WILLIAMSBURG -- William Julius Wilson, the University of Chicago sociologist, is here to push an oddball notion: If we are to figure out what to do about the burgeoning underclass, we'd better pay some attention to how it came to be.

The existence of the black underclass, at least in its present magnitude, is a fairly recent phenomenon, according to Wilson, one of nearly a hundred private- and public-sector welfare experts brought together at Colonial Williamsburg for a two-day conference.

In 1970, he said, 16 of Chicago's 77 community areas had poverty rates of at least 20 percent. Eight of these had rates of 30 percent or more, and only one exceeded 40 percent. By 1980, 25 community areas had poverty rates in excess of 20 percent, 14 had rates of 30 percent or higher, two had rates above 50 percent, and one topped 60 percent.

In 1970, only five of Chicago's community areas had unemployment rates of 15 percent or more; by 1980, 25 did, and of these all 10 predominately black areas had jobless rates of 20 percent or higher.

What happened during that fateful decade? The fundamental factor, according to Wilson and his colleague Lo c J. D. Wacquant, was drastic change in the American economy.

"In the last 35 years, Rust Belt central cities lost close to half of their total manufacturing employment and upwards of six-tenths of their production workers in this sector alone," according to a Wilson-Wacquant paper based on their new book, "The Truly Disadvantaged."

Manufacturing jobs fell by two-thirds in New York City and Chicago, by 61 percent in Philadelphia and Detroit, and by 54 percent in Cleveland and Baltimore, with the sharpest cuts coming after 1967.

It is that "deindustrialization," not the growth of welfare, some new "culture of poverty" or some unexplained increase in racism that accounts for the jobless, hopeless central city element now known as the underclass.

But why have urban blacks been hit so disproportionately hard by the shift to service-producing industries and by the relocation of manufacturing plants to the suburbs, the South and abroad?

The reason, say the University of Chicago researchers, is that blacks -- particularly blue-collar blacks -- have been disproportionately concentrated in the inner cities, partly due to racial discrimination.

Thus: "Racism may help to explain why there was a large black group that was peculiarly vulnerable to these changes in the economy, but the changes themselves are the critical key."

But the term underclass invokes not just joblessness but also single-parent households, school failure, crime and general discouragement. Can economic shifts account for the entire range of problems?

Wilson and Waquant have no doubt.

Single-parent households are, in large measure, a direct result of the joblessness of black men, they note. "Black women generally, but especially young black females residing in large cities, are facing a shrinking pool of 'marriageable' -- that is, economically secure -- men."

In 1950, for example, there were between six and seven employed males for every 10 adult women in the neighborhoods that make up the heart of Chicago's black ghetto. By 1980, when the proportion of marriageable men had dropped to 56 percent in Chicago overall, it was 24 percent in Grand Boulevard, 29 percent in Washington Park and 19 percent in Oakland.

As for school failure, they note, rampant joblessness "weakens the perception of a meaningful relation between education and work and thus decreases academic aspirations."

Worse, the economic transformation of the inner cities eliminates the normal channels of employment (people tend to learn about employment opportunities through employed friends and family members), and the relocation of local businesses eliminates a traditional source of full- and part-time jobs.

"Illegal activities such as drug dealing or fencing stolen goods are often the only readily available means by which teen-agers from these communities can get the income they and their families need. As a result, many inner-city adolescents routinely become involved with crime rather than work at an early age."

Wilson and Wacquant are perhaps at their weakest when it comes to prescribing remedies -- not necessarily because they are wrong but because the present political climate makes their prescription -- the adoption of a full-employment policy, the creation of WPA-type jobs and an expanded earned income tax credit -- unlikely to be filled.

But they are on the mark when they insist that the problem of the underclass is less the result of individual shortcomings than of the economy and that it cannot be solved so long as inner-city joblessness remains the norm.