The findings are further evidence for what the British Medical Journal, which published the studies, describes as a "big idea" - "that what matters in determining mortality and health in a society is less the overall wealth of that society, and more how evenly wealth is distributed."
Measures which reduce inequality are more likely to be effective in improving health than measures which increase overall wealth but also increase inequalities - the trend that has been underway in both Britain and the US over the past two decades, the journal says.
The studies from Harvard and Berkeley add "new authoritative data" to the argument that it is not just the absolute standard of living which matters but relative differences within a society - possibly as a result of "loss of social cohesion, depression, isolation, insecurity and anxiety".
Past data has shown that within the EU, for example, life expectancy has increased most in countries where the distribution of income has become more equal. The studies now show something similar among US states; those with the most unequal share of household income had the highest death rates, and the wider the income distribution the smaller their decline in mortality between 1980 and 1990. The same is found to hold true for specific causes of death which included heart disease, cancer and homicide.
"Policies which deal with the growing inequities in income distribution may have an important impact on the health of the population," the authors of the Harvard study say.
The Berkeley study says its findings give "cause for alarm, given the increasing inequality of income and wealth in the United States", while in a commentary, George Watt, Professor of General Practice at the University of Glasgow, argues that with the same thing happening in the UK self-interest may eventually make the better-off interested in greater equality. "The advantages of living in a cohesive society may outweigh those of living in a free market," he said.
"If we share the resources of our country more fairly, we shall have a more cohesive society and reduce inequalities in health. It will not happen the other way around."