A major public health mystery arose in 1984 when a British team reported a linear negative correlation between British civil servants' age at death and highest attained job level (Lancet, Vol. 323, No. 8384): All other factors being equal, the higher up you are on the corporate food chain, the longer you live, and nobody was quite sure why.

The usual explanations of poverty and lack of health-care access didn't explain it. Most people thought it was related to stress, but that's a difficult variable to measure, and one that sometimes gives contradictory results. Nancy Adler, PhD, professor of medical psychology at the University of California, San Francisco, explored this mystery and shared her strategy for getting more meaningful measurements at APA's 2009 Annual Convention.

With her colleagues, Adler has explored the repeated exposure to stress that's often associated with lower socioeconomic status (SES). Over time, that stress builds and makes one more vulnerable to disease—a process known as allostatic load. Chronic stress can affect the functioning of the central nervous and endocrine systems, undermining people's immune systems and making them more prone to diabetes, cardiovascular disease and obesity, Adler said. And for people of lower SES, stressors are much more frequent and threatening.

For example, a misfortune like a flat tire might be a trivial event for a person of higher SES, but it's much more stressful to someone who then has to make the decision between feeding his or her family or getting a new tire. Furthermore, Adler said, even ambiguous events look more threatening to people of lower SES. Constantly being vigilant against potential harm is itself very taxing. All of those factors increase people's allostatic load, and consequently tax their health.

The three factors comprising SES—income, education and occupation—each predict health outcomes on their own, but they're not interchangeable. Each provides a slightly different picture, which makes SES measurement hazy. To assess people's sense of where they stand overall, Adler developed the MacArthur Scale of Subjective Social Status, which asks respondents to report what they consider to be their SES on a ladder scale.

To see if lower subjective status is linked to greater stress response, volunteers in one study were given stressful public speaking assignments over three days, then measured for the amount of the stress hormone cortisol in their blood. Sure enough, people who reported themselves lower on the SES ladder had higher levels of cortisol than those who said they were higher on the ladder. "One of the reasons that the ladder works is because it's a more nuanced look at SES," Adler said. "People understand and get a gut feel for what exactly is being measured." Traditional measures of income, education and occupation don't take that into account, she added.

Better understanding of the interplay between SES and health is the first step toward decreasing health disparities, Adler said.