That rapid, under-the-political-radar expansion has turned the program into a massive budget item. As of 2010, its monthly cash payments accounted for nearly one out of every five Social Security dollars spent, or about $124 billion. In 1988, by comparison, it accounted for just one out of eight Social Security dollars. Because disabled workers qualify for Medicare, they also added $59 billion to the government's healthcare tab.

Are disabilities just becoming more common? According to economists such as MIT's David Autor, the evidence says no. The workforce is indeed getting older, and thus more ailment prone. But Americans over 50, who make up most disability cases, report much better health today than in the 1980s. And demographers have found that the percentage of Americans older than 65 suffering from a chronic disability has fallen drastically since then. In the end, economists Mark Duggan and Scott Imberman estimate that, at most, the graying of America's workers explained just 4 percent of the increase in the rate of disability program participation for women, and 15 percent for men, through 2004.

Instead, it seems two things have happened: Qualifying for disability got easier, and finding work got harder. As the Planet Money piece puts it, "there's no diagnosis called disability." According to the letter of the law, disability recipients must prove they are too physically or mentally impaired to hold a job. And early in the program's life, the most commonly reported ailments were easy-to-diagnose problems such as heart-disease, strokes, or neurological disorders. But after the Reagan administration began trying to thin out the program's rolls in the early 80s, an angry Congress reacted by loosening its criteria. Suddenly, subjective measures like self-reported pain or mental health problems earned more weight under Social Security's formula. Today, the most common diagnoses are musculo-skeletal issues, such as severe back pain, and mental illnesses, such as mood disorders -- health problems where the line between a disability and a mild impairment is far blurrier.

Just as the bar for disability fell, the economy turned on the working class. Factories laid off their assembly workers. The service sector picked up the slack. Wages stagnated for anyone without a college diploma. These changes have made disability more attractive for reasons both obvious and subtle. Although program's payments are small -- the average benefit is a bit over $1,000 per month -- they're not much worse than a minimum wage job. Better yet, they're indexed to inflation, meaning they sometimes rise faster than wages, and come with generous government healthcare. For former blue-collar workers who feel they've lost all hope of finding employment, or who don't want to spend their last years leading to retirement standing all day at McDonald's, disability isn't a bad offer.