In the largest study of child care and development in the U.S. (1,364 children), researchers found the more the child care, the more behavioral problems at school. In 2001, the first phase of the study found that children in child care were more likely to be aggressive and defiant. Follow-up now when the children are in 5th and 6th grades show that behavioral problems continue.
The latest study isn't available free online yet (why not, as it's a federally-funded study, one might ask...), but the NIH released a 62 page booklet summarizing the first phase of the study last year (available here.
The correlation of child care and problems at school behavioral problems can be noted below, but also note that poor parenting quality (observing parent-child interactions, interviews) also seemed to have a role.
Importantly, parenting quality was a stronger predictor than early childcare experience of school success.
MSNBC: Daycare and School Behavioral Problems
Are There Long-Term Effects of Early Childcare? Abstract March 07
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