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."
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