WASHINGTON – Since its inception in 1991, the largest and longest-running study of American child care has generated plenty of controversial conclusions about the effects on kids of early care outside the family.

The latest findings of the federally funded Early Child Care Research Network are certain to be no exception. At age 15, according to a study published Friday in the journal Child Development, children who spent long hours in day care as preschoolers are more impulsive and more prone to take risks than are teenagers whose toddler years were spent largely at home.

To be sure, the differences between kids who logged long hours in day care and those who did not were slight. Filling out inventories that measured their impulsiveness, teenagers rated themselves about 16 percent more rash in their behavior for every additional 10 hours they spent per week in day care as a preschooler.

RISKY BEHAVIOR

In terms of risk-taking, the link to time spent in day care was more marginal: 10 more hours a week in day care prompted the average teenager to answer one out of 30 questions with an admission of more risky behavior.

But the study’s authors defended the findings as significant and, in some ways, surprising.

For starters, the behavioral differences between day care veterans and those who spent more time in the care of a parent spanned the income and class spectrum. Those differences were evident even at 15 years of age — more than a decade after Mom or Dad had picked them up at day care for the last time.

And the effects are spread across vast swaths of the American population.

That, says psychologist Jay Belsky, an author of the latest study, makes small behavioral shifts potentially far-reaching in their impact, especially as preschoolers mature into adolescents and as peers become the pre-eminent force in a child’s life.

“You end up with contagion effects,” said Belsky, a professor of psychology at Birbeck University of London.

In classrooms and peer groups populated by kids who may be just a little more impulsive or risk-taking, “these small effects end up being spread and bounce off each other,” he said. “The dynamic becomes, ‘I dare you to take a risk, you dare me to take a risk.’

“Nobody knows what the threshold here is, when the little becomes a lot,” he said.

WARNINGS STIR ANXIETY

Earlier, such warnings from Belsky and other child care researchers have stirred anxiety and guilt among parents — especially mothers, whose large-scale entry into the work force spurred an epochal shift in child care patterns starting in the 1970s.

Belsky said he has been “crucified” for sounding the alarm about the Early Child Care Research Network’s earlier findings, which revealed a link between the amount of time a child spent in day care and an increase in aggressive and disobedient behavior throughout elementary school.

He said that the research, carried out at 21 academic institutions across the United States, has gleaned important insights. In doing so, it has helped shift researchers and policy makers from an exclusive focus on the quality of care to consider what Belsky called the “dosage effect” — or time spent in care.

Deborah Lowe Vandell, the study’s lead author, acknowledged that the behavioral effects uncovered among kids with long hours in day care may worry some parents. At the same time, she said, those findings should help parents, as well as child care providers and policy makers, with some guideposts to ensuring better care for their kids.

EXTENDING PAST FINDINGS

“We might be much more attentive to issues of helping children in navigating social settings and in teaching them more about behavioral regulation,” said Vandell, a professor of educational psychology at the University of California, Irvine. This and other research suggests that smaller group sizes in day care centers may make it easier for kids to learn important self-control skills, she said.

The latest study results echoed and extended the network’s past findings on the importance of good-quality child care — of attentive, trained and well-compensated caregivers, clean facilities and stimulating activities.

For teenagers who had had such care, the study published Friday found strong advantages in terms of academic performance, and some behavioral benefits. But kids reaping those benefits were clearly the exception.DAY CARE CENSUS FIGURES

Some 2.3 million American kids under 5 are cared for at day care centers — about a quarter of preschoolers whose mothers are employed, according to the U.S. Census.

Another 17 percent, roughly, are in the care of someone who is not related to them in family day care settings and other, less formal arrangements.