Study: Day Care Kids Aggressive?

The longer young children spend in day care, the more likely they are to be overly aggressive by the time they reach kindergarten, say researchers who first made the link two years ago.

Results of the 10-year, 10-city federally financed study made headlines at a child development meeting in 2001. On Wednesday, the same findings were published in a medical journal, Child Development.

The study followed more than 1,000 children from birth to age 4½ in a variety of settings, from care with relatives to large day-care centers.

The more time children spent in care away from their mothers, the more likely caregivers were to report aggression and disobedience. Seventeen percent of children in day care for more than 30 hours a week had problem behaviors.

CBS News Correspondent Elizabeth Kaledin spoke to Dr. Sarah Friedman at the National Institutes of Health, one of the study's authors.

"There is an association between hours and problem behavior, that is, the more time in care, the more problem behaviors," Friedman said.

That news might cause working mothers to worry, but the study goes on to say the problem behaviors witnessed in the vast majority of children in day care were all considered well within the normal range, so what are mothers to believe?

CBS' Kaledin also spoke to Ellen Galinsky, an expert who's been following the debate for decades. Galinsky says the conflicting results underscore the main problem with American childcare. Parents are being asked to work longer hours, but day care remains inadequate, and a low priority for policy makers.

"We need to make changes in work and in the supports that we offer families rather than turn around and attack the people who are doing their best to earn a living for their children," said Galinsky.

The study also focused on children's responses to their mothers' behaviors.

The strongest predictor of child behavior was not day care but how sensitive mothers were to their child's needs. Researchers said children of sensitive mothers appeared more competent socially and less likely to be involved with conflicts.

A separate study published Wednesday tracked levels of the stress hormone cortisol in 55 infants and toddlers. For a third of infants and almost three-fourths of toddlers, cortisol levels rose throughout the day when they were in day care. For most, cortisol levels dropped when they stayed home.

Toddlers who played with their peers had lower cortisol levels than those described as fearful.

The University of Minnesota researchers had previously found the same patterns in preschoolers, and said they show that day care may be challenging for some children.