The more time toddlers spend in child care, the more likely they are to display behavior problems in kindergarten, according to the results of the largest and most authoritative long-term study of child care in the United States.

That was true regardless of the quality or type of child care, the study found.

While those findings are sure to send shudders through millions of American homes, the study from the National Institute of Child Health and Human Development also contained good news for working parents, showing that children who are enrolled in high-quality child care performed better on tests of language, knowledge and memory than did children who spent more time at home with their mothers or in lower-quality child care.

The government-sponsored research found a direct correlation between time spent away from mothers and children who exhibited behavior that was more aggressive, disobedient and defiant in kindergarten.

Researchers said this held true whether the children were looked after by child-care centers, relatives, nannies or even the children's fathers. It held true regardless of the size of the centers, gender of the children or the financial circumstances of the family.

The reason for the behavior problems was unclear, researchers said. The researchers could not say whether time away from mothers actually brought on the behavior problems or the problems resulted from some other factors such as the stresses on the lives of two-income families.

The researchers said they had no idea whether the problem behavior persisted as the children moved to higher grades, and cautioned against reading too much into the study, because most of the children's behavior fell within the normal range.

"We don't know what the implications are," said Sarah Friedman, a psychologist at the National Institute of Child Health and Human Development and one of the lead investigators on the study. "We don't understand why we got these findings."

The study has tracked more than 1,300 children in 10 cities across the country since their births in 1991. Most of the children are now in fourth grade; it takes academics years to analyze their data.

Many child development experts praised the research.

Marcy Whitebook, a researcher at UC Berkeley's center for the study of child-care employment, said the study was one of the most comprehensive ever done on the subject in the United States.

Even so, the findings sparked an animated, and often outraged, response from some women's advocates.

"It's like just yank every woman out of the job and tell her to get back in her house and take care of her kids," said state Sen. Jackie Speier (D-San Francisco), an advocate for greater funding for child-care programs. "We are living in the 21st century. Everyone's got to get over that."

At the same time, the study was touted by some who endorse the idea of mothers staying home.

John Fugatt, executive director for the Christian Coalition of California, said: "It is better in the early years for kids to have a mom at home if at all possible. Why would you want to pay someone who doesn't care as much about your child as you do? You know what your child's needs are and can focus on those needs rather than someone who is watching eight to 10 kids and doesn't give them individual attention."

One researcher cautioned against overreaction in either direction.

"We can't tell parents, 'Oh, don't worry, you can wash your hands of the responsibility of looking at where your kids spend their days,' " said Virginia Allhusen, a research professor at UC Irvine who participated in the study. "But at the same time, it's not such a cause for alarm that we need to tell parents 'Quit your job immediately and stay home with your kids.'"

The results will be formally presented today at a meeting of the Society for Research in Child Development in Minneapolis.

The researchers found that 17 percent of children who were away from their parents for more than 30 hours a week were rated as having problem behaviors by their child-care providers or kindergarten teachers as early as age 4½. Among children who spent less than 10 hours a week in child care, only 6% had similar problems.