Low social status 'can damage immune system'
Simply being at the bottom of the social heap directly alters the body in ways that can damage health, a study at Duke University in the US suggests.
Monkey experiments showed low status alters the immune system in a way that raises the risk of heart disease, diabetes and mental health problems.
One expert said the findings were "terrifically applicable" to people.
The findings, in Science, had nothing to do with the unhealthy behaviours that are more common in poorer groups.
The gulf in life expectancy between the richest and poorest is huge - in the US it is more than a decade for women and 15 years for men.
Part of the explanation is that people from poorer backgrounds are more likely to have a worse lifestyle - including smoking, little exercise and diets containing junk food.
But the latest study goes further to show low status - with all of those other factors stripped out - still has an impact on the body.
Looking at 45 non-human primates allowed scientists to adjust only social status to assess its impact - something impossible to do in people.
The captive Rhesus monkeys - who were all female, unrelated and had never met before - were divided one-by-one into nine new groups of five.
The newest member nearly always ended up at the bottom of the social order and became "chronically stressed", received less grooming and more harassment from the other monkeys.
A detailed analysis of the monkeys' blood showed 1,600 differences in the activity levels of genes involved in running the immune system between those at the top and bottom.
It had the impact of making the immune system run too aggressively in those at the bottom. High levels of inflammation cause collateral damage to the body to increase the risk of other diseases.
One of the researchers, Dr Noah Snyder-Mackler, told the BBC News website: "It suggests there's something else, not just the behaviours of these individuals, that's leading to poor health.
"We know smoking, eating unhealthily and not exercising are bad for you - that puts the onus on the individual that it's their fault.
"Our message brings a positive counter to that - there are these other aspects of low status that are outside of the control of individuals that have negative effects on health."
Further experiments showed the immune system was not fixed and could be improved, or made worse, by mixing up the social rankings.
Sir Michael Marmot, one of the world's leading experts on health inequalities and based at University College London, said the findings were "extraordinarily interesting" and underpinned much of his own research.
He told the BBC News website: "This is hard science saying there's a plausible biological mechanism that results in clear differences depending where you are in the hierarchy.
"The gateway through which the social environment impacts health is the mind. Whether it is unhealthy behaviours or direct stress, the mind is crucial and this study is lending real credence to that."
'Governments don't get it'
While Rhesus macaques do form strict societies, they are far more simplistic that human ones.
But Prof Graham Rook, from University College London, told the BBC News website: "All the evidence is showing the findings are terrifically applicable to humans."
He pointed to evidence suggesting people at the bottom end up with worse health when the top gets richer, even if they themselves do not get any poorer.
He said: "It is something governments just don't understand; they think people at the bottom have got cars, have got TVs, so compared with people in India they're enormously wealthy.
"But that really isn't the point, they feel they are at the bottom of the heap."
Hierarchies are a fixture of society. However, the researchers believe more can be done to ease the health problems coming from being bottom of the pile.
Dr Snyder-Mackler said: "Status is always relative, but if we could flatten the slope so the differences between the highest and lowest weren't as much, or find ways to focus attention on poorer social environments so that they are less stressful, we could mitigate some of these effects.
"It's a hard problem that might never be fixed, but it might be possible to make it less worse."
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