As a result, by April there were a total of 10.8 million people on disability, according to Social Security Administration data released this week. Even after accounting for all those who've left the program — about 700,000 drop out each year, mainly because they hit retirement age or died — that's up 53% from a decade ago.

To be sure, disability rolls have grown steadily as a share of the workforce since the 1990s (see nearby chart).

The main causes of this broader trend, according to a study by economists David Autor and Mark Duggan, are the loosening of eligibility rules by Congress in 1984, the rise in disability benefits relative to wages, and the fact that more women have entered the workforce, making them eligible for disability.

Their research found that the aging of the population has contributed only modestly to the program's growth.