Most studies that have examined growing levels of health disparity in the United States have focused on the gap between the "haves" and the "have-nots" in terms of socioeconomic factors such as education and income.
But a new study has found, for the first time, that health disparities have grown dramatically among people in the same socioeconomic groups often times more than the disparities have grown between groups.
For example, researchers found that health disparities between employed and unemployed men decreased over the course of the study, while disparities among men who were employed actually increased.
"People with the same socioeconomic status differ much more from each other in terms of health than they did 20 or so years ago," said Hui Zheng, lead author of the study and assistant professor of sociology at Ohio State University.
"This is an issue that really hasn't been studied before and needs to be examined more closely."
Zheng conducted the study with Kenneth Land of Duke University. Their results appear in the March 2012 issue of the journal Social Science Research.
The study is based on data from the National Health Interview Survey for the 24-year period from 1984 to 2007. The survey, which includes about 30,000 people each year, is conducted by the National Center for Health Statistics.
The survey asked respondents to rate their own health on a five-point scale from poor to excellent. While this is a self-report and not based on any objective health data, previous studies have shown that self-reported health status is a good indicator of objective health and is actually better at predicting mortality among the elderly than assessments by doctors, Zheng said.
For one part of the study, the researchers separated the respondents into 432 groups based on various combinations of family income, education, employment status, age, race and marital status.
"Health scholars often emphasize between-groups disparities in health, but here we found that within-group disparities often far exceed the between-groups disparities, and the changes in total health disparities in the last two decades are mainly driven by the changes in the within-group disparities," Zheng said.
The data from this study can't say why within-group health disparities have increased over the past 24 years. Several factors may be in play, depending on the group. For example, the increase in disparity among employed men may be partially due to an increase in the number of men employed only part-time, which may reduce their access to health care services compared to full-time employees.
Overall, other studies have suggested that within-group income instability has increased over the past decades, due to declining unionization and increasing number of people with lower-wage, insecure jobs. This type of instability may lead to greater health disparities within groups, he said.
"We need to find out more about how rising within-group income instability affects rising within-group health disparities," he said.
Zheng said the study identified another source of the growing health gap in the overall population, and that has to do with changes in population composition.
Generally, the researchers found that there are fewer Americans in groups that enjoy the best levels of self-rated health. That was especially true for men. Over the 24 years of the study, men became increasingly less likely to be employed, and less likely to attain middle or upper-income levels both of which are associated with less health disparity.
In contrast, women saw increases in college attainment, employment rate, and middle and upper-income attainment, all of which led to declining health disparities for women in this time period.
These results show how positive changes in the nation's population composition can help reduce the health gap, Zheng said.
"If we can increase educational attainment and labor force participation, and increase social mobility, we will also probably decrease health disparities in our population," he said.
"So, in a sense, economic policy and social policy can also be viewed as health policy."