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.

The study did not take into account employee turnover, a reality in many day care centers that can have a negative effect on children, said Marci Young, deputy director of the Center for the Child Care Workforce, which represents day care workers. Most employees are “egregiously underpaid and have no benefits,” Ms. Young said, and when they leave for other work, “children experience this as a loss, and that does have an effect on them.”

The study, a $200 million project financed by the National Institute of Child Health and Human Development, recruited families in 10 cities from hospitals, after mothers gave birth. The researchers regularly contacted the mothers to find out where their children were being cared for, and visited those caregivers to see how attentive and how skilled they were with the youngsters.

In 2001, the authors reported that children who spent most of their day in care not provided by a parent were more likely to be disruptive in kindergarten. But this effect soon vanished for all but those children who spent a significant amount of time in day care centers.

Every year spent in such centers for at least 10 hours per week was associated with a 1 percent higher score on a standardized assessment of problem behaviors completed by teachers, said Dr. Margaret Burchinal, a co-author of the study and a psychologist at the University of North Carolina.

The Children’s Defense Fund estimates that 2.3 million American children under age 5 are in day care centers, many starting as toddlers and continuing until they enter kindergarten. Some 4.8 million are cared for by a relative or a nanny, and 3.3 million are at home with their parents.

The study was not designed to explain why time in day care could lead to more disruptive behavior later on. The authors and other experts argue that preschool peer groups probably influence children in different ways from one-on-one attention. In large groups of youngsters, disruption can be as contagious as silliness, studies have found, while children can be calmed by just the sight of their own mother.