Married couples, whose numbers have been declining for decades as a proportion of American households, have finally slipped into a minority, according to an analysis of new census figures by The New York Times.

The American Community Survey, released this month by the Census Bureau, found that 49.7 percent, or 55.2 million, of the nation’s 111.1 million households in 2005 were made up of married couples — with and without children — just shy of a majority and down from more than 52 percent five years earlier.