The middle class that President Obama identified in his State of the Union speech last week as the foundation of the American economy has been shrinking for almost half a century.

In the late 1960s, more than half of the households in the United States were squarely in the middle, earning, in today’s dollars, $35,000 to $100,000 a year. Few people noticed or cared as the size of that group began to fall, because the shift was primarily caused by more Americans climbing the economic ladder into upper-income brackets.

But since 2000, the middle-class share of households has continued to narrow, the main reason being that more people have fallen to the bottom. At the same time, fewer of those in this group fit the traditional image of a married couple with children at home, a gap increasingly filled by the elderly.