The study was conducted by the National Institute on Aging (NIA), part of the National Institutes of Health, in collaboration with the University of California, Berkeley, and the University of Toronto.

The researchers report that those living below the poverty line are the most likely to say they have functional limitations, and, up to age 84, the odds of having such limitations drops with each incremental increase in income. They also note that older people are less likely to report functional limitations with each increase in educational level, a measure that is closely tied to income.

"We found that a 'gradient of disability' exists across the full socioeconomic spectrum, as functional limitations proved inversely related to household income," says senior author Jack M. Guralnik, M.D., Ph.D., chief of the NIA?s Laboratory of Epidemiology, Demography and Biometry.

Improved understanding of the relationship between socioeconomic status and disability is critical as the U.S. population ages, Guralnik notes. The rate of disability decreased 1 to 2 percent annually during the 1980s and 1990s, when trends were last reported, and the rate of decline was smaller among those in the poorest socioeconomic groups.

Guralnik and co-authors Meredith Minkler, D.P.H., University of California, Berkeley, and Esme Fuller-Thomson, Ph.D., University of Toronto, analyzed data for more than 335,000 community-dwelling people 55 and older who participated in the Census 2000 Supplementary Survey. Nearly one in four respondents reported having a functional limitation, defined as a long-lasting condition that substantially limits one or more basic physical activities, such as walking, climbing stairs, reaching, lifting or carrying.

Functional limitation differences by income level were evident among those 55 to 64 years, 65 to 74 years, and 75 to 84 years, but differed more dramatically in the younger age groups. Among all respondents under age 85, even those whose incomes were at six times the poverty threshold had significantly higher odds of reporting functional limitations, compared with the wealthiest group.

The poverty threshold in 2000, the year the data were collected, was $8,259 for a person age 65 or older who lived alone and $17,761 for a four-person household. The highest income category used in the analysis — 700 percent or more of the poverty line — began at $57,813 for an older adult living alone and $124,327 for a four-person household.