In 1910, Henry Van Dyke wrote a book called “The Spirit of America,” which opened with this sentence: “The Spirit of America is best known in Europe by one of its qualities — energy.”

This has always been true. Americans have always been known for their manic dynamism. Some condemned this ambition as a grubby scrambling after money. Others saw it in loftier terms. But energy has always been the country’s saving feature.

So Americans should be especially alert to signs that the country is becoming less vital and industrious. One of those signs comes to us from the labor market. As my colleague David Leonhardt pointed out recently, in 1954, about 96 percent of American men between the ages of 25 and 54 worked. Today that number is around 80 percent. One-fifth of all men in their prime working ages are not getting up and going to work.

According to figures from the Organization for Economic Cooperation and Development, the United States has a smaller share of prime age men in the work force than any other G-7 nation. The number of Americans on the permanent disability rolls, meanwhile, has steadily increased. Ten years ago, 5 million Americans collected a federal disability benefit. Now 8.2 million do. That costs taxpayers $115 billion a year, or about $1,500 per household. Government actuaries predict that the trust fund that pays for these benefits will run out of money within seven years.