"In 1955," says R. Morton Darrow, "10 percent of household units were single people. Today, 25 percent of all households are single people."

Darrow, former senior executive with Prudential Insurance Co. and now head of a national consulting firm, sees continued growth for this segment of the population.

Already, he says, we have left behind what once was considered the traditional nuclear family -- "two people of different sex a husband as wage-earner and a wife as homemaker linked together by a marriage contract, and two or more dependent children" -- and are moving toward recognition of new family definitions.

The old-style nuclear family now makes up only 10 percent of all households today. "Now," says Darrow, "two people of the same sex may constitute a family; a single person with children may constitute a family and six to eight people living together in a communal arrangement are a family."

Darrow's findings and predictions are contained in "The State of Families 1984-85," a report he compiled for Family Service America, a New York-based nonprofit organization representing 280 family-service organizations throughout the U.S. and Canada.

Among conditions identified in the report:

*The average size of families will decrease over the next 25 years, but so will the number of involuntarily childless families and individuals, thanks to the increasing "technology of fertility."

*The baby-boomers, now coming into their own, "will dominate society for another 50 years."

*Baby-boomers could have a powerful effect on the national economy. "If fundamental factors push the inflation growth rate to 9 percent, we are likely to see baby-boomer psychology push the rate to 18 percent and beyond in a very short time as families get rid of their dollars."

The data in the report, notes FSA president Geneva Johnson, "reflects the experience of more than 1 million families who sought help from FSA agencies in 1983."

The four areas of concern most mentioned by family members were unemployment, problems related to being a single parent, government cutback of human services and family violence.