FRIDAY, Feb. 8 (HealthDay News) -- Parents who send their children to day care may be able to breathe a sigh of relief. New research finds that children in child care do not have an increased risk of behavioral problems.

The catch? The new study was conducted in Norway, which has a vastly different child care system than the United States -- where studies have been conducted that did find increased behavioral problems.

Differences between several studies finding behavioral troubles such as aggression -- including a 2007 U.S. National Institutes of Health study -- and this one from Norway noting no such link may be attributable to vastly different systems of child care in the two countries, authors of the new study suggested.

The researchers assessed behavioral problems in more than 75,000 children attending day care, including nearly 18,000 siblings, at ages 18 months and 3 years.

Without adjusting for factors such as family characteristics, the authors initially found a small association between children who spent very long hours in day care (more than 40 hours a week) and an increased risk of behavioral problems.

But the increased risk probably was not "clinically meaningful," meaning it wouldn't necessarily be apparent to the average observer, said Eric Dearing, co-author of the study, which appeared online recently in the journal Child Development.

But that finding changed when additional analyses were done, particularly when the authors zeroed in on children who were siblings.

"Once we moved beyond simple associations and began to compare siblings from the same families and individual children [in the same family] whose quantity of day care changed over time, we saw no evidence of an association," said Dearing, who is a psychologist and associate professor at Boston College's Lynch School of Education.

Because this study used more rigorous methods than commonly seen in U.S. studies (such as looking at siblings), at this point it's not possible to know why the Norwegian study had different results.

Certainly it could be due to the dramatic difference between the Norwegian child care system and that of the United States, Dearing said.

In Norway, new parents get one year of paid parental leave. As a result, children don't enter day care until after they're 1 year old. In the United States, the average age of entering child care is 3 months, Dearing said.

Norway also offers near universal access to subsidized child care and has stringent quality standards for all child care centers.

Another expert noted the variations between Norway and the United States.

"There are substantial differences in the approach to early child care between the two countries, which limits to some extent the [ability to extrapolate] their findings to our society," said Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Cohen Children's Medical Center of New York, in New Hyde Park.

The study also had other limitations that may have affected the results, said Alan Hilfer, director of psychology at Maimonides Medical Center, in New York City.

For instance, the follow-up period -- until the children were 36 months of age -- was relatively short and behavioral problems could first appear later than that, Hilfer said.

Also, Hilfer pointed out, the authors relied only on mothers' reports of whether their child was showing other behavioral disturbances, which is not always a reliable measure.

Still, study author Dearing said, it's possible that "providing access to high-quality care ... and generous parental-leave policies may be critical to realizing the benefits of care without realizing the harm."

More information

The U.S. National Library of Medicinehas more on child care.