A dramatic increase in the hours worked by wives and mothers kept many families above water in the past decade but about 40 percent of all families lost income, a study released Sunday showed.

"This study leaves little doubt that American families have been under greater economic pressure in the past decade than in any decade of the post-war period," said Jeff Faux, president of the Economic Policy Institute.The study found that average American per capita income growth declined from 2.1 percent in the boom years of the `50s and `60s, to 1.5 percent in the `70s, then down to 1.2 percent in the `80s.

The study, "Family Income Changes in the 1980s: New Pressure on Wives, Husbands and Young Adults" by economists Stephen Rose and David Fasenfest, found that husbands' salaries declined in 80 percent of married couple households, with a steeper drop among lower income families.

The average husband's wage has fallen 4 percent in recent years, the survey of Census Bureau data found.

Between 1979 and 1986, wives in intact married couples increased their working time in the paid labor force by 18 percent, the study found. If they had not done so, family incomes would have grown by one-third less.

"Since 72 percent of the population live in these families, the increased work effort by wives was the major reason many families have maintained living standards in a period of declining wages," the study said.

The study also found that young adults aged 25 to 34 years old who did not enter or finish college are doing substantially worse than their counterparts in earlier years.

A young man without any college education earned nearly 15 percent less in 1986 than he would have earned in 1979, the study said, and added that the proportion of young men without any college education rose from 50 percent in 1979 to 54 percent in 1986.

In order to cope with lower wages, more young adults are living at home than forming their own households, with 1.5 million more young adults "returning to the nest" in 1986 than in 1979, the study said.

The 40 percent of the population with total family incomes below $15,500 saw their living standards decline by more than 5 percent during the 1980s, the study found.

Those middle income families who managed to achieve some income growth in the `80s did so by working longer hours, the study showed.

"This increased income, however, is only available at the cost of increased child care expenses (or foregoing having children,) loss of leisure time and the need to purchase services that were previously provided by family members," the study said.