Most people who are poor aren't that way very long. Researchers have found that between a third and half the families below the federal poverty line in one year rise above it in the next. In the 10 years 1969-78, it is estimated that a fourth of all Americans had a spell of poverty, from which most recovered. But some people, the research has shown, are persistently poor. These tend to be concentrated in female- headed families, particularly black ones. A new report reminds that there is a large such population in this city.

The report, from the Greater Washington Research Center, says that in 1980, the last census year, 113,000 people in the city were officially poor, almost a fifth of the population. Just under 100,000 were black, of whom around 50,000 were in female-headed families. Such families make up a rising share of the city population: 29 percent of black families in the District were headed by women in 1970, and 42 percent by 1980. Reflecting in part the lack of earning power in these households, 22 percent of the blacks in the District and 31 percent of black children were in poverty in 1980. Poor children become, in turn, a responsibilty of the schools.

The pattern -- less pronounced here than in most other cities -- is familiar enough. It makes everyone uncomfortable. Americans think of their social history as mostly a record of assimilation and upward mobility, not resistant poverty. What should the city government and others in the field do to ease these mostly black and young outsiders into the mainstream?

In part this debate always comes back to a question of public versus personal responsibility. The report observes that two-thirds of the black women who were heads of families in poverty in 1980 had not finished high school. They had children instead. ng many other things, that combination plainly handicapped them in any search for work. There is a problem of schooling to be dealt with. But are the schools a particularly useful place to start?

Consider in the same vein the report's grim statistics on black males. Twenty-seven percent of adult black males in the District had no jobs in 1970, were either unemployed or out of the labor force; by 1980 it was 41 percent -- 62,130 idle men, a tenth of the city's population. In 1970, for every 100 black women in their twenties in the city there were 65 black men of similar age working full-time. By 1980 there were only 53. What is the relationship between black family structure and what the report calls the "limited pool of economically marriageable black men" in the city? Are jobs the starting point?

These are long-run -- and in some ways theoretical -- questions. For the near term, the report says that the only way to make people less poor is to ive them more money -- adopt "some form of enhanced income support." The city's welfare program is not generous; the cash allowance for a welfare mother and three children is $4,788 a year (though on top of that come food stamps and Medicaid); the poverty line for a family of four was $10,609 last year. City welfare benefits have lagged behind inflation; their purchasing power has fallen 35 percent in the last 16 years. But an increase -- a proposal to index benefits is before a city council committee -- would be costly. Too often in the past this subject has been shunted aside. The new report may help keep it center- stage this time, where it belongs.