Here is a charming statistic: divide the U.S. by race, sex and county of residence, and differences in average life expectancy across the various groups can exceed 30 years. The most disadvantaged looks like denizens of a poor African country: a boy born on a Native American reservation in Jackson County, South Dakota, for example, will be lucky to reach his 60th birthday. A typical child in Senegal can be expected to live longer than that.

America is not alone in this respect. While the picture is extreme in other rich nations, health inequalities based on race, sex and class exist in most societies and are only partly explained by access to healthcare.

But fresh insights and solutions may soon be at hand. An innovative project in Chicago to unite sociology and biology is blazing the trail, after discovering that social isolation and fear of crime can help to explain the alarmingly high death rate from breast cancer among the city's black women. Living in these conditions seems to make tumors more aggressive by changing gene activity, so that cancer cells can use nutrients more effectively.

We are already familiar with the lethal effect of stress on people clinging to the bottom rungs of the societal ladder, thanks to pioneering studies of British civil servants conducted by Michael Marmot of University College London. What's exciting about the Chicago project is that it both probes the mechanisms involved in a specific disease and suggests precise remedies. There are drugs that may stave tumors of nutrients and community coordinators could be employed to help reduce social isolation. Encouraged by the U.S. National Institutes of Health, similar projects are springing up to study other pockets of poor health in populations ranging from urban black men to white poor women in rural Appalachia.

To realize the full potential of such projects, biologists and sociologists will have to start treating one other with a new respect and learn how to collaborate outside their comfort zones. Too many biomedical researchers still take the arrogant view that sociology is a "soft science" with little that's serious to say about health. And too many sociologists reject any biological angle-fearing that their expertise will be swept aside and that this approach will be used to bolster discredited theories of eugenics, or crude race-based medicine.

It's time to drop these outdated attitudes and work together for the good of society's most deprived members. More important, it's time to use this fusion of biology and sociology to inform public policy. This endeavor has huge implications, not least in cutting the wide health gaps between blacks and whites, rich and poor.