Q. What’s the No. 1 cause of blindness in older adults in the United States?

A. “It sounds like a simple question, but there’s no perfect answer,” said Dr. Susan Vitale, a research epidemiologist at the National Eye Institute of the National Institutes of Health. “It depends on age, how blindness is measured and how statistics are collected.” For example, some studies have relied on the self-reported answer to the vague question: “Do you have vision problems?”

The best available estimates, she said, come from a 2004 paper aggregating many other studies, some in the United States and some in other countries, updated by applying later census data.

This paper and others have found striking differences by age and by racial and socioeconomic groups, Dr. Vitale said. In white people, she said, the major cause of blindness at older ages is usually age-related macular degeneration, progressive damage to the central portion of the retina. In older black people, the major causes are likely to be glaucoma or cataracts. In older people of working age, from their 40s to their 60s, the major cause, regardless of race, is diabetic retinopathy, damage to the retina as a result of diabetes.

Many studies have shown that white people are more likely to have age-related macular degeneration, Dr. Vitale said, but as for cataracts, for which blindness is preventable by surgery, there are questions about access to health care and whether those affected can get the needed surgery. It is not known why black people are at higher risk of glaucoma. There are also some gender differences, she said, with white women more likely than white men to become blind. Studies have not found the same difference by gender in black and Hispanic people.