Washington ? Since its inception in 1991, the largest and longest-running study of American child care has generated plenty of controversial — and to many working parents, infuriating — 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 being published today 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 teens 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, teens 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.

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 teen 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 experienced day care veterans and those who spent more time in the care of a parent appeared across 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: 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 a nonrelative in family day care settings and other, less formal arrangements.

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 adolescence 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,” Belsky said in an interview. “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 added.

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.