WE Americans believe in progress, and yet progress is often a double-edged sword. The benefits and adventures of change often vie with the shortcomings and disruptions, leaving us in a twilight zone of ambiguity and doubt about the ultimate outcome.

Few subjects better illustrate this than the decline of marriage, as Isabel Sawhill shows in her sobering book Generation Unbound: Drifting into Sex and Parenthood without Marriage.

Even those who know marriage is on the skids - presumably, most of us - may be surprised by the extent of its decline. A little history helps. To Americans coming of age in the 1950s, the expectation was that most would marry. It was part of society's belief structure. And most did. Now these powerful social pressures have faded and, for many, disappeared.