New research indicates that levels of health disparity have increased substantially for people born in the United States after 1980.
The study also found that health disparity tends to increase as people move into middle age, before declining as people reach old age.
These two results suggest that the gap between the healthiest and least healthy people in the United States as a whole will grow larger for the next one or even two decades as the younger generations grow older and replace previous generations.
A lot will depend on whether future generations will continue the trend, seen in post-baby boomers, of large health disparities.
"If that trend continues, as I expect it will, health disparities in the whole population will increase in the coming decades," Zheng said.
The health gap has not always been growing, according to the study. Health disparities continuously declined from those born early in the 20th century to the baby boomer cohort, before increasing for post-baby boomer cohorts, especially those born after 1980.
Zheng conducted the study with Yang Yang of the University of North Carolina-Chapel Hill and Kenneth Land of Duke University. Their results appear in the December issue of the American Sociological Review.
This study provides one of the clearest, most comprehensive pictures ever of health disparities in the United States because of a methodological innovation, Zheng said.
Zheng and his colleagues combined two statistical models that allowed them, for the first time, to disentangle how health disparity over time is affected by three factors: people's age, when they were born, and the time period when their health is assessed.
"We have never before been able to look at all three of these factors together and see how each interacts with the others to affect changes in health disparities," Zheng said.
The study is based on data from the National Health Interview Survey for the 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 is a good indicator of objective health and is actually better at predicting mortality among the elderly than doctor assessments, Zheng said.
The researchers took into account a variety of other factors that may affect health, including gender, race, marital status, work status, education and income.
Overall, the study found that late baby boomers—those born from 1955 to 1964—reported better health than any other generation. In addition, self-rated health has significantly declined since the late 1990s.
One of the key findings was the large gap in self-reported health that opened up for people born since 1980. That means people are more spread out among the five health categories, from excellent to poor, Zheng said.
This data can't explain why health disparities grew, but research by other sociologists provides potential explanations, Zheng said.
For one, income inequality increased dramatically in the past three decades in the United States, which could impact accessibility to health care and other important resources.
Also, an increase in immigrants, both documented and undocumented, has probably changed the distribution of health ratings in the country, while the growing obesity crisis has added to those in poor health.
Finally, a growing "digital divide" in access to medical and health information on the internet has created disparities in health knowledge among different populations, which can affect health choices and outcomes.
The main reason that health disparity is expected to grow in the whole population in the coming decades has to do with what is happening among young adults born since 1980, Zheng said.
Current young adults have a larger health gap than preceding cohorts and, in addition, disparity rises as people move from youth to middle age, peaking at about age 55. Disparity then declines among the elderly, according to the study.
Those two factors will work together to increase disparity in the whole population as young adults replace their elders in the population.
Most young people are generally healthy, which keeps disparities low, Zheng said. As people age and some develop health problems and diseases, disparity grows. But these disparities fall again in old age as sicker people die and only the healthier people remain. The narrower disparities in old age may also result from the fact that all older people suffer frailty and tend to share the same health risk factors. Another factor may be equalization of health care usage and protections through Medicare coverage after age 65.
The study also found gender differences in health across the lifespan. There is a relatively large gap in early adulthood, with men being healthier than women. This gap narrows until about age 61, as men are more likely to experience more severe forms of chronic conditions, such as heart disease.
The gap widens again after age 61, as only the healthier men survive and there is a relatively larger share of women with poorer health alive at older ages.
While this study focused on health, Zheng said this new model can be used to study other types of inequality, such as income, education or wealth.
"This model provides a powerful framework to identify and study the evolution of inequalities across age, period and cohorts," Zheng said.