Nearly half of African Americans born to middle-income parents in the late 1960s plunged into poverty or near-poverty as adults, according to a new study -- a perplexing finding that analysts say highlights the fragile nature of middle-class life for many African Americans.

Overall, family incomes have risen for both blacks and whites over the past three decades. But in a society where the privileges of class and income most often perpetuate themselves from generation to generation, black Americans have had more difficulty than whites in transmitting those benefits to their children.

Forty-five percent of black children whose parents were solidly middle class in 1968 -- a stratum with a median income of $55,600 in inflation-adjusted dollars -- grew up to be among the lowest fifth of the nation's earners, with a median family income of $23,100. Only 16 percent of whites experienced similar downward mobility. At the same time, 48 percent of black children whose parents were in an economic bracket with a median family income of $41,700 sank into the lowest income group.

This troubling picture of black economic evolution is contained in a package of three reports being released today by the Pew Charitable Trusts that test the vitality of the American dream. Using a nationally representative data source that for nearly four decades has tracked people who were children in 1968, researchers attempted to answer two questions: Do Americans generally advance beyond their parents in terms of income? How much is that affected by race and gender?

"We are attempting to broaden the current debate" beyond the growing gap between higher- and lower-income Americans, said John Morton, Pew's managing director for program planning and economic policy. "There is little out there on the question of mobility across generations, and we wanted to examine that."

The data source, called the Panel Study of Income Dynamics, followed 2,367 people from across the country, including 730 African Americans, since 1968. The study participants have been repeatedly interviewed about their economic status through the years, allowing for income comparisons across generations.

The Pew reports found that in many ways the American dream is alive and well. Two out of three Americans are upwardly mobile, meaning they had higher incomes than their parents. About half the time, moving up meant not only that they earned more money than their parents, but also that they were better off in relation to other Americans than their parents were.

That growth was most evident among lower-income people. Overall, four out of five children born into families at the bottom 20 percent of wage earners surpassed their parents' income. Broken down by race, nine in 10 whites were better-paid than their parents were, compared with three out of four blacks.

Median family income for adults now in their 30s and 40s rose by 29 percent, to $71,900 between the two generations covered in the reports. And as incomes grew, households shrank, from an average of 3.1 individuals in 1969 to 2.3 in 1998 -- meaning that income per person grew even more.

Julia B. Isaacs, a researcher at the Brookings Institution who authored the three reports, noted that between 1974 and 2004, the median income for men in their 30s actually dropped 12 percent. But because more women entered the workforce, and earned much more than their mothers, median income for women more than tripled during the period, to $20,000.

"The growth we've seen in family incomes is because of the increase in women's income," Isaacs said. "Without that, we would not have seen an increase, because men's earnings have been flat and even declined."

Again, the reduction has been more dramatic for black men than whites. And income for white women, who were less likely than black women to work outside the home a generation ago, has grown faster than it has for black women. Black women earned a median income of $21,000 in 2004, almost equal to that of white women. Black men had a median income of $25,600, less than two-thirds that of white men.