It could be the greatest social experiment of our time, in which millions of parents are unwitting participants.

So says Stanley Greenspan, George Washington University child psychiatrist, about the current state of day care. In just 25 years, American families have been radically restructured as the number of women in the workforce has nearly doubled. Instead of parents providing early child care, it is outsourced to virtual strangers. An estimated 12 million American infants, toddlers and preschoolers—more than half of children in this age group—attend day care. The majority of these kids spend close to 40 hours per week in day care; many start when they are only weeks old.

The raging debates around maternal , work/family balance, money and childrearing often drown out scientific insights into the developmental impact of day care. But the latest findings, from a huge, long-term government study, are worrisome. They show that kids who spend long hours in day care have behavior problems that persist well into elementary school. About 26 percent of children who spend more than 45 hours per week in day care go on to have serious behavior problems at kindergarten age. In contrast, only 10 percent of kids who spend less than 10 hours per week have equivalent problems.

Developmental psychologists are sweeping this information under the rug, hoping studies will churn out better data soon, argues Jay Belsky, a researcher at London's Birbeck College and a longtime critic of his fellow scientists. He contends that the field of developmental psychology is monopolized by women with a "liberal progressive feminist" . "Their concern is to not make mothers feel bad," he says.

Belsky's fellow researchers—many of them women—take a different view. They emphasize that research findings about day care's effects are a complicated mixture of good and bad news, and that it's too early to judge whether too many hours in child care is inherently dangerous.

Few parents have heard about the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care, an ongoing $100 million survey of 1,100 children. It's the largest and most rigorous examination of day care in history, taking into account family income and the quality of day care. Evidence from the study shows that the total number of hours a child is without a parent, from birth through preschool, matters. The more time in child care of any kind or quality, the more aggressive the child, according to results published in Child Development. Children in full-time day care were close to three times more likely to show behavior problems than those cared for by their mothers at home.

Belsky contends that the current results clearly show children benefit from fewer hours in child care, especially at a very young age, and parents should be advised to limit the hours their young children spend there. NICHD researcher Sarah Friedman—who, along with Belsky and many other scientists, works on the NICHD study—emphasizes that 83 percent of children who spend 10 to 30 hours in day care did not show higher levels of . Friedman says the study results so far don't tell her anything.

"It's so obvious that this is the worst of politically correct rhetoric," counters Belsky, adding that if researchers were studying the amount of time spent in poverty and they got the same results, no one would dare put a neutral spin on the data.

Belsky has been outspoken on the issue for decades. In the 1980s, his studies showed that children spending long hours in day care had higher levels of aggression than those raised by their mothers. Detractors excoriated him then for using bad science to criticize working women.

Belsky argues that other NICHD scientists gloss over the finding that the aggressive children in the study were more than just a little defiant. They were in the "at risk" range, meaning their behavior was close to the threshold requiring . The children with problematic behavior were regularly observed being disobedient. Parents and caregivers noted frequent hitting of other children and caregivers, arguing, cheating, destruction of objects and demands for excessive .

Although Belsky's harsh words haven't won him many friends, some researchers think he has a point. Kathy McCartney, an professor at Harvard and another NICHD day care researcher, concedes the aggression results are significant, but won't offer cautionary advice without more research. "So far it is looking like he's right," says McCartney, who criticized Belsky's claims in the past. "Long hours in child care are associated with behavior problems."

Still, McCartney questions what parents and policy makers should do with the information since solutions aren't obvious. "I don't think there is a simple answer to the question 'Does child care pose a risk?' because it clearly does for some children, but not all children," she says.

The real question is whether parents can afford to wait years for more answers. What if, Belsky asks, "kids experiencing long hours in day care are more likely to use drugs, are less ambitious and have trouble with relationships? Parents will say, 'How come no one warned me?' It is our scientific responsibility to tell people what they may not want to know."

Bad Behavior

Data from a government study show a strong link between the total hours per week a child spends in day care and behavior problems at age 5. The 113 behaviors surveyed include frequent arguing, temper tantrums, , hitting and unpredictable conduct.

Number of hours in day care equals a percentage of bad behavior*

Less than 10: 10 percent.

10 to 30: 17 percent.

More than 45: then 26 percent.

*As reported by mothers. Source: NICHD.