"Compared to many homes, preschool centers are richer settings in terms of enriched language, reading and math," said Bruce Fuller, a co-author of this report, "The Influence of Preschool Centers on Children's Development Nationwide: How Much Is Too Much?"

The report, by sociologists at Stanford and the University of California, found that cognitive skills in prereading and math were strongest when children entered a center-based program from age 2 to 3.

But it also found that on average, the earlier a child enters center-based care, the slower the pace of social development. The greatest effect was among high-income children. Youngsters who were from families with income of at least $66,000 and who spent more than 30 hours a week in center-based care had the weakest social skills -- including diminished levels of cooperation, sharing and motivated engagement in classroom tasks, along with greater aggression -- compared with similar children who remained at home with a parent.

Another study, being published today in The American Sociological Review, is apparently the first broad research into safety in child care. It found that the rate of death among children receiving care in private homes was 16 times that of children in child care centers.

"Fatalities are the tip of the iceberg, and they're fortunately very rare, but they do reveal something about what goes on in centers," said Julia Wrigley, a sociologist and acting associate provost at the Graduate Center of the City University of New York, who wrote the report with Joanna Dreby. "No one kind of child care is clearly and definitely better for all children, but in terms of safety there are advantages in centers."

Infants are by far the most vulnerable children in care, Professor Wrigley found, and most often die from being shaken, usually by a caregiver stressed by constant crying. In a database she put together from state records, legal cases and news reports covering 1989 to 2003, she found 203 shaken-baby deaths in care in a private home and not a single one in a child care center.

It was a shaken-baby death that prompted her project.

"I was on the Phil Donahue show talking about a book I'd written on the relationship between parents and the caregivers they hire in their home, and another woman on at the same time had a baby who was shaken to death by the nanny," Professor Wrigley said. "As a sociologist, that got me thinking that with eight million kids in paid child care every day, there must be some safety data out there somewhere. But there wasn't."