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The poor live shorter lives than the rich, and this link between social position and health has been been well documented for more than 150 years.

But up until the 1980s,  most research on health inequality focused on the effects of poverty. Studies generally assumed that higher rates of illness and death among the poor arose from material deprivation (lack of medical care, inadequate food, greater exposure to pollution). And public policy hinged on the assumption that there must be a threshold at which further increases in income or socioeconomic status have little or no effect on health.

That changed with the Whitehall Study, a decades-long survey comparing the health and life spans of 17,000 British civil servants of differing pay grades. The study found that not only did those at the bottom of the scale have worse health and higher mortality than those above them, but it also found gains in health and longevity at each step up in job status all the way to the top of the ladder. There was no threshold at which the link between higher social status and better average health stopped.

The Whitehall study, and others that followed, demolished the assumption that material deprivation alone was the cause of social class differences in health. And it fueled a debate that continues to this day about what we should be doing to improve the health and longevity of disadvantaged populations.

Sociologists and public health researchers in recent years have learned much from studying the so-called “social determinants” of health, that is, how lack of education, low income, and the characteristics of neighborhoods and social networks can harm health through complex pathways.

Here’s why it will pay off for journalists to dig into this subject:

The health gap appears to be getting worse. Socioeconomic, race and ethnic disparities in infant death rates and premature mortality shrank between 1966 and 1980 across the U.S. But since then, the relative health inequities have grown wider, according to numerous studies. Virtually all gains in life expectancy since 1980 occurred among the highly educated. Steep reductions in infant mortality have benefitted white Americans more than African Americans, among whom infant death rates are now more than double the rate among whites. In the 1950s, black and white Americans had comparable death rates for heart disease and cancer, but now death rates for both diseases are significantly higher among blacks. Researchers at the Harvard School of Public Health estimate that 14 percent of premature deaths among whites and 30 percent of premature deaths among blacks between 1960 and 2002 would not have occurred if everyone had experienced the mortality rates of whites with incomes in the top one-fifth. Among industrialized countries, the U.S. ranking for life expectancy has dropped from 14th in 1980 to 27th in 2010. That’s stirred debate about the reasons why the U.S. is falling behind. Some blame trends in individual behavior or failures of the health care system. But others suspect that the root cause is worsening social inequality.

Scientists are making fascinating discoveries about the biological ways that social determinants shape health. Prolonged exposure to stress can trigger the release of hormones, such as cortisol and epinephrine, that undermine immunity, boost inflammation, and increase vulnerability to conditions such as diabetes and heart disease. Stress during fetal development, from a mother’s poor diet or exposure to pollutants, for example, may set the stage for diseases decades later in life by altering metabolism or triggering lasting changes in the activity of genes.

Some studies suggest that these “epigenetic” changes in gene expression can be passed on to children and influence the occurrence of disease in more than one generation.

Health reform won’t solve the problem. Expanding health insurance coverage (the focus of the federal Affordable Care Act) is important, but it’s unlikely to compensate for all of the social determinants of health. In countries that established universal health coverage decades ago, for example, lower social status still correlates with worse health and shorter lives. The research on social determinants suggests that progress is likely to require broader social changes, such as improving access to education, boosting economic opportunity, and making disadvantaged neighborhoods safer and and more vital.

Political battles loom. How best to educate children, extend economic opportunity to the disadvantaged, and regulate urban development are controversial questions, to say the least. When it comes to the social determinants of health, political leaders on the conservative end of the spectrum tend to emphasize the role of individual behaviors such as unhealthy eating, lack of physical exercise, smoking, and abusing drugs. And, in fact, a lot of questions remain unanswered. There is not enough evidence yet to say, for instance, whether school funding, tax credits, or income support could do more to improve health than policies more narrowly focused on changing behavior.