WASHINGTON — Married couples have dropped below half of all American households for the first time, the Census Bureau says, a milestone in the evolution of the American family toward less traditional forms.

Married couples represented just 48 percent of American households in 2010, according to data being made public Thursday and analyzed by the Brookings Institution. This was slightly less than in 2000, but far below the 78 percent of households occupied by married couples in 1950.

What is more, just a fifth of households were traditional families — married couples with children — down from about a quarter a decade ago, and from 43 percent in 1950, as the iconic image of the American family continues to break apart.

In recent history, the marriage rate among Americans was at its highest in the 1950s, when the institution defined gender roles, family life and a person’s place in society. But as women moved into the work force, cohabitation lost its taboo label, and as society grew more secular, marriage lost some of its central authority.