Families constitute a smaller part of the American population, with the percentage of U.S. households that include a family falling by one-fifth over the last 40 years.

The Census Bureau reported Monday that 71.5 percent of all households are made up of families currently; in 1948 the corresponding figure was 90.3 percent.Census demographer Steve Rawlings said a variety of factors are involved in the change, including the increasing numbers of widowed elderly living alone, unmarried couples and young people postponing families to pursue education and careers.

The Census Bureau reported that, overall, there were 91.1 million households in the United States as of March, of which 65.1 million were families. That's up from 89.5 million households, including 64.5 million families, a year earlier.

By definition, a household is one or more persons occupying a housing unit. A family includes at least two people related through birth, adoption or marriage.

The preliminary 1988 report on families and households also disclosed that:

- There were 31.9 million families that had a child under age 18 living at home, compared to 33.2 million without a child. Families without children at home surprassed those with children in 1985 for the first time.

- Two-parent families are declining, slipping from 40 percent of all households in 1970 to just 27 percent this year. There are slightly more married couples with one or two children, but these were more than offset by the decline in those with three or more.

- There are 21.9 million one-person households, up 20 percent from 1980. Women account for six out of every 10 people living alone.

- Unmarried couples living together totaled 2.6 million, up 63 percent from 1980. Some 31 percent of these couples have children at home.

- The median age at which women marry for the first time is 23.6 years, the same as last year and the highest on record. For men, first marriage occurs at 25.9 years, tying 1900 as the highest in this century.

The youngest median marriage ages were 22.5 for men in 1959 and 20.1 for women in 1956.