A much-anticipated report from the largest and longest-running study of American child care has found that keeping a preschooler in a day-care center for a year or more increased the likelihood that the child would become disruptive in class — and that the effect persisted through the sixth grade.

The effect was slight and well within the normal range for healthy children, the researchers found. And as expected, parents' guidance and their genes had by far the strongest influence on how children behaved.

But the finding held up regardless of the child's sex or family income and regardless of the quality of the day-care center.

On the positive side, the authors also found that time spent in high-quality day-care centers was correlated with higher vocabulary scores through elementary school.

The research, being reported today as part of the federally financed Study of Early Child Care and Youth Development, tracked more than 1,300 children in various arrangements, including staying home with a parent; being cared for by a nanny or a relative; or attending a large day-care center. Once the subjects reached school, the study used teacher ratings of each child to assess behaviors like interrupting class, teasing and bullying. The findings are certain to feed a long-running debate over day care, experts say.

The debate reached a high pitch in the late 1980s, during the so-called day-care wars, when social scientists questioned whether it was better for mothers to work or stay home. Day-care workers and their clients, mostly working parents, argued that it was the quality of the care that mattered, not the setting. But the new report affirms similar results from several smaller studies in the past decade suggesting that setting does matter.

Others experts were quick to question the results. The researchers could not randomly assign children to one kind of care or another; parents chose the kind of care that suited them. That meant there was no control group, so determining cause and effect was not possible. And some said that measures of day-care quality left out important things.