The composition of the American family, particularly the black family, has undergone major changes over the past two decades - statistically, at least - according to a new study by the U.S. Census Bureau.

Traditional families - those headed by married couples - have decreased from 87 percent of all family households in the United States in 1970 to 79 percent in March of this year, the study showed. Meanwhile, the number of families maintained by women with no husband present in the house doubled over the past 20 years - from 5.5 million in 1970 to 10.9 million today.But the figures on the number of black families maintained by women alone showed a more radical development, as those concerned with black-family structure have known for some time. The number is approaching a point where such families will represent nearly half of all black families; it is now 44 percent.

Additionally, 6 percent of all black families are now maintained by men alone.

Two areas spotlighted by the study, titled "How We're Changing - The Demographic State of the Nation: 1990," were Americans' incomes and the extent of poverty in the United States.

The median income of white families has risen slightly since 1970 - from $32,713 for a total of 46.5 million families to $35,980 for 56.6 million families in 1989, the study found. The median income for black families has shown little change: $20,067 for a total of 4.9 million families in 1970, and $20,210 for 7.5 million families in 1989.

On a per capita basis, however, the study found that blacks have done better.

Their per capita income increased from $5,973 in 1970 to $8,750 in 1989, an increase of 46 percent. Whites' per capita income, meanwhile, rose from $10,719 in 1970 to $14,900 in 1989, up 39 percent.

The study showed, nevertheless, that blacks' goal of "parity" appears to be a long way off.

Major civil rights and black affairs organizations, including the National Urban League and the Joint Center for Political and Economic Studies, have set a goal of black-white parity in income by the year 2000. But the study showed that blacks are not only nowhere near that stage, but are moving toward it at a rate that would bring them considerably short of their goal.

Black families earned only about two-thirds - 61 percent - of the median income of white families in 1970, according to the study, and they have been unable to keep pace even with that low share.

Today, black-family median income, according to the study, represents only 56 percent of what a white family earns.