Starting with the Bush I recession in 1990, the numbers of Americans on disability rolls has surged. The Great Recession has further accelerated the trend. Now disability spending > (food stamps + welfare combined), with 14M Americans participating (9% of the labor force).
Of course some people are truly disabled and need public assistance to survive, but many others suffer from the same "ailments" that working people have. I am not saying that these people are lazy "takers", and in fact there is a higher prevalence in southern Red states (and of course is correlated with education levels and local economic conditions). And that is kind of the point - due to structural changes in the US economy and labor force, tens of millions of Americans are now effectively unhirable in the 21st Century. Often they come from blue-collar non-desk jobs that are more physical, so it is likely that they have various pains and elements. So it is easy for a physician to approve many of them for disability. But the problem is that once they are approved and on the rolls, there is next to zero incentive for them to find a job and go through the effort of retraining. It's not like welfare and unemployment with a finite time span of benefits. And speaking of welfare - that is a driver too. "Reforms" under Clinton and Newt effectively shifted people on welfare to other programs such as disability, so it didn't really save the country much money or motivate the labor force. Probably America would get more bang for its buck if it invested in worker transition and retraining programs rather than just hide/relegate these people to the disability space (the US spends about $260B/year on the 2 major disability programs). I am not advocating kicking legitimate people off these programs, but I think it behooves us to at least perform a cost and impact analysis, and propose improvements/alternatives. Disability is just a band-aid to mask our larger labor and health care problems.
There is even a cottage industry now ("disability industrial complex") to advise and help people to get approved for disability - similar to the folks who help kids get into college or help immigrants get their green cards (one firm took in $70M in fees in 2012 alone). And some families may now see their kids as "cash cows" too, by enrolling them and collecting additional benefits. The number of kids on disability has grown 4X since 1990. Clearly medicine has improved since then, and doctors are better able to diagnose disabilities, but it is pretty much impossible to attribute that huge rise to actual increases in disability diagnoses and incidence.