Between 1996 and 2015, the number of working-age Americans who subsist on federal disability benefits grew rapidly, becoming one of the country’s most hotly debated social benefits. The rise has become another indicator of the divide between urban and rural America, where disability benefit rates are nearly twice as high.

Here are answers to some of the most important questions about this form of public assistance.

Q: What programs serve Americans with disabilities?

A: Two federal programs provide benefits to people with disabilities. One is Social Security Disability Insurance (SSDI), which was signed into law in 1956 and serves disabled workers. It is available only to people with enough work experience. The other, Supplemental Security Income (SSI), which began paying benefits in 1974, serves the disabled poor and is a means-tested benefit.

Q: If the number of people receiving disability has increased so much, it must be easy to get on the rolls, right?

A: Wrong. Disability benefits, for both SSI and SSDI applicants, are very difficult to secure. In fact, only about four in 10 applications are approved. It can take as long as two years, after several layers of appeals, to win approval.

Q: So what has driven the growth then?

A: This is more controversial. Most experts agree that the primary factor in SSDI’s growth is demographics. Because of the aging of the baby-boom generation, more people are at an age when disability is more likely. More women also are in the workforce. Then there is simple population growth. Some experts think those factors represent nearly all of the increase. But others point to a congressional act that broadened the definition of disability, economic factors such as recessions and approvals they criticize as too lenient as contributing factors.

Q: Is the number of people on disability still rising?

A: No. In 2015, for the first time in decades the number of people on disability decreased slightly as the demographic factors that drove the rise started to subside and older Americans aged into their retirement years.

Q: In that case, what’s the problem?

A: The program for disabled workers, which Congress had to rescue from insolvency in 2015, is estimated to go broke again sometime over the next decade or so. The government this year is expected to spend $192 billion on disability payments — more than the combined total that will be spent on welfare, unemployment benefits, housing subsidies and food stamps.

Q: What sort of income, individually, does this provide for recipients?

A: A meager one. The maximum monthly payment for an SSI beneficiary is $735. The amount of money SSDI beneficiaries receive depends on how much money they made. The average monthly check is less than $1,200. For most beneficiaries, the checks are not taxable.

Q: Do they receive any other services?

A: Beneficiaries receive health insurance through Medicare, for SSDI recipients, and Medicaid, for SSI recipients.

Q: Do beneficiaries ever return to work?

A: Rarely. Some die within a few years of starting benefits. Other disabled workers try to return to work, but don’t keep their jobs for long. Or they continue working, but make less than the income threshold that would put their benefit at risk. Less than 4 percent of disabled workers get off disability within 10 years of their first payment.

Q: Are disability beneficiaries spread evenly nationwide?

A: The majority of disability recipients live in densely populated urban and suburban areas, but they are disproportionately prevalent in rural America — where, on average, 9.1 percent of the working-age population receives disability, compared to the national average of 6.5 percent and an urban rate of 4.9 percent. Beneficiaries are even more overrepresented in the Southeast and central Appalachia. These are places economists have called “disability belts.”