Researchers have long known that social class is one of the most powerful predictors of health, more powerful than genes, smoking, alcohol intake, or other health risks.

The lower a person is on the social ladder — as measured by income, education and other markers of relative status — the higher the risk of heart disease, cancer, diabetes, psychiatric disorders and a host of other illnesses. One recent study based on income data from 1.4 billion tax records found that people in the top 1 percent income bracket had life expectancies that were as much as 10 to 15 years longer than those in the bottom 1 percent.

But investigators do not know for sure whether lower social status, which often comes with less access to health care, a lack of control over one’s life circumstances and a variety of other stresses, causes people to end up sicker, or whether being less healthy leads to lower social status. And although researchers have speculated how social class might influence health, they still have little evidence for what those mechanisms might be.

The answers to these questions are unlikely to come from studies of humans, since human subjects cannot be randomly assigned to a social class or moved up and down the social ladder to see what happens.