How Economic Inequality Is (Literally) Making Us Sick

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Imagine there was one changeable factor that affected virtually every measure of a country’s health— including life expectancy, crime rates, addiction, obesity, infant mortality, stroke, academic achievement, happiness and even overall prosperity. Indeed, this factor actually exists.

It’s called economic inequality. A growing body of research suggests that such inequality — more so than income or absolute wealth alone — has a profound influence on a population’s health, in every socioeconomic group from rich to middle class to poor.

As the protests against increasing inequities between rich and poor spread from Wall Street, it’s clear that the issue is crucial — not least for the understanding of human health. On Wednesday in Rio, in fact, a World Health Organization conference focused on related issues is being attended by high-level health officials from at least 60 nations around the world.

Economic inequality is measured by looking at the distribution of wealth and income in a society, not the general wealth of a country. At a basic level, a country’s overall economic success does predict its people’s well-being, but the healthiest and happiest countries in the world are not the richest. Rather, they are countries where wealth is shared widely and more equally.

One obvious way in which economic equality may improve health is by reducing barriers to health care among the poor. But it turns out that the health gap exists even in countries with national health services, so the roots of the problem appear to reach deeper than that. Indeed, they may go back to the dominance hierarchies of our primate ancestors.

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As studies of wild baboons in Africa have shown, there are certain key side effects of inequality — namely, stress. Baboons have a rigidly enforced social hierarchy in which fights to win alpha status are common and higher-ranking males constantly abuse and bully those below them. Not surprisingly, this results in chronically elevated levels of stress hormones in the lower ranks.

Chronically high stress hormone levels are bad news, for both humans and baboons. While these hormones can be helpful in short-term fight-or-flight situations, if they are elevated over long periods of time, they increase the risk of virtually all major mental and physical illnesses, including stroke, heart disease, diabetes, depression, infectious disease, many cancers and, of course, all types of addictions.

In humans, in fact, differences in health linked to social status — which tracks closely with economic status — have often been attributed only to addictions and to the generally bad health habits of the poor, such as eating a lousy diet. But baboons don’t have these “lifestyle factors” and yet increased mortality in the lower ranks is still seen.

Human studies bear out the association between low socioeconomic status and ill health. Sir Michael Marmot, professor of epidemiology at University College London, has spent decades looking at the effects of status differences in a population very different from baboons: British civil servants working in the government bureaucracy based in Whitehall Street in London.

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Marmot found large health differences across the social scale. “There are two striking findings. The first was that there was about a threefold difference between the top and bottom in mortality. That’s absolutely enormous,” he says. “The second was that it wasn’t just a difference between the top and bottom; it was what I called a social gradient. There was a stepwise relationship between your socioeconomic position and your health.”

He adds, “Your readers who are not at the bottom [may want to know] that the second from the top had worse health and higher mortality than the top and the third was worse than the second.”

The effect is much more profound than that seen in baboon societies. As Stanford biologist Robert Sapolsky, who led much of the research in stress in African baboons, once told me: “When humans invented inequality and socioeconomic status, they came up with a dominance hierarchy that subordinates like nothing the primate world has ever seen before.”

Moreover, Marmot points out, the people who were on the “bottom” in his research were not poor or unemployed. “In Whitehall, we’re not dealing with poverty. We’re dealing with people in stable white collar employment and [still we see] this graded relationship between where they are in the hierarchy and health,” he says.

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When Marmot and his colleagues controlled for lifestyle factors like smoking and lack of exercise in the lower socioeconomic groups, the gradient remained. “Those things accounted for about a third of the gradient,” he says, noting that you have to look for the “causes of the causes” — the reasons that the lower classes might be driven to smoke, drink, take drugs or indulge in sweet, fatty foods. “It’s now been described the world over,” Marmot says.

Indeed, in country-to-country comparisons, researchers find that the greater the difference between the richest and the poorest in a society, the worse off everyone in that society seems to be. “It looks as if what inequality does is amplifies the effect of social status differences,” says Richard Wilkinson, an epidemiologist and co-author of the British bestseller The Spirit Level: Why Greater Equality Makes Societies Stronger.

In countries such as Japan and those in Scandinavia, which have the lowest levels of socioeconomic inequality in the developed world, the populations also enjoy overall greater life expectancy, lower infant mortality, reduced obesity, heart disease and mental illnesses, and lower rates of murder and addictions, compared with nations with high inequality like the U.K., and even more so, the U.S. Countries with wider equality also have higher levels of academic achievement and happiness.

One study in Finland found that mortality rates were twice as high in the poorest people as in the richest, a much less stark difference than that seen in Whitehall, where bottom-of-the-ladder folks had a threefold higher mortality risk than those at the top. Marmot cautions that such comparisons are hard to make accurately and that even in relatively equal countries, some status differences still exist. Still, he says, “Fairly detailed comparisons done across Europe suggest that the magnitude of the gradient is less in Nordic countries.”

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So how exactly does inequality lead to so much stress? By degrading our relationships with one another. “The most important pathway is the effect of inequality on the quality of social relationships in a society,” says Wilkinson. “I think the more unequal you get, the more competitive social relationships become, because there’s more status competition.”

And because good relationships are crucial for protecting people from stress, socioeconomic inequality creates an environment in which people with the most stress can’t counter its negative health effects. This social competition degrades relationships and erodes trust across the socioeconomic spectrum — and not just among those on the bottom. (More equal societies also have higher levels of trust.)

Although researchers like Marmot haven’t been able to study the top 1%, at least one recent finding in baboons should give pause even to the very well-off.

The study found that alpha males — those at the very peak of the hierarchy — were actually just as stressed as their lowest-ranked followers. The least stressed among the baboon troops were the beta males, the No. 2’s in the hierarchy. Unlike the alphas, they could chill out and reap the perks of power without constantly having to fight off challengers.

Even when you’re at the top, then, inequality affects you and a gradient always remains for those who are successively lower down.  But it’s not inevitable: in one baboon troop Sapolsky studied, rank appeared not to affect stress and health. What happened there was that the meanest top-ranked animals had died off due to eating tainted meat, leaving the mellower ones in charge, which reduced rank-related bullying.

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In the U.S., inequality has been rising since the 1980s. Between WWII and Ronald Reagan’s election to the presidency, average income grew by $19,000. The bottom 90% of the country received 65% of that increase.

Between 1981 and 2008, however, average income grew by around $12,000. About 96% of that went to the top 10% richest people in the country. The ratio of pay between CEOs and average workers also became much more extreme over the same time period: in 1980, it was around 35 to 1. Today it is about 185 to 1.

If we want to be healthier and happier, we can’t ignore inequality.

Maia Szalavitz is a health writer at Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.