Add one more theory to the long list of theories explaining the high and persistent rate of joblessness amoung young urban blacks.

You've heard the theory, dear to political conservatives, that the key problem is the minimum wage; the kids simply aren't worth what the law says they must be paid. You've heard the liberal theory that the problem is government's reluctance to subsidize employers for hiring disadvantaged youth.

You've heard the demographic theory (the youth are in the central cities while the growth in job opportunity is in the suburbs), the educational theory (the jobless youngsters have too few vocational or academic skills) and the racial theory (the problem is discrimination, though that theory leaves unexplained why the job gap between white and black youth is wider now than when racism was far more blatant).

Now try this one: the youngsters are unemployed because employers don't want them around.

I've just seen a report by the Manpower Demonstration Research Corporation of New York that seems to make just that point.

Working under grants from the Rockefeller Foundation, the Rockefeller Brothers Fund and the U.S. Department of Labor, the researchers undertook to discover the extent to which private businesses are willing to hire disadvantaged youths and the reasons why.

As a part of the demonstration, nearly 6,000 private businesses were recruited as work sponsors, with the prime sponsors -- CETA agencies -- subsidizing the workers" pay at rates ranging up to 100 percent.

"All the youths were paid from a central payroll, managed by the prime sponsor, so that work sponsors would not have the paperwork burden of carrying youths on their payrolls, withholding taxes or covering them with Workmen's Compensation Insurance," the researchers reported.

"Furthermore, all prime sponsors save one opted to subsidize the full minimum-wage cost of enrollees assigned to private businesses. Thus a youth who worked a full year with a private employer (10 to 20 hours per week in the school year, 30 to 40 hours per week in the summer), with a full wage subsidy, would have brought more than $2,500 in subsidized wages to an employer at the 1980 minimum wage.

"These arrangements probably marked as attractive a bargain as a prime sponsor could strike in recruiting a business to provide work experience for youths."

In addition, several of the prime sponsors enlisted the assistance of such organizations as the Chamber of Commerce and the National Alliance of Business in recruiting private employers. The results:

Only 18 percent of the employers who were offered a 100 percent wage subsidy agreed to participate in the project. When the subsidy was set at 75 percent, the participation rate fell to 10 percent. At a 50 percent subsidy, the participation rate was only 5 percent.

The most frequently cited reason for non-participation was that the employer simply didn't have enough work or that the work available required higher skills than high school students could offer.

Among those firms that did participate, the availability of cheap labor seemed to be a less important factor than sheer altruism -- the chance to do something to help jobless youths.

It seems clear that employers (at least those involved in this relatively small demonstration) tended to see the youngsters as not worthwhile -- even when no money was involved. On the other hand, 80 percent of those who did participate found the youths' work habits, attitudes and willingness to work average or better; three-fourths found their performance improving over time, and two-thirds said the young workers required no more supervision than they had originally anticipated.

Indeed, one-fifth of the employers said they had put enrollees on their own payrolls after the subsidy expired.

In terms of policy development, it's hard to know what to make of the findings. Clearly the adoption of either a special "youth minimum wage" or government subsidies would not improve the employ-ability of youths who weren't wanted even at no cost. Nor does the accessibility of the work place seem to be an important barrier. And perhaps most significant, the findings cast serious doubt on the Reagan administration's emphasis on private-sector jobs as a cure for youthful unemployment.

The most upbeat finding in the whole dismal report is that when employers do give the youngsters a chance, they tend to work out quite all right