American households have shrunk to their smallest size ever, containing just half as many people as at the start of the Civil War.

The Census Bureau reports that the average household contained 2.64 people as of last July 1, continuing a long-term decline. By comparison, the 1860 census found an average household of 5.28 peopleUtah had the largest household size, 3.19 in 1987 and 3.20 in 1980. Hawaii had the second largest, 3.02 in 1987, and the District of Columbia had the smallest size, 2.35.

Idaho's household size was 2.74 in 1987 and 2.85 in 1980.

While the household size has declined nationally, the number of households has continued to climb, topping 90 million for the first time in 1987, the bureau said Monday.

Household size is shrinking because the number of households is increasing faster than the overall population. This is occurring, said bureau population expert Campbell Gibson, because of changes in the country's age structure.

The many Americans born in the post-World War II baby boom are now in their 20s and 30s, when they are most likely to set up households on their own, he said.

The number of households rose from 80.4 million in 1980 to 88.8 million in 1986 and is now past the 90 million mark.

At the same time the average number of persons per household declined from 2.75 in 1985 to 2.65 in 1986 and now to 2.64, Gibson said.

By comparison, the 1970 Census found the average household contained 3.14 people. In 1930 households averaged more than 4 people, and they averaged more than 5 in 1880. The nation's first census in 1790 found an average household size of 5.79 people.

While the growing number of ever smaller households continues a trend, Gibson pointed out that household growth isn't outpacing population increases as fast as in the 1970s.

The same age factors that are increasing households now also occurred then, Gibson said. But in addition, in the 1970s the proportion of households in each age group was also growing. That has not continued in the 1980s, he said.

"This could be occurring for a number of reasons," Gibson said.

For example, in this decade more young people have chosen to remain home with their parents instead of setting up housekeeping on their own, as many had done in the 1970s. This could be caused by the increasing cost of housing, he said.

Delays in marriage, while young people pursue careers and school, have also been widely reported as a factor.