Americans are falling out of love with marital bliss. Look at the data. As of 2010, only 51 percent of Americans 18 or older were married, compared with 72 percent in 1960. Exacerbated by a weak job market, the drop is starker still among the young. Today, just a fifth of Americans ages 18 to 29 have a spouse, down from roughly three-fifths in 1960. The number of marriages performed in the United States fell by 5 percent from 2009 to 2010, according to the Pew Research Center. That was partly the result of a sagging economy, but it also represents an acceleration of longer-term trends seen in the United States, Europe, and elsewhere in the developed world.