smoking — did not account for all of the differences. Also, even the lowest-ranked civil servants in Marmot’s research were employed, meaning that those on bottom rungs weren’t impoverished, simply less well-off.

The reason for the differences, say Marmot, Stanford neuroscientist Robert Sapolsky, Rockefeller University’s Bruce McEwen and a growing number of their peers, is found instead in the stress system, which in primates seems fine-tuned to rank and hierarchy — not absolute poverty, but relative social position.

“For the poor, more inequality means more anger at what they don’t have and more cognitive load from the worry about how to keep up,” Sapolsky says. “For the wealthy, it’s more fear about the menace of the have-nots and more effort put into walling themselves off from them. For everyone, there’s less social support — by definition, the more widely-spread and unequal a hierarchy, the fewer peers one has, and true social support requires the symmetry of peers.”