Financed with Social Security taxes, SSDI provides cash assistance to people who are permanently disabled and unable to work, and who previously worked and paid Social Security payroll taxes. Although the number of work credits required to qualify for SSDI depends on the age of disability onset, an individual must typically have 40 credits, of which 20 must be from the last 10 years (four work credits can be earned per year). The size of the monthly benefit depends on the beneficiary’s earnings record. Widows, widowers and adults who are blind or disabled since childhood are also eligible for SSDI.