One of five children under 18 in America today lives in a one-parent family, and for blacks the proportion is about one in two, according to new Census Bureau reports.

The reports, derived from the 1980 decennial census and other surveys, show that of about 63 million children, 12.6 million are living with one parent only, usually the mother.

The current figures contrast sharply with 1960, when only 9 percent of children lived in single-parent households. In that year, 93 percent of white children and 75 percent of black children lived in two-parent homes.

The increase in single-parent families is one of the most striking social developments of the past generation. About 19 percent of families with children (over double that for black families) are now headed by a single parent.

Single parenthood contributes massively toward poverty and dependency on welfare. The 1980 census shows that while the proportion of American families living in poverty in 1979 was 9.6 percent, it was 40 percent for families with children headed by single women, or 2.3 million of 5.6 million such households.

Another survey shows that for white single-parent families, the poverty rate was 36 percent in 1980, for black families of this type it was 56 percent and for Hispanic-origin families almost 57 percent. In 1979, the median income of families with children headed by single black women was only $6,610, or one-third the $19,961 figure for all U.S. families.

These reports mirror profound changes in the American family in recent decades.

There has been a drop in the proportion of Americans living in traditional "mom-and-pop" families, a sharp rise in divorces and in births to unmarried women, especially black women, a huge jump in one-parent families, and an increase in the number of women in the labor force. There have also been increases in the number of black-white married couples (only 166,000, but triple 1970), and in unmarried couples living together (1.6 million in 1980).

In 1960, 84 percent of the population lived in households headed by a married couple (including remarried ones), consisting of parents, children and other relatives.

By 1970, the proportion of Americans living in these "mom-and-pop" households had dropped to 82 percent, and in 1980, to 73.5 percent.

The marriage rate (including remarriages) was 10.3 per 1,000 population in 1910 and was estimated at 10.6 in 1981. But divorce has increased enormously. In 1910 it was only 0.9 per 1,000 population, reached 2.2 in 1960, 3.5 in 1970 and now is about 5.3 per 1,000.

The past generation has also witnessed a sharp rise in births to unmarried women, including the "explosion" of teen-age pregnancies. In 1950, 1.7 percent of children born to white women were born out of wedlock; the figure was 16.8 percent for non-white women. By 1979, the figure for whites had reached 9.4 percent, and for non-whites 48.8 percent; for black women alone the 1979 out-of-wedlock rate was 55 percent.

In 1940, according to Census Bureau figures, the proportion of families with children headed by single women among blacks was 18 percent of all such families; by 1970 it was about 30 percent, and today about 42 percent.

Among white female-headed families with children the figure was 8.3 percent in 1970 and 12.6 percent today, a big rise even though starting from a smaller base than blacks. The overall figure for single-parent families today is 19 percent.

All these phenomona have increased enormously the proportion of children living with one parent, from 9 percent in 1960 to 20 percent today. By 1990, 25 percent is projected.

Broken down by race the figures are startling: 75 percent of black children lived in two-parent households in 1960; today, only 42.7 percent. Another 46 percent live in one-parent households and the rest with other relatives or in other living arrangements.

In 1960, 93 percent of white children under 18 were living in a two-parent household. By 1981, the figure had dropped to 82.3 percent. Another 15.5 percent were in one-parent families and the remainder were with other relatives or in other living arrangements.

The sexual revolution and the rapid entry of women into the labor force (in 1940 a quarter of the women were in the labor force, today it is 52 percent) are often cited as reasons for some of these trends.

But there is disagreement among experts on why the figures on illegitimacy and one-parent families are higher for blacks than whites.

Some observers say that low earnings, lack of economic security and relatively high unemployment among black men, as compared to white men, probably discourage marriage by teen-age girls who become pregnant and thus contribute to the high illegitimacy and one-parent family rates.

"The males have little economic future; they can't count on a good income," said Herrington Bryce, president of the National Policy Institute. As a result, marriage is often economically unappealing and impractical and the woman bears the baby alone.

Robert B. Hill, former director of research of the National Urban League and now a senior research associate at the Bureau of Social Science Research, also cited economic difficulties of black men, particularly during the 1970s, when some of the social changes have become the most evident.

Several recessionary periods created extremely high unemployment during the 1970s for many blacks in urban areas, Hill said. Harsh conditions, he said, make it more likely that there will be family breakup when there is personal strife.

But he also said there is something about moving out of the rural environment of the South into the nation's big cities that leads to family breakup and instability, and blacks made that move rapidly and are now more urbanized than whites.

In 1940, half the nation's blacks lived in urban areas compared with three-fifths of the whites. But by 1970, 81 percent of the blacks and only 72 percent of the whites did, and most blacks lived in central cities while a high proportion of the whites lived in the surrounding suburbs.

The earlier era of farm life made for more stability and the male was the breadwinner, he said. But that stability and that role was often lost in the labor market conditions of the central cities.